Risk Assessment of Thyroid Follicular Cell Tumors

Hill,1 M. Pamela M. Hurley,' Rosenthal2 and Dharm V. Sing2f

1Office of Prevention, Pesticides and Toxic Substances, and 2Office of Research and Development, U.S. Environmental Protection

Agency, Washington, DC 20460 USA

Thyroid folliclar celi tumor arise in rodents from mutations, perturbations of thyroid and pituitay hormone status with increased sution of thyroid cell growth by thyroid-stimulating

hormone (TSH), or a combination of the two. The only kown human thyroid carcinogen is ionizing radiation. It is not known for certain whether chemicals that affect thyroid cell growth lead

to human thyroid cancer. The U.S. Environmental Protection Agency applies the following science policy positions: 1) chemical induced rodent thyroid tumors are presumed to be relevnt to

humans; 2) when interspecies information is lacking the default is to asme comparble carcinogenic senitivity in rodents and humans; 3) adverse rodent noncancer thyroid effe due to chemically induced thyroid-pituity disruption are presumed to be relevant to humans; 4) linear

dose-response considerations are applied to thyroid cancer induced by chemical substaces that

either do not disrupt thyroid functioning or lack mode of action information; 5) nonlinear thyroid cancer dose-response considerations are applied to chemicals that reduce thyroid hormone

levels, increase TSH and thyroid cell division, and are judged to lack mutanic acivity, and 6)

nonlinear conideions may be applied in thyroid cancer dose-response assessments on a caseby-case basis for chemicalls that disrupt thyroid-pituitary functioning and demonstrate some

mutagenic activity. Required data for risk asessment purposes is mode of action information on

mutgenicity, increases in follicular cell growth (cell size and number) and thyroid gland weight,

thyroid-pituitary honrones, site of action, correlations between doses producing thyroid effects

and cancer, and reversibility of effet when dosing ceases. Key won iodide pump, microsomal

enzyme induction, 5'-monodeiodinase, ris assessment, thyroid follicular cell tumors, thyroid

hormone (T4, T3), thyroid p ida, throid-smulating hormone (TSH), UDP glucuronosyl

trnsferase. Enwron Healt Perspect 106:447-457 (1998). [Online 29 June 1998]

hbtap//ehpnal.niehs.nihgo.ldo/1998/106p447457hiWlbsaract.html

The U.S. Environmental Protection Agency

(EPA) conducts risk assessments on chemicals for carcinogenicity under the guidance

provided in its cancer assessment guidelines

(1,2). From time to time, scientific developments cause the agency to reexamine procedures that are generally applied. That is the

case with the review of some chemicals that

have produced thyroid follicular cell tumors

in experimental animals. In this paper, we

will present a summary of thyroid carcinogenesis and describe the procedures the

EPA will use to evaluate these tumors and

the data that are needed to make these

judgments. Experience in applying the thyroid policy to a set of pesticides has been

reported (3).

The proposed EPA cancer assessment

guidelines () generally operate on the rebuttable premise that findings of chemically

induced cancer in laboratory animals signal

potential hazards in humans. Likewise, for

dose-response analyses, the guidelines first

call for use of the most biologically appropriate means for dose extrapolation. In the

absence of such knowledge, assessors are

directed toward the use of default science

policy positions, either low-dose linear,

nonlinear, or both procedures. Male rat kidney tumors due to accumulation of ax2uglobulin were the first instance in which the

EPA developed generic guidance for the

assessment of a specific tumor type (4); thyroid tumors are the second.

Although the present effort is directed

specifically at tumors of the thyroid follicular cells, it is recognized that endocrine

organs that secrete hormones into the

blood or lymph respond to stimuli via

common pathways, and other organs may

develop tumors by disruption of hormonal

balance. Therefore, it may be possible that

some of the lessons learned with the analysis of thyroid follicular cell tumors will be

applicable to other endocrine tumor sites

in the future (5,6).

In 1988, the EPA developed a review

of the existing science on thyroid follicular

cell carcinogenesis and a draft science policy position covering the evaluation of

chemicals that have induced thyroid

tumors in experimental animals. The external EPA Science Advisory Board approved

the science review (7) and tentatively

embraced the policy position that

dose-response relationships for some thyroid tumors could be assessed using nonlinear considerations. After revision and a second review, the policy document was finalized (8). This paper presents the underlying

science policy for thyroid follicular cell

tumors.

Overview of Thyroid

Carcinogenesis

Circulating thyroid hormone determines

the level of operation of most cells of the

body (9); too much or too little hormone

results in disease. Control of the concentration of this endocrine hormone in the

blood is mainly regulated by a negative

feedback involving three organs: the thyroid

gland, which produces thyroid hormone,

and the pituitary gland and hypothalamus,

which respond to and help maintain optimal levels of thyroid hormone (Fig. 1). The

hypothalamus stimulates the pituitary

through thyrotropin-releasing hormone

(TRH) to produce thyroid-stimulating hormone (TSH), which then prompts the thyroid gland to produce thyroid hormone.

The stimulated thyroid actively transports

inorganic iodide into the follicular cell and

converts it to an organic form and then into

thyroid hormone molecules, which can

influence target organs throughout the

body. Thyroid hormone, in tissues peripheral to the thyroid, can be converted from a

less active thyroxine (T4) to a more active

triiodothyronine (T ) form. Thyroid hormone is also metaboized by the liver, largely by conjugation reactions, and excreted

into the bile.

Cells in the hypothalamus and pituitary

gland respond to levels of circulating thyroid

hormone, such that when thyroid hormone

levels are high, there is a signal to reduce the

output of TRH and TSH. Likewise, when

thyroid hormone levels are reduced, the

Address correspondence to R.N. Hill, Office of

Prevention, Pesticides and Toxic Substances, U.S.

Environmental Protection Agency (7101), 401 M

Street, S.W., Washington, DC 20460 USA.

This paper is dedicated to the memory of Orville E.

Paynter, who began this project in 1985.

Received 29 September 1997; accepted 27 February

1998.

Environmental Health Perspectives * Volume 106, Number 8, August 1998 447
Reviews * Hill et at.

14

m:..

...... , .. U.

p.

-11X}

Exicndon

Figure 1. Hypothalamic-pituitary-thyroid axis.

Abbreviations: TRH, thyrotropin-releasing hormone; TSH, thyroid stimulating hormone; T3, triiodothyronine; T4, thyroxine.

hypothalamus and pituitary are prompted

to deliver more TSH to the thyroid gland

to increase the output of thyroid hormone.

This negative feedback loop helps the body

to respond to varying demands for thyroid

hormone and to maintain hormone homeostasis. Circulating T4, T3, and TSH can

readily be monitored in the serum of experimental animals and humans by radioimmunoassay and serve as biomarkers of exposure and effect of agents that disrupt thyroid-pituitary status.

In mammals, when demands for more

thyroid hormone are small, existing thyroid

follicular cells can meet the demand. With

increased need, as a result of certain chemical exposures or iodide deficiency, the thyroid responds by increasing the size (hypertrophy) and number (hyperplasia) of thyroid follicular cells to enhance hormone

output. With continued TSH stimulation

there is actual enlargement of the thyroid

gland (goiter) and, at least in rodents, eventually neoplasia of the thyroid follicular

cells. Since TSH-producing pituitary cells

are also stimulated, they too sometimes

undergo hyperplasia and neoplasia.

For details about thyroid follicular cell

carcinogenesis, consult Hill et al. (7) and an

update of the science since 1988 (10), as well

as a number of relevant reviews (11-25). Rich

historical literature is also valuable (26-33).

