# Human Physiology/The endocrine system

## Introduction To The Endocrine System

The endocrine system is a control system of ductless
glands that secrete hormones within specific
organs. Hormones act as \"messengers,\" and are carried by the
bloodstream to different cells in the body, which interpret these
messages and act on them.

It seems like a far fetched idea that a small chemical can enter the
bloodstream and cause an action at a distant location in the body. Yet
this occurs in our bodies every day of our lives. The ability to
maintain homeostasis and respond to stimuli is largely due to hormones
secreted within the body. Without hormones, you could not grow, maintain
a constant temperature, produce offspring, or perform the basic actions
and functions that are essential for life.

The endocrine system provides an electrochemical connection from the
hypothalamus of the brain to all the organs that control the body
metabolism, growth and development, and reproduction.

There are two types of hormones secreted in the endocrine system:
Steroidal (or lipid based) and non-steroidal, (or protein based)
hormones.

The endocrine system regulates its hormones through negative feedback,
except in very specific cases like childbirth. Increases in hormone
activity decrease the production of that hormone. The immune system and
other factors contribute as control factors also, altogether maintaining
constant levels of hormones.

## Types of Glands

!Major endocrine glands. (Male left, female on the right.) **1.**
Pineal gland **2.** Pituitary gland **3.** Thyroid gland **4.** Thymus
**5.** Adrenal gland **6.** Pancreas **7.** Ovary **8.**
Testis 1. Pineal gland 2. Pituitary gland 3. Thyroid gland 4. Thymus 5. Adrenal gland 6. Pancreas 7. Ovary 8. Testis"){width="227"}

**Exocrine Glands** are those which release their cellular secretions
through a duct which empties to the outside or into the lumen (empty
internal space) of an organ. These include certain sweat glands,
salivary and pancreatic glands, and mammary glands. They are not
considered a part of the endocrine system.

**Endocrine Glands** are those glands which have no duct and release
their secretions directly into the intercellular fluid or into the
blood. The collection of endocrine glands makes up the endocrine system.

:   1, The main endocrine glands are the pituitary (anterior and
    posterior lobes), thyroid, parathyroid, adrenal (cortex and
    medulla), pancreas and gonads.

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:   2, The pituitary gland is attached to the hypothalamus of the lower
    forebrain.

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:   3, The thyroid gland consists of two lateral masses, connected by a
    cross bridge, that are attached to the trachea. They are slightly
    inferior to the larynx.

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:   4, The parathyroid glands are four masses of tissue, two embedded
    posterior in each lateral mass of the thyroid gland.

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:   5, One adrenal gland is located on top of each kidney. The cortex is
    the outer layer of the adrenal gland. The medulla is the inner core.

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:   6, The pancreas is along the lower curvature of the stomach, close
    to where it meets the first region of the small intestine, the
    duodenum.

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:   7, The gonads (ovaries and testes) are found in the pelvic cavity.

## Hormones and Types

A **hormone** is a type of chemical signal. They are a means of
communication between cells.

The endocrine system produces hormones that are instrumental in
maintaining homeostasis and regulating reproduction and development. A
hormone is a chemical messenger produced by a cell that effects specific
change in the cellular activity of other cells (target cells). Unlike
exocrine glands (which produce substances such as saliva, milk, stomach
acid and digestive enzymes), endocrine glands do not secrete substances
into ducts (tubes). Instead, endocrine glands secrete their hormones
directly into the surrounding extra cellular space. The hormones then
diffuse into nearby capillaries and are transported throughout the body
in the blood.

The endocrine and nervous systems often work toward the same goal. Both
influence other cells with chemicals (hormones and neurotransmitters).
However, they attain their goals differently. Neurotransmitters act
immediately (within milliseconds) on adjacent muscle, gland, or other
nervous cells, and their effect is short-lived. In contrast, hormones
take longer to produce their intended effect (seconds to days), may
affect any cell, nearby or distant, and produce effects that last as
long as they remain in the blood, which could be up to several hours.

In the following table there are the major hormones, their target and
their function once in the target cell.