Role ofTSH in Rodent

Carcinogenesis

Several experimental findings in rodents

that perturb thyroid-pituitary homeostasis

and lead to elevated TSH levels indicate

the central role of TSH in inducing thyroid

carcinogenic effects: 1) loss of thyroid cells

through partial thyroidectomy led to a sustained inability of the thyroid to meet the

demands for thyroid hormone; 2) iodide

deficiency decreased the thyroid's ability to

synthesize adequate supplies of thyroid

hormones; and 3) transplantation of pituitary tumors that autonomously secrete

TSH added more of the trophic hormone

to graft recipients (7). These experimental

manipulations were done in the absence of

any exogenous chemical treatment but

demonstrate the seminal qualitative role

that TSH plays in thyroid carcinogenesis.

Quantitatively, its significance is demonstrated by the correlation between the TSH

level and the number of tumors/gland in

an initiation-promotion study (15).

If a goitrogenic stimulus that would

lead to thyroid tumor formation in rodents

is removed early in the process, effects

reverse toward normal (34). Likewise, if a

goitrogenic stimulus is given in conjunction with adequate amounts of exogenous

thyroid hormone (35) or after hypophysectomy to remove TSH-secreting cells (36),

then hypertrophy, hyperplasia, and tumors

of the thyroid do not develop. It follows

from these observations that if TSH levels

are chronically elevated, there will be thyroid cell hypertrophy, hyperplasia, and

some potential for neoplasia, but under

conditions where thyroid-pituitary homeostasis is maintained, the steps leading to

tumor formation are not expected to develop, and the chances of tumor development

are negligible.

Genetic Influences

Rodent studies indicate an interplay

between genetic and nongenetic events in

the development of thyroid tumors.

Evidence indicates that carcinogenesis

often proceeds through a number of operational steps: initiation, promotion, and

progression (37). Initiation seems linked to

genetic events with the induction of DNA

mutations, whereas promotion includes at

least nongenetic events that lead to the

expansion of a clone of initiated cells via

repeated cell division. Progression is associated with the accumulation of cell behaviors (such as enzymatic destruction of basement membranes and increased mobility)

that allow cells to invade locally and metastasize distally, probably in part due to still

other mutations. Some genetic influences

do not result in mutations, but in changes

in gene expression that can affect the carcinogenic process.

Treatment regimens that produce thyroid tumors in rodents can be conceptualized

in regard to initiation and promotion. These

steps do not themselves describe carcinogenic

mechanisms but, instead, present a framework for viewing experimental findings.

*In two-stage experiments in which a

mutagenic agent such as radioactive

iodide is followed by treatment with a

nonmutagenic goitrogen (e.g., a chemical

inhibitor of thyroid hormone synthesis),

the first agent acts like an initiator and

the second behaves as a promoter (38).

* When treatment with a goitrogen alone

leads to tumor formation, TSH increases

cell division among normal cells, which

leads to increases in the overall chance of a

spontaneous initiating mutation, and then

promotes the altered cells, which retain

responsiveness to TSR; carcinogenesis in

these cases would be free of chemically

induced mutagenic effects (39).

* Some chemicals appear to have both initiating and promoting activity, as they are

mutagenic in many test systems and have

significant antithyroid activity (e.g., 4,4'-

oxydianiline) (40).

* Still other agents, such as x irradiation (41)

and certain chemicals (e.g., some nitrosamines) (42), are definitely mutagenic but

lack intrinsic goitrogenic activity.

These agents can easily initiate the carcinogenic process, but tumor formation

would be independent of strong promotional

activity from antithyroid effects. TSH may

still play a permissive role, as these agents can

induce cell injury and cell death, which lead

to reductions in the output of thyroid hormone and increases in TSH-induced cell

division of initiated thyroid cells.

It thus appears that thyroid cancer in

experimental animals may be due to mutagenic influences that lead to DNA changes

(X rays, 1311, mutagenic chemicals), to hormone perturbation that leads to growth

stimulation, or both. Hormone perturbation directly increases the number of thyroid cells and indirectly leads to mutations

(partial thyroidectomy, transplantation of

TSH-secreting pituitary tumors, iodide

deficiency, chemicals inhibiting iodide

uptake by the thyroid, chemicals inhibiting

thyroid peroxidase, chemicals inhibiting

release of thyroid hormone from the thyroid gland, chemicals damaging thyroid follicular cells, chemicals inhibiting conversion

of T4 to T3, chemicals increasing hepatic

thyroid hormone metabolism and excretion). In those cases where increases in cell

number are dominant, the inciting agent or

procedure may be seen as the carcinogenic

stimulus, but the proximate carcinogenic

influence is TSH. For those with a dominant mutagenic influence, TSH may play

an enhancing role in the carcinogenic

process.

Possible Mechanistic Steps

The precise molecular steps in the carcinogenic process leading to thyroid follicular cell

cancer have not been totally elucidated,

although significant insights into the problem

448 Volume 106, Number 8, August 1998 * Environmental Health Perspectives
Reviews * Assessment of thyroid tumors

have been described (25,43). Normal cell

division in the thyroid seems to be affected by

an interplay among several mitogenic factors,

namely TSH, insulin-like growth factor 1

(IGF-1), insulin, epidermal growth factor

(EGF), and possibly fibroblast growth factor

(FGF). Stil other factors such as transforming growth factor , (TGFP), certain interferons, and interleukin 1 may inhibit growth.

TSH communicates with the cell's interior by activating adenylate cydase to raise levels of cyclic AMP. It also functions through

the phosphatidyl-inositol/Ca2+ signal transduction cascade, which activates phospholipase C. This latter system expresses itself

through two pathways: inositol triphosphate,

which releases calcium from cellular stores,

and 1,2-diacylglycerol, which activates protein kinase C. Signal transduction continues

following protein kinase C activation through

several steps, including the ras protooncogene

and various kinases, culminating in the activation of nuclear transcription factor genes

(e.g., c-fs), which leads to cellular proliferation. The diacylglycerol pathway may

account for the fact that the phorbol ester

tumor promoters, which increase protein

kinase C, also stimulate thyroid cell division.

EGF, insulin, and IGF-1 act through

tyrosine kinase receptors. TSH increases

EGF binding to its receptor and enhances

cell division. IGF-1 and high doses of

insulin may influence the TSH receptor.

Iodide decreases thyroid cell adenylate

cyclase and calcium levels, and reduced

iodide enhances TSH effectiveness. In sum,

the actual control of normal cell division in

follicular cells may, in fact, represent some

interaction of all these factors and possibly

other growth regulatory substances.

Under conditions of thyroid-pituitary

imbalance, there is no question that TSH

plays a significant role in stimulating DNA

synthesis and cell proliferation; however,

there is a controversy concerning the extent

that normal follicular cells can proliferate.

One research group claims that cells have

limited capacity to respond to the growthinducing effects of TSH (44). In this case,

tumor formation would entail mutational

steps that free cells from their growth limiting potential. Another group of investigators thinks that follicular cells are innately

heterogeneous, with some of them having

stem cell-like proliferation potential while

others are more restricted (45). The stem

cell-like follicular cells would continue to

respond to TSH stimulation and eventually

give rise to tumors. It seems possible that

both positions might actually apply.

Neoplastic transformation appears to

occur in single cells that then expand clonally. Under TSH stimulation, the yield of

mutations that may influence transformation

increases, even in the absence of an increase

in the mutation rate per cell. This is because

the repeated cell divisions lead to an

increased number of cells at risk for mutation, or rapid cell turnover leaves some spontaneous DNA damage unrepaired.

The precise genetic alterations that

accumulate in thyroid follicular cells have

not been clearly established in humans or

in experimental systems, although mutations involving the ras protooncogene, the

p53 tumor suppressor gene, and various

chromosome aberrations have been reported in the follicular variety of epithelial

tumors. These changes in gene expression

could lead to uncontrolled cellular growth

and allow cells to attain the ability to

invade adjacent tissues and metastasize

(Fig. 2). For the papillary variety of thyroid

epithelial tumors, changes in expression of

other factors have been noted, namely protooncogenes PTC/ret, trk, and met (25,43).

Transformed rodent cells that are stimulated to proliferate under the influence of

continuing antithyroid stimulation retain

their responsiveness to TSH. Interestingly,

human thyroid cancer cells often retain

TSH receptors and the ability to respond

to TSH, although their receptors dissipate

as tumors become more anaplastic. Tumor

cells that attain maximal malignant potential usually lose their dependence on TSH.

Although interesting observations concerning growth regulation associated with thyroid carcinogenesis have been made, clearly

more work is needed.

Human Thyroid Carcinogenesis

Clinically manifest thyroid cancer in

humans in the United States is uncommon

and largely nonfatal; only about 17,200 new

cases occur each year (an incidence rate of

around 3 per 100,000 persons, with about

1,200 deaths annually (>90% 5-year survival, which constitutes only around 0.5%

of all cancer deaths) (46,47). In contrast to

clinically apparent disease, small occult thyroid cancers are noted at autopsy in a small

percentage of persons in a number of surveys and up to about 50% in other investigations (48,4p). The incidence in autopsy

studies is more like that noted in rats in the

National Toxicology Program, where about

0.4% of control rats are diagnosed with thyroid cancers at 2 years of age (50). However,

this comparison is somewhat misleading.

Detailed histological examinations of

human and rodent thyroids are not routinely performed by all investigators.

Histological criteria for tumors differ over

time and across reviewers. In addition, thyroid follicular cell cancer in humans is most

often diagnosed histologicly as a papillary

pattern and less often as a follicular pattern,

TSH, insulin, IGF-1, EGF, FGF

ras, Chromosome aberrations

of 5,7 and 12. gap

Loss of heterozygosity at 3p

roid (follicular) carcinogenesis. Abbreviations:

TSH, thyroid-stimulating hormone; IGF-1, insulinlike growth factor 1; EGF, epidermal growth factor; FGF, fibroblast growth factor; gsp, GTP-binding protein mutation.

whereas in rodents the follicular cell tumors

are of a follicular pattern. The aggressiveness

of tumors varies: rodent thyroid neoplasms

rarely metastasize; human cancers frequently

metastasize. These differences regarding histology, along with the shortcomings of

information from descriptive and analytical

epidemiologic investigations, help to

emphasize the diffilculty in comparing

human and rodent cancer incidence data.

For years, the only known human thyroid carcinogen was x irradiation, which

caused an increase in papillary tumors

(41,51). It was questioned whether ionizing

radiation from diagnostic or therapeutic use

of 1311 was carcinogenic in humans (52,53),

although more recently, children exposed to

radioiodine (1311) following the Chernobyl

reactor accident in the Ukraine have developed thyroid cancer; iodide deficiency is

also common in the region and may augment the response to 1311I (54). Most of the

human chemical carcinogens appear to be

mutagenic and cause tumors in more than

one site; some are steroid hormones. To

date, no chemical has been identified as

being carcinogenic to the human thyroid.

Rumans respond as do experimental animals in regard to short- and mid-term disturbances in thyroid functioning from various

antithyroid stimuli such as iodide deficiency,

partial thyroidectomy (surgically or 1311

induced), and goitrogenic chemicals (e.g.,

thionamides): when circulating thyroid hormone levels go down, the TSR level rises and

induces thyroid hypertrophy and hyperplasia.

Rowever, the long-term consequences of

antithyroid action are harder to interpret,

and there is controversy regarding whether

the enlarged human thyroid gland undergoes

Environmental Health Perspectives * Volume 106, Number 8, August 1998 449

1

1

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Reviews * Hill et al.

conversion to cancer. Thyroid enlargements

and nodules have been implicated as possible antecedents to thyroid cancer in

humans, but direct evidence of conversion

of these lesions to malignancy is lacking.

The evidence that does exist for possible

conversion of preneoplastic to neoplastic

lesions is only indirect in nature. Persons

who live in iodide-deficient areas of the

world are unable to synthesize adequate levels of thyroid hormones; they develop elevated TSH levels, very enlarged thyroid

glands, and lesions typified as adenomatous

hyperplasia. There is conflicting evidence

whether thyroid cancer is increased in these

people (55,56). In contrast to these observations, it seems that domestic animals but

not wild animals in iodide deficient areas

developed elevated incidences of thyroid

tumors (57), and tumor incidence disappeared in dogs following the advent of

using iodized salt (58). Iodide may have

some influence on the histological type of

thyroid cancer, with follicular being more

common in iodide deficient areas and papillary being more common in iodide rich

areas (59). People with various inborn

errors of metabolism that are unable to synthesize enough effective thyroid hormone

develop very enlarged thyroids, but few

cases of cancer have been reported. There

are no reports of thyroid tumors among

persons with resistance to thyroid hormone

(60,61). In persons with the autoimmune

disorder Graves' disease, there are often

immunoglobulins that stimulate thyroid

cells in ways analogous to TSH, even

though TSH levels per se are very low. It is

not known whether there is an increase in

thyroid cancer among these patients; some

studies seem to indicate either that cancer

incidence may be increased or that thyroid

tumors in these patients may be more

aggressive (62,63). Overall, this qualitative

information suggests that prolonged stimulation of the human thyroid under certain

circumstances may lead to cancer, as in the

presence of inherited metabolic conditions

or long-term immunologic abnormalities,

but there is uncertainty in this conclusion.

In epidemiologic studies, goiter and

thyroid nodules have been shown to be risk

factors for thyroid cancer. The specific

causes of these enlargements are not known

but, where studied, do not appear to be

due to hypothyroidism (64,65). Some

researchers believe that part of the association may be due to the close medical scrutiny given to persons with suspicious thyroid

enlargements (66,67).

In spite of the potential qualitative similarities, there is evidence that humans may

not be as quantitatively sensitive to thyroid

cancer development from thyroid-pituitary

450

disruption as rodents. Rodents readily

respond to reduced iodide intake with the

development of cancer; humans develop

profound hyperplasia with adenomatous

changes with only suggestive evidence of

malignancy. Even with congenital goiters

due to inherited blocks in thyroid hormone

production, only a few malignancies have

been found in humans.

The reasons for differences in perceived

interspecies sensitivity are not fully known.

However, one factor that may play a role in

interspecies quantitative sensitivity to thyroid

stimulation deals with the influence of protein carriers of thyroid hormone in the blood

(Table 1). Both humans and rodents have

nonspecific low affinity protein carriers of

thyroid hormone (e.g., albumin). However,

in humans, other primates, and dogs there is

a high affinity binding protein, thyroxinebinding globulin, which binds T4 (and T3 to

a lesser degree); this protein is missing in

rodents and lower vertebrates. As a result, T4

bound to proteins with lower affinity in the

rodent is more susceptible to removal from

the blood, metabolism, and excretion from

the body. In keeping with this finding, the

serum half-life of T4 is much shorter in rats

(<1 day) than in humans (5-9 days); this difference in T4 half-life results in a 10-fold

greater requirement for endogenous T4 in

the rat thyroid than in the adult human (68).