  Endocrine Gland           Hormone Released                                 Chemical Class         Target Tissue/Organ               Major Function of Hormone
  ------------------------- ------------------------------------------------ ---------------------- --------------------------------- ---------------------------------------------------------------------------------------------
  **Hypothalamus**          Hypothalamic releasing and inhibiting hormones   Peptide                Anterior pituitary                Regulate anterior pituitary hormone
  **Posterior Pituitary**   Antidiuretic (ADH)                               Peptide                Kidneys                           Stimulates water reabsorption by kidneys
                            Oxytocin                                         Peptide                Uterus, mammary glands            Stimulates uterine muscle contractions and release of milk by mammary glands
  **Anterior Pituitary**    Thyroid stimulating (TSH)                        Glycoprotein           Thyroid                           Stimulates thyroid
                            Adrenocorticotropic (ACTH)                       Peptide                Adrenal cortex                    Stimulates adrenal cortex
                            Gonadotropic (FSH, LH)                           Glycoprotein           Gonads                            Egg and sperm production, sex hormone production
                            Prolactin (PRL)                                  Protein                Mammary glands                    Milk production
                            Growth (GH)                                      Protein                Soft tissue, bones                Cell division, protein synthesis and bone growth
  **Thyroid**               Thyroxine (T4) and Triiodothyronine (T3)         Iodinated amino acid   All tissue                        Increase metabolic rate, regulates growth and development
                            Calcitonin                                       Peptide                Bones, kidneys and intestine      Lowers blood calcium level
  **Parathyroids**          Parathyroid (PTH)                                Peptide                Bones, kidneys and intestine      Raises blood calcium level
  **Adrenal Cortex**        Glucocorticoids (cortisol)                       Steroid                All tissue                        Raise blood glucose level, stimulates breakdown of protein
                            Mineralocorticoids (aldosterone)                 Steroid                Kidneys                           Reabsorb sodium and excrete potassium
                            Sex Hormones                                     Steroid                Gonads, skin, muscles and bones   Stimulates reproductive organs and brings on sex characteristics
  **Adrenal Medulla**       Epinephrine and norepinephrine                   Modified amino acid    Cardiac and other muscles         Released in emergency situations, raises blood glucose level, "fight or flight" response
  **Pancreas**              Insulin                                          Protein                Liver, muscles, adipose tissue    Lowers blood glucose levels, promotes formation of glycogen
                            Glucagon                                         Protein                Liver, muscles, adipose tissue    Raises blood glucose levels
  **Testes**                Androgens (testosterone)                         Steroid                Gonads, skin, muscles and bone    Stimulates male sex characteristics
  **Ovaries**               Estrogen and progesterone                        Steroid                Gonads, skin, muscles and bones   Stimulates female sex characteristics
  **Thymus**                Thymosins                                        Peptide                T lymphocytes                     Stimulates production and maturation of T lymphocytes
  **Pineal Gland**          Melatonin                                        Modified amino acid    Brain                             Controls circadian and circannual rhythms, possibly involved in maturation of sexual organs

Hormones can be chemically classified into four
groups:

1.  **Amino acid-derived**: Hormones that are modified amino acids.
2.  **Polypeptide and proteins**: Hormones that are chains of amino
    acids of less than or more than about 100 amino acids, respectively.
    Some protein hormones are actually glycoproteins, containing glucose
    or other carbohydrate groups.
3.  **Steroids**: Hormones that are lipids synthesized from cholesterol.
    Steroids are characterized by four interlocking carbohydrate rings.
4.  **Eicosanoids**: Are lipids synthesized from the fatty acid chains
    of phospholipids found in plasma membrane.

Hormones circulating in the blood diffuse into the interstitial fluids
surrounding the cell. Cells with specific receptors for a hormone
respond with an action that is appropriate for the cell. Because of the
specificity of hormone and target cell, the effects produced by a single
hormone may vary among different kinds of target cells.

Hormones activate target cells by one of two methods, depending upon the
chemical nature of the hormone.

-   **Lipid-soluble** hormones (steroid hormones and hormones of the
    thyroid gland) diffuse through the cell membranes of target cells.
    The lipid-soluble hormone then binds to a receptor protein that, in
    turn, activates a DNA segment that turns on specific genes. The
    proteins produced as result of the transcription of the genes and
    subsequent translation of mRNA act as enzymes that regulate specific
    physiological cell activity.