Serum T3 levels also show a species difference; the half-life is around 6 hr in rats and

24 hr in humans (69,70). There is a morphological consequence to these hormone

differences. High thyroid hormone synthetic

activity is demonstrated in follicles in

rodents: they are relatively small, surrounded

often by cuboidal epithelium. Follicles in primates demonstrate less activity and are large

with abundant colloid, and follicular cells are

relatively flattened (low cuboidal) (16).

The accelerated production of thyroid

hormone in the rat is driven by serum TSH

levels that are probably about 6- to 60-fold

higher than in humans. This assumes a basal

TSH level in rats and humans of 200 ng/ml

and 5 pU/mI, respectively, and a potency of

human TSH of 1.5-15 U/mg of hormone

(71). Thus, it appears that the rodent thyroid

gland is chronically stimulated by TSH levels

to compensate for the increased turnover of

thyroid hormone. It follows that increases in

TSH levels above basal levels in rats could

more readily move the gland toward

increased growth and potential neoplastic

change than in humans. Interestingly, adult

male rats have higher serum TSH levels than

females (72, and they are often more sensitive to goitrogenic stimulation and thyroid

carcinogenesis. In humans, there is no sex

difference in hormone levels, but females

more frequently develop thyroid cancer (46).

In addition to considerations about the

influence of serum thyroid hormone carrier

proteins, there are differences between

humans and animals in size, lifespan, basal

metabolic rate, and pharmacokinetics and

pharmacodynamics of endogenous and

exogenous chemicals. Any comparison of

thyroid carcinogenic responses across species

should be cognizant of all these factors.

The guidance given here on thyroid

tumors is not unique. Other authorities have

recognized and incorporated advances in the

understanding about carcinogenic mechanisms into their assessments of cancer risks

(73-78).

Science Policy Guidance

Position Statements

Rodents and humans share a common physiology in regard to the thyroid-pituitary

feedback system. Short-term perturbation in

this system often leads to similar effects in

both groups, resulting in increases and

decreases in circulating thyroid and pituitary

hormones. It is well established in rodents

that disruption of thyroid-pituitary status

with elevation of TSH levels is associated

with thyroid tumor and sometimes related

pituitary tumor development. This is true

whether it is due to deficiency in iodide,

reduction in thyroid mass, presence of TSHproducing pituitary tumors, or administration of goitrogenic chemicals. An increase in

TSH stimulation of the thyroid is a final

common development. Likewise, administration of exogenous thyroid hormone or

removal of a TSH-increasing stimulus

reduces the effects in the thyroid.

The role of thyroid-pituitary disruption

in cancer development in humans is much

less convincing than in animals. Iodide deficiency has been associated with increases in

thyroid cancer in some studies but not in

Table 1. Interspecies and intraspecies differences

Parameter Human Rat

Thyroxine binding Present Essentially

globulin absent

T4 half-life 5-9 Days 0.5-1 Day

T3 half-life 1 Day 0.25 Day

T4 production rate/ 1 x 10 x that in

kg body weight humans

Serum TSH 1 x 6-60 x that in

humans

Follicular cell Low cuboidal Cuboidal

morphology

Sex differences

Serum TSH M = F M<2xF

Cancer sensitivity F = 2.5 x M M > F

Abbreviations: T4, thyroxine; T3, triiodothyronine; TSH,

thyroid stimulating hormone; M, male; F, female.

Volume 106, Number 8, August 1998 * Environmental Health Perspectives
Reviews * Assessment of thyroid tumors

others. Similarly an association between either

inborn errors of metabolism affecting thyroid

hormone output or autoimmune-related

Graves' disease and cancer has been suggested

but not proven. TSH may at least play some

permissive role in carcinogenesis in humans.

Accordingly, we cannot qualitatively reject the

animal model; it seems reasonable that it may

serve as an indicator of a potential human

thyroid cancer hazard. However, to the extent

that humans are susceptible to the tumorinducing effects of thyroid-pituitary disruption and given that definitive human data are

not available, humans appear to be quantitatively less sensitive than rodents to developing

cancer from perturbations in thyroid-pituitary status. Recognizing these things and

based upon thyroid carcinogenesis mode of

action considerations, the EPA adopted the

following three science policy positions:

1. It is presumed that chemicals that produce

rodent thyroid tumors may pose a carcinogenic hazard for the human thyroid.

2. In the absence of chemical-specific data,

humans and rodents are presumed to be

equally sensitive to thyroid cancer due to

thyroid-pituitary disruption. This is a

conservative position when thyroid-pituitary disruption is the sole mode of

action in rats because rodents appear to

be more sensitive to this carcinogenic

mode of action than humans. When the

thyroid carcinogen is a mutagenic chemical, the possibility that children may be

more sensitive than adults needs to be

evaluated on a case-by-case basis.

3. Adverse rodent noncancer thyroid effects

(e.g., thyroid gland enlargements) following short- and long-term reductions in

thyroid hormone levels are presumed to

pose human noncancer health hazards.

Some chemicals that have produced thyroid follicular cell tumors in laboratory

rodents appear to work by producing a disturbance in thyroid-pituitary homeostasis,

others appear to act primarily through a

mutagenic mode of action, and still others

seem to show a combination of both modes

Table 2. Default dose-response procedures for

thyroid carcinogens

Array of data DoseExample Antithyroid Mutagenic methodology

1 Unknown Unknown Linear

2 No Yes Linear

3 Yes No Nonlinear

(MOE)

4 Yes Yes Linear and

nonlinear

(MOE)

MOE, margin of exposure.

of action. The question then becomes how

to evaluate the risks of thyroid tumors for

humans given exposure to any of these

chemicals. If the animal tumors are due to

chemical doses that produce imbalances in

thyroid-pituitary functioning, the chance of

cancer is anticipated to be minimal under

conditions of hormonal homeostasis.

Tumors that seem to arise from relevant

mutagenic influences (e.g., gene mutations

and structural chromosome aberrations)

without perturbation in thyroid-pituitary

status may pose some chance of cancer across

a broader range of doses. Consequently, until

such time that biologically based models and

data become available, the EPA has adopted

five other science policy positions for conducting dose-response assessments of chemical substances that have produced thyroid

follicular cell (and related pituitary) tumors

in experimental animals:

1. A linear dose-response procedure should

be assumed when needed experimental

data to understand the cause of thyroid

tumors are absent and the mode of

action is unknown (Table 2, Example 1).

2. A linear dose-response procedure should

be assumed when the mode of action

underlying thyroid tumors is judged to

involve mutagenicity alone (Table 2,

Example 2).

3. A nonlinear dose-response relationship

(margin of exposure) should be used when

thyroid-pituitary disruption is judged to

be the sole mode of action of the observed

thyroid and related pituitary tumors

(Table 2, Example 3). Thyroid-pituitary

perturbation is not likely to have carcinogenic potential in short-term or highly

infrequent exposure conditions. The margin of exposure procedure generally

should be based on thyroid-pituitary disruptive effects themselves, in lieu of tumor

effects, when data permit. Such analyses

will aid in the development of combined

noncancer and cancer assessments of toxicity. Results of the margin of exposure

procedure will be presented in a way that

supports risk management decisions for

exposure scenarios of differing types (e.g.,

infrequent exposure, short durations).

4. Consistent with EPA risk characterization principles, both linear and margin of

exposure considerations should be

assumed when both mutagenic and thyroid-pituitary disruption modes of action

are judged to be potentially at work

(Table 2, Example 4). The weight of evidence for emphasizing one over the other

should also be presented. The applicability of each to different exposure scenarios

should be developed for risk management

consideration.

5. Dose-response relationships for neoplasms other than the thyroid (or pituitary) should be evaluated using mode of

action information bearing on their

induction and principles laid out in current EPA cancer risk assessment guidelines. There is an association between

thyroid and liver tumors in rodent cancer

studies (79,80). The reason(s) for this

relationship has not been generically

established but should be carefully

assessed for chemicals on a case-by-case

basis. Some may be due to induction of

hepatic microsomal enzymes.

Most of the focus in implementing

this policy is devoted to answering the following questions: 1) Does an agent that

shows thyroid carcinogenic effects have

antithyroid activity? 2) Can modes of

action other than thyroid-pituitary disruption account for thyroid tumor formation by this chemical? and 3) How can

one express thyroid dose-response relationships? Adequately answering these

questions is dependent upon a data-rich

information base for the chemical under

review. To the extent practicable, effort

should be made to review such information before deciding upon the possible

mode of chemical action underlying the

thyroid tumors and their consequences for

risk assessment.

The procedures and considerations

developed in this report embody current

scientific knowledge of thyroid carcinogenesis and evolving science policy positions.

Should significant new information become

available that would change these positions,

the EPA will update its science policy positions accordingly.

Evidence for Antithyroid Activity

Data needs. Different types of information

on a chemical may indicate that it has

antithyroid activity, that is, whether it

works via disruption of thyroid-pituitary

status. These include effects manifest in the

thyroid gland per se, various tissues peripheral to the thyroid, and/or the liver. All

available factors are assembled into an overall evaluation of the likelihood that the

chemical is acting via disruption of the thyroid-pituitary axis. A number of chemicals

have been assessed (3).

Special mechanistic studies are needed

to demonstrate chemically induced perturbations in thyroid-pituitary functioning.

Repeat dose (e.g., 2-4 week and 13 week)

studies that have simultaneously evaluated a

number of end points can often provide

critical information for evaluating qualitatively whether antithyroid activity exists,

the cause of the activity, and quantitatively

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what dose-response relationships may be

appropriate. Such studies should be carefully designed to encompass multiple doses

from above those clearly associated with

tumors in chronic studies down to those

below which there is no indication of disturbance in critical thyroid-pituitary parameters, so that dose-response relationships

can be defined. Special attention should be

given to the time of sampling of thyroid

and pituitary hormones because of the compensatory action of homeostatic mechanisms and the difficulty in discerning

changes after compensation occurs.

Hormone sampling should also be conducted at the same time during the course of a

day to minimize circadian hormonal fluctuations, and efforts should minimize stress in

handling animals. Effects measured only at

the end of chronic rodent studies are often

difficult to evaluate and, alone, seldom provide adequate information.

The determination of the antithyroid

activity of a chemical requires empirical

demonstration of the following items.

Demonstration of increases in thyroid

growth and changes in relevant thyroid and

pituitary hormones are considered to be the

most important. Location of the site(s) of

antithyroid action documents where in the

body the chemical under assessment leads to

perturbations in thyroid-pituitary functioning. Dose correlations among various effects

determine where the growth curve for the

thyroid gland deviates from the normal pattern of cell replacement and how this relates

to doses producing tumors. Reversibility of

effects following treatment cessation during

the early stages of disruption of the thyroid-pituitary axis shows that permanent,

self-perpetuating processes have not been set

into motion.

The following three items are desirable

in determining antithyroid activity: lesion

progression, structure-activity analysis, and

other studies. Each provides supporting

information that can add profoundly to the

assessment of an agent's ability to produce

antithyroid effects.

Increases in cellular growth (evidence

required). Agents that affect thyroid-pituitary function may stimulate thyroid

enlargement. Most parameters commonly

measured include, but are not limited to,

increases in absolute or relative thyroid

gland weight or histological indications of

cellular hypertrophy and hyperplasia, morphometric documentation of alteration in

thyroid cellular components, and changes

in the proliferation of follicular cells detected by DNA labeling or mitotic indices.

Hormone changes (evidence required).

With a disruption in thyroid-pituitary functioning, there is typically a reduction in both

circulating serum T4 and T3 concentrations

and an increase in TSH levels within days or

a few weeks of chemical administration. In

some cases, T4 levels may be lowered while

T3 levels are maintained within normal limits. In addition, sometimes hormone levels

may return to normal over time for mild

goitrogenic agents because of the homeostatic compensatory increase in thyroid activity

and mass. Statistical tests can help evaluate

the significance of hormone perturbations,

but it is the constellation of changes in both

thyroid and pituitary hormones that indicate

whether the negative feedback loop between

the thyroid and hypothalamus/pituitary has

been perturbed.

Site of action (evidence required).

Chemicals that produce thyroid tumors

alone or after administration of a mutagenic

initiator produce interference with

thyroid-pituitary function at various sites in

the body (Fig. 3). Effects have been found at

one or more of the following anatomical

locations: intrathyroidal and various

extrathyroidal sites, induding the liver and

other sites. Clues to the site of action can

sometimes be deduced by analysis of structurally related compounds. Generally,

enough information on a chemical should be

given to be able to identify the sites that contribute the major effect on thyroid-hypothalamus-pituitary function. Given experience to date, it appears that most often the

liver is the site of action, followed by the thyroid, where thyroid peroxidase is affected;

other sites of action seem to be less common.

Intrathyroidal. Several different effects

in the thyroid gland have been associated

with the development of antithyroid activity

and the formation of thyroid tumors in

rodents. Iodide pump inhibition by chemicals like thiocyanate and perchlorate ions

leads to a decrease in uptake of inorganic

iodide into the thyroid gland. Thyroid peroxidase inhibition blocks the incorporation

of active iodide into iodotyrosines and their

coupling to form the nascent thyroid hormones. Agents that are known to reversibly

or irreversibly inactivate this enzyme include

various thionamides such as 6-propylthiouracil and ethylene thiourea, certain aromatic amines, e.g., some of the sulfonamides, and miscellaneous compounds such

as amitrole. Toxicity to thyroid cells, as has

been seen with polychlorinated biphenyls,

may affect the gland's ability to manufacture

and secrete thyroid hormones. Inhibition of

thyroid hormone release, with agents like

lithium and excess iodide, results in a retention of hormones within the colloid and a

paucity released into the circulation (81).

Extrathyroidal. Several tissues and

organs of the body, including skeletal muscle, kidneys, and liver, contain different

deiodinases that remove iodine atoms from

thyroid hormones. Inhibition of 5'-monodeiodinase, the enzyme that normally converts T4 to T3, leads to a reduction in circulating T3 and an increase in the rT3 level via

3 -deiodination. Compounds like FD&C

Red No. 3 (erythrosine), iopanoic acid, and

6-propylthiouracil act by competitive inhibition of this enzyme or interaction with its

sulfhydryl cofactor. The deiodinase system

in the pituitary is somewhat different from

that in the periphery and may respond differently to certain chemicals (82).

A significant amount of thyroid hormone normally is metabolized by the liver.

Certain chemicals induce liver microsomal

enzymes and enhance thyroid hormone

metabolism and removal. T4 conjugation

with glucuronic acid is enhanced by agents

that induce hepatic glucuronosyl transferase

(19). In these cases, thyroid hormone may

also show increased binding to hepatocytes,

increased biliary excretion, and increased

plasma clearance. Other common manifestations of microsomal induction include

enlargement of hepatocytes in the centrolobular region, an increase in hepatic cell

smooth endoplasmic reticulum, an increase

Figure 3. Antithyroid effects that influence thyroid carcinogenesis. Abbreviations: TH, thyroid hormone;

TSH, thyroid stimulating hormone.

452 Volume 106, Number 8, August 1998 * Environmental Health Perspectives
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in P450-associated metabolism of various

chemical substrates, and an increase in biliary flow.

Disparate chemical and functional classes like polyhalogenated hydrocarbons (e.g.,

2,3,7,8-tetrachlorodibenzo-p-dioxin), barbiturates (e.g., phenobarbital), and various

individual compounds (e.g., the pesticide

clofentezine, the drug spironolactone, and

the histamine antagonist SK&F 934790) are

known to enhance thyroid hormone excretion via effects on microsomal enzymes.

Conjugation also may occur with sulfate,

usually associated with deiodination; deamination, oxidative decarboxylation, and

ether-link cleavage are minor degradative

pathways. Interestingly, phenobarbital has

been shown to be a promoter in the rodent

thyroid (35), but there is no indication it

produces thyroid cancer in humans (83).

Chemicals may bind thyroid hormone

receptors and produce certain effects. For

instance, agents (e.g., salicylates) can displace thyroid hormone from plasma carrier

proteins and result in reductions in effective

thyroid hormone (84,85). They could bind

receptors in target organs (e.g., pituitary)

but result in inactive complexes. These possibilities plus other potential sites of action

(e.g., affecting TRH, thyroid hormone

responsive elements on the DNA) may be

conceived as influencing thyroid-pituitary

functioning.

Dose correlations (evidence required).

Confidence in an antithyroid mode of

action is enhanced by evidence of a correlation between doses of a chemical that do

and do not jointly perturb thyroid-pituitary hormone levels, produce various histological changes in the thyroid, or produce

other effects, including thyroid cancer.

These are important steps in evaluating the

significance of thyroid-pituitary disruption

in thyroid carcinogenesis and in evaluating

dose-response relationships.

Reversibility (evidence required).

Chemicals working through an antithyroid

mode of action induce changes in thyroid

cell morphology and number and in thyroid-pituitary hormones that are reversible

upon cessation of chemical dosing.

Lesion progression (evidence desirable).

Evidence for a progression of histological

lesions over time is commonly noted during

the carcinogenic process. These include cellular hypertrophy and hyperplasia, followed

by focal hyperplasia and, eventually, neoplasia (benign and possibly malignant tumors).

Structure-activity analyses (evidence

desirable). Analysis of chemical structure

may show that an agent belongs to a class

of compounds that induces thyroid tumors

via thyroid-pituitary imbalance (e.g.,

agents that inhibit thyroid peroxidase, liver

microsomal enzyme inducers). This allows

for the scientific inference that the chemical under review may act similarly. In addition, generic information developed on a

group of analogues can be used to support

the assessment of the agent under review.

Other studies (evidence desirable). Many

other studies bearing on thyroid-pituitary

imbalance can provide a range of findings

from strong ancillary information to only

suggestive indications. For example, a few of

these studies indude suppression of induced

effects by concurrent administration of thyroid hormone; absence of initiating activity

but presence of promoting activity in twostage carcinogenicity tests; localization of certain chemicals in the thyroid (e.g., thionamides); influences on hypothalamic responsiveness to thyroid hormone levels and output of TRH; changes in TSH mRNA transcripts in the pituitary; and alteration of thyroid hormone nuclear receptor number or

synthesis.

Other Modes of Carcinogenic Action

Another critical element in the evaluation

of a thyroid carcinogen is a determination

of whether mutagenicity may account for

the observed tumors. Primary emphasis

should be placed on those end points that

have mechanistic relevance to carcinogenicity. DNA reactivity is a prime predictor of

potential mutagenic carcinogenicity. Many

of these compounds belong to particular

chemical classes (e.g., aromatic amines,

nitrosamines, polyaromatic hydrocarbons).

These chemicals or their metabolites bind

to DNA, and they often induce gene mutations and structural chromosome aberrations. In recognition that organs and tissues may have unique metabolic activity, it

is helpful to know in addition to traditional short-term test results whether there is

evidence of DNA reactivity in target tissues

(e.g., DNA adducts, unscheduled DNA

synthesis, single strand breaks).

Mutagenic effects other than those

associated with direct DNA reactivity need

to be carefully evaluated in regard to their

mechanistic implications; some may have

different cancer dose-response considerations than do direct DNA reactive agents.

These effects include the ability of the

chemical under review to induce indirect

effects on DNA, such as through influence

on the cell division spindle or production

of reactive oxygen. Agents also should be

evaluated for the presence of structural

alerts that are often predictive of chemical

reactivity or potential carcinogenicity. All

of these findings are then melded into an

overall appraisal of an agent's ability to

influence genetic processes relevant to carcinogenesis in the thyroid or other sites.

It is possible that information on carcinogenic modes of action other than mutagenicity or thyroid-pituitary derangement

will become available. If so, this information

also needs to be incorporated on a case-bycase basis into the evaluation of a chemical's

ability to produce tumors of the thyroid (and

of other sites).

Dose-Response Considerations

Evaluation of potential dose-response relationships for thyroid tumors depends on an

evaluation of the chemical's expected mode

of carcinogenic action. Major determinations include whether the thyroid tumors

appear to be due, at least in part, to thyroid-pituitary imbalance and whether other

modes of action (e.g., relevant mutagenicity) may be pertinent to their formation.

Other case-specific factors may provide crucial information, such as the extent to

which data gaps and uncertainties prevail. A

rationale should accompany the selection of

any dose-response method. Guidance is

provided below to illustrate ways to evaluate

data and make judgments about potential

thyroid cancer dose-response relationships.

High to low dose extrapolation. If

antithyroid influences are operative in the formation of thyroid tumors, attention should

be directed to biologically based procedures

that embody the mechanistic influences, if

they are available. In their absence, default

procedures should be employed that incorporate nonlinear or linear considerations. When

thyroid-pituitary imbalance is not operative,

other modes of action or default considerations should be used; generally low-dose linear extrapolations are appropriate. All extrapolation procedures should be consistent with

the guidance given in EPA cancer risk assessment guidelines. Finally, when thyroid

tumors seem to arise from both a chemical's

antithyroid activity and its mutagenic potential, dose-response relationships may be projected in ways that express concerns for both

possible modes of action.

When tumors other than thyroid follicular cell (and related pituitary) tumors are

found along with thyroid tumors, mode of

action and other considerations should help

guide the selection of the appropriate

dose-response extrapolation method(s).

Separate dose-response extrapolations may

apply for the different tumor types, depending on the specifics of the case.

Biologically based. Optimally, mechanistic considerations that underlie thyroid tumor

formation should be incorporated into biologically based extrapolation models. They

should indude physiologically based pharmacokinetic considerations for the chemical and

its interactions with and effects upon cells.

The trouble is that generic biologically based

Environmental Health Perspectives * Volume 106, Number 8, August 1998 453
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dose-response extrapolation models have yet

to be developed and validated for the thyroid. Fortunately, work in this area is beginning, and a model has been developed to

explain effects associated with 2,3,7,8-tetrachlorodibeno-p-dioxin (86). Until other

mechanistic models become generally available and chemical-specific data have been

produced, the EPA assessments will employ,

as discussed below, one of three default procedures in its evaluation of thyroid cancer

risks: nonlinear, linear, or both nonlinear

and linear.

Nonlinear. For cases in which tumors

arise from chemically induced disturbances

in thyroid-pituitary functioning, tumors are

considered to be secondary to the adverse

effects on thyroid gland function that precede them. As exposures to such agents

decrease, the likelihood of cancer decreases;

risks may be seen as minimal at doses at

which there is no effect on thyroid-pituitary

homeostasis. Generally, homeostasis is considered to apply when serum T4, T3, and

TSH levels and thyroid and pituitary morphology and growth are within their normal

limits. Risk assessments on agents should

contain case-specific and generic information that support the contention that nonlinear dose-response relationships apply.

Empirically there is some support for

thyroid follicular cell tumors having a

dose-response curve that is less than linear

(curvilinear upward). Slope functions were

calculated using the Weibull model to give

some indication of the shape of the

dose-response curve and the rodent tumor

incidence database of the National Cancer

Institute/National Toxicology Program.

Incidence for tumors in general was more

consistent with a quadratic rather than a

linear dose-response curve shape (87,88).

However, when slopes for thyroid follicular

cell tumors were compared with those of

thyroid C cell tumors and all other tumor

sites, those from the thyroid follicular cells

were even more curvilinear than the others

(Portier, personal communication).

One argument for assuming low-dose

linearity in dose-response assessments is the

concept of additivity to background. If a

given chemical acts in the same way or augments an endogenous or exogenous background factor that contributes to tumor

development, the effect of the chemical will

add to that of the background factor. The

result is low-dose linearity up to at least a

doubling of the background rate (85). The

concept may not be applicable to certain

processes subject to hormonal regulation,

such as with the thyroid gland. Normally

the level of circulating thyroid hormone is

adjusted carefully by the negative feedback

with the pituitary. Elevations and reductions

in thyroid hormone are met with adjustments in the amount of TSH released from

the pituitary so as to bring thyroid hormone

values back into the normal range. These

excursions are not part of an ongoing carcinogenic process; instead, they represent

the body's means of maintaining thyroid

hormone homeostasis. Small doses of a

potentially antithyroid chemical may not

result in any perturbation in hormone levels

or stimulation of thyroid follicular cell

growth simply because homeostatic mechanisms will drive thyroid hormone levels back

into the normal range.

The EPA acknowledges that it may be

difficult to establish a precise dose where

there is negligible response for a specific toxicologic effect given the sensitivity of methods to evaluate various parameters and the

variability in measurement of end points. In

recognition of this, it is incumbent upon the

scientific community to help in the transfer

of various molecular techniques from the

research laboratory to testing facilities (e.g.,

measurement of hormone receptor mRNA

production and receptor content, occupancy, and turnover) that may be more sensitive

indicators of thyroid-pituitary malfunction.

The way the EPA has dealt with nonlinear phenomena is to express concern for

human exposure (risk) as a margin of exposure (MOE), the ratio between a dose point

of departure for the critical effect and the relevant estimate of anticipated human exposure (incorporating dose, frequency, and

time). Large MOEs are attended with less

concern than are small ones. Traditionally,

the point of departure is expressed as a no

observed adverse effect level (NOAEL), and

the critical effect is the relevant toxicological

end point occurring at the lowest doses in a

toxicological study. More recently, alternatives to the NOAEL have been proposed to

serve as points of departure for MOE calculation. In April 1996, the EPA proposed a

revision of its existing cancer risk assessment

guidelines (4. A dose point of departure is

determined by extrapolating effects in the

observed part of the dose-response curve. It

is used, depending on the expected mode(s)

of action, as the starting point for either linear extrapolation to the origin or calculation

of an MOE in the case of nonlinear extrapolation. Generally, tumor or nontumor end

point (e.g., hyperplasia) incidence is extrapolated to the 10% effect level. The 95% confidence levels are placed on that dose. The

lower 95% confidence limit on that dose

may be used as the point of departure for risk

estimation below the observed data. There is

some discussion about using the point estimate at the 10% effect level in lieu of the

lower bound estimate in the proposal. Other

means of determining departure points are

also proposed (2). Final cancer guidelines

will darify these points. Procedures should

be employed for thyroid tumors that are

consistent with EPA cancer risk assessment

guidelines and practices that are applicable at

the time.

Assessments should include adequate

information to aid in interpreting the significance of MOEs, such as taking into

consideration the variability in sensitivity

among individuals within a species, the

sensitivity of humans relative to experimental animals, and other strengths, weaknesses, and uncertainties that are part of the

assessment. Decision makers must then

judge the adequacy of the MOE for their

risk management purposes.

Low-dose linear. For those assessment

cases in which thyroid tumors do not seem

to be due to thyroid-pituitary imbalance

(e.g., mutagenic agents), existing case-specific mode-of-action information and default

considerations should be used to develop

dose-response relationships. In other cases,

there may be an absence of mode-of-action

information for an agent. Generally a lowdose linear default for the thyroid tumors

may be contemplated in these two circumstances in accordance with current EPA procedures. Recent cancer risk assessment

guideline proposals suggest that linear

extrapolation would involve calculation of a

dose point of departure with a straight line

extrapolation from there to the origin (2).

Low-dose linear and nonlinear. Finally,

careful review is warranted when both

antithyroid and other determinants seem to

apply to the observed thyroid tumors, such as

when there are certain mutagenic influences

(e.g., structural chromosome aberrations).

Judgments with accompanying scientific reasoning should be presented on the most

appropriate way(s) to evaluate thyroid risk:

either linear or nonlinear or, when the two

positions are about equally tenable, both.

When both procedures are presented, assessors should state the relative merits of each

procedure. In some cases, one of the two

methods may be preferable and should be

given more weight; the rationale for condusions should be expressly presented. Projected

risks using linear extrapolation often give rise

to concerns at doses lower than those when

nonlinear techniques are applied. Thus, these

two techniques usually can be seen as putting

lower and upper bounds on exposures ofconcern. Assessments should indude guidance

for decision makers in interpreting concerns

for exposure when both extrapolation techniques are presented.

End points to be employed Optimally,

one would have access to data on various

preneoplastic end points that would be evaluated following short-term (e.g., 2-4 week)

454 Volume 106, Number 8, August 1998 * Environmental Health Perspectives
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and subchronic (e.g., 13 week) studies and

compared with the doses that have produced

tumors in chronic studies. End points that

should regularly be evaluated and presented

in dose-response analyses include 1) changes

in levels of T4 and T3; 2) increases in TSH;

3) the incidence of thyroid follicular cell

hypertrophy, hyperplasia, and neoplasia; 4)

increases in cell proliferation and thyroid

weight; and 5) specific end points associated

with thyroid-pituitary disturbance at the

site(s) of chemical action (e.g., inhibition of

thyroid peroxidase, increased metabolism

and clearance of thyroid hormone). A host

of other effects as discussed above could be

monitored in the thyroid, pituitary, or thyroid hormone-responsive organs and included on a case-by-case basis. Care should be

taken to ensure that studies to evaluate these

parameters are conducted for adequate periods of time and at doses that clearly define

dose-response relationships. Attention also

needs to be given to procedures that help to

reduce the variability in responses among

animals (e.g., time and means of animal sacrifice and tissue sampling).

Estimation ofpoints of departure. For

the important toxicity studies, a point of

departure (e.g., NOAEL) is determined for

each thyroid toxicity end point and exposure duration. Doses associated with tumors

should also be noted. The departure point

may be a study dose or an estimated dose.

For instance, when data permit, the departure point can be estimated by extrapolation of doses associated with observed

responses (e.g., TSH levels) to those attended with no significant deviation from the

control range. In other cases, an appropriate

observed study NOAEL may be selected or

other procedures in accordance with EPA

guidance may be used.

Considerations for the selection of the

critical end point to be used to project thyroid cancer risk (i.e., calculation of the MOE)

include 1) the nature of the end point and its

relationship to the perturbations in endocrine

balance and carcinogenicity; 2) the presence

of good dose-response or dose-severity of

effect relationships; 3) the sensitivity of the

end point vis-a-vis other potential end points;

and 4) the length of the dosing period and its

relevance to making judgments about the

consequences of potential chronic exposures.

Interspecies Extrapolation

Many considerations are relevant in

attempting to extrapolate thyroid carcinogenic effects in experimental animals to

humans. The relative sensitivity of humans

and rodents to the carcinogenic effects of

elevated TSH are not firmly established, but

important observations have been made.

Given that the rodent is a sensitive model

for measuring the carcinogenic influences of

TSH and that humans appear to be less

responsive, one would expect that projections of potential risk for rodents would

serve as conservative potential indicators of

risks for humans.

Rodent cancer studies typically include

doses that lead to toxicity, induding perturbation in thyroid-pituitary functioning,

over a lifetime. The relevance of such experimental conditions to anticipated human

exposure scenarios (i.e., dose, frequency, and

time) should be considered and presented in

the final characterization of risk. This is

especially true because thyroidal effects are

not necessarily expected at all doses. In addition, chemically induced effects that are produced by short-term disruption in thyroid-pituitary functioning appear to be

reversible when the stimulus is removed.

Although it appears that humans are less

sensitive to the carcinogenic perturbations

of thyroid-pituitary status than rodents

(e.g., iodide deficiency), such determinations should be made on a case-by-case

basis. This would depend upon a host of

factors involving the agent, including the

depth and breadth of the database, the congruence of the information supporting a

given mode of action, and the existence of

information on humans. Decision makers

should be apprised of risk assessment judgments and their rationales. In the absence of

chemical-specific information, the default

position is that humans should be considered to be as sensitive to carcinogenic effects

as are rodents. That is to say, a factor of one

would be used when extrapolating effects in

rodents to those in humans.

Human intraspecies evaluations. Thyroid

hormones are regulated within rather narrow

ranges. Normal human serum values are as

follows: T4, 4-11 pg/dl and T3, 80-180

ng/dl; TSH levels extend over a broader

range: 0.4 to 8 pU/ml due to the incorporation in recent years of more sensitive laboratory methods that have extended the normal

range to lower values (90,91). To the extent

that data exist in humans as to their thyroid-pituitary status, one should carefully

review central value parameters between

chemically exposed and unexposed groups as

well as the distribution of values between

both groups.

Extended deviations in human thyroid

hormone levels either above or below the

normal range are associated with the disease

states hyperthyroidism and hypothyroidism,

respectively. In the United States, most cases

are associated with some autoimmune problem. Worldwide, iodide deficiency is the

most prominent cause of thyroid disease generally and hypothyroidism specifically.

People develop characteristic symptoms and

signs that readily cause them to seek medical

attention. The goal of therapy for such conditions is to bring persons back into normal

thyroid-pituitary balance, which secondarily

would greatly minimize any potential for carcinogenic effects. Overt hypothyroidism,

with reduced thyroid hormone and increased

TSH levels, requires treatment; it has an

incidence of about 0.2% in women and less

in men. Subclinical hypothyroidism may

have an incidence of 5% among women.

Men are affected less often; incidence

increases significantly with age. There is not

agreement as to whether these people need

treatment (92). Some with hypothyroidism

may go for some time before diagnosis and

treatment. The possible consequences of

chemical exposure on this subpopulation of

individuals may warrant consideration.

The human thyroid is susceptible to

ionizing radiation, the only known human

thyroid carcinogen. Children are more sensitive than adults to the carcinogenic effects

of ionizing radiation (41,53). In keeping

with the EPA position on the evaluation of

childhood risks (93), the impact of mutagenic chemicals that produce thyroid

tumors should be evaluated in context of

the information available on the subject as

well as agent-specific information. The

extent to which a chemical may act as ionizing radiation does should be evaluated on

a case-by-case basis and factored into decisions accordingly. It seems possible that

chemicals producing mutagenic effects like

those of radiation may pose some accentuated risk to children. More research is

needed in this area (94).

Combined effects. It is not known if there

are human subpopulations that are sensitive

to thyroid cancer development from exposure to antithyroid chemicals. Seemingly, the

thyroid status of persons living in an iodidedeficient area or those who are hypothyroid

due to other causes may further be harmed

by simultaneous exposure to naturally occurring chemicals or xenobiotics that can further

disrupt thyroid-pituitary functioning (95).

This potential to possibly increase cancer risk

following chemical exposure should be kept

in mind in evaluating the composition of

human populations, the numbers and nature

of sensitive individuals, the magnitude and

pattern of chemical exposure, estimates of

risk to the general population, and the

potential risk consequences to persons with

some increase in sensitivity.

In addition to possible sensitive subpopulations, persons may be exposed to

more than one antithyroid chemical at a

time and by more than one route of exposure. These combined exposures may worsen thyroid-pituitary status and influence

thyroid cancer potential (96). To the

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Rviews's Hill et al.

extent practicable on a case-by-case basis,

consideration should be given to combined

exposures to chemicals that influence thyroid-pituitary functioning. Attention can

be directed toward 1) an understanding of

whether the chemicals affect the same or

different sites of antithyroid action, 2)

whether mutagenicity or other modes of

action may be operative, and 3) the ways

various antithyroidal and other effects may

combine to influence potential cancer risks

by the same and different routes of exposure. Some guidance for evaluating these

cases is provided in the EPA mixtures

assessment guidelines (94).

Unlike tumors at many sites, modes of

action for thyroid follicular cell tumors can

be readily explored through laboratory

experimentation. By using this cancer

mode-of-action information along with considerations about intraspecies and interespecies variability, chemicals can be assessed

as to the consequences of human exposure.

REFERENCES AND NoTEs

1. U.S. EPA. Guidelines for carcinogen risk assessment.

Fed Reg 51:33992-34003 (1986).

2. U.S. EPA. Proposed guidelines for carcinogen risk

assessment Fed Reg 61:17960-18011 (1996).

3. Hurley PM, Hill RN, Whiting RJ. Mode of carcinogenic

action for pesticides inducing thyroid follicular cell

tumors in rodents. Environ Health Perspect

106:437-445(1998).

4. U.S. EPA. Alpha2p-Globulin: Association with

Chemically Induced Renal Toxicity and Neoplasia in the

Male Rat. Risk Assessment Forum. EPN625/3-91/019F.

Washington, DC:U.S. Environmental Protection Agency,

1991.

5. latropoulos MJ. Endocrine considerations in toxicologic pathology. Exp Toxicol Pathol 45:391-410(1993/94).

6. Capen CC, Dayan AD, Green S. Receptor-mediated

mechanisms in carcinogenesis: an overview. Mutat

Res 333:215-224 (1995).

7. Hill RN, Erdreich LS, Paynter OE, Roberts PA,

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