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-   **Water-soluble** hormones (polypeptide, protein, and most amino
    acid hormones) bind to a receptor protein on the plasma membrane of
    the cell. The receptor protein, in turn, stimulates the production
    of one of the following second messengers:

Cyclic AMP (cAMP) is produced when the receptor protein activates
another membrane-bound protein called a G protein. The G protein
activates adenylate cyclase, the enzyme that catalyzes the production of
cAMP from ATP. Cyclic AMP then triggers an enzyme that generates
specific cellular changes.

Inositol triphosphate (IP3) is produced from membrane phospholipids.
IP3, in turn, triggers the release of CA2+ from the endoplasmic
reticulum, which then activates enzymes that generate cellular changes.

Endocrine glands release hormones in response to one or more of the
following stimuli:

1.  Hormones from other endocrine glands.
2.  Chemical characteristics of the blood (other than hormones).
3.  Neural stimulation.

Most hormone production is managed by a negative feedback system. The
nervous system and certain endocrine tissues monitor various internal
conditions of the body. If action is required to maintain homeostasis,
hormones are released, either directly by an endocrine gland or
indirectly through the action of the hypothalamus of the brain, which
stimulates other endocrine glands to release hormones. The hormones
activate target cells, which initiate physiological changes that adjust
the body conditions. When normal conditions have been recovered, the
corrective action - the production of hormones - is discontinued. Thus,
in negative feedback, when the original (abnormal) condition has been
repaired, or negated, corrective actions decrease or discontinue. For
example, the amount of glucose in the blood controls the secretion of
insulin and glucagons via negative feedback.

The production of some hormones is controlled by positive feedback. In
such a system, hormones cause a condition to intensify, rather than
decrease. As the condition intensifies, hormone production increases.
Such positive feedback is uncommon, but does occur during childbirth,
where hormone levels build with increasingly intense labor contractions.
Also in lactation, hormone levels increase in response to nursing, which
causes an increase in milk production. The hormone produced by the
hypothalamus causing the milk let down and uterine contraction is
**oxytocin**.

## Endocrine Glands

### Pituitary gland

The hypothalamus makes up the lower region of the diencephalons and lies
just above the brain stem. The pituitary gland (hypophysis) is attached
to the bottom of the hypothalamus by a slender stalk called the
infundibulum. The pituitary gland consists of two major regions, the
anterior pituitary gland (anterior lobe or adenohypophysis) and the
posterior pituitary gland (posterior lobe or neurohypophysis). The
hypothalamus also controls the glandular secretion of the pituitary
gland.

The hypothalamus oversees many internal body conditions. It receives
nervous stimuli from receptors throughout the body and monitors chemical
and physical characteristics of the blood, including temperature, blood
pressure, and nutrient, hormone, and water content. When deviations from
homeostasis occur or when certain developmental changes are required,
the hypothalamus stimulates cellular activity in various parts of the
body by directing the release of hormones from the anterior and
posterior pituitary glands. The hypothalamus communicates directives to
these glands by one of the following two pathways: The pituitary gland
is found in the inferior part of the brain and is connected by the
pituitary stalk. It can be referred to as the master gland because it is
the main place for everything that happens within the endocrine system.
It is divided into two sections: the **anterior** lobe (adenohypophysis)
and the **posterior** lobe (neurohypophysis). The Anterior pituitary is
involved in sending hormones that control all other hormones of the
body.

#### Posterior pituitary

Communication between the hypothalamus and the posterior pituitary
occurs through neurosecretory cells that span the short distance between
the hypothalamus and the posterior pituitary. Hormones produced by the
cell bodies of the neurosecretory cells are packaged in vesicles and
transported through the axon and stored in the axon terminals that lie
in the posterior pituitary. When the neurosecretory cells are
stimulated, the action potential generated triggers the release of the
stored hormones from the axon terminals to a capillary network within
the posterior pituitary. Two hormones, oxytocin and antidiuretic hormone
(ADH), are produced and released this way. Decreased ADH release or
decreased renal sensitivity to ADH produces a condition known as
diabetes insipidus. Diabetes insipidus is characterized by polyuria
(excess urine production), hypernatremia (increased blood sodium
content) and polydipsia (thirst). Oxytocin is secreted by
paraventricular nucleus and a small quantity is secreted by supraoptic
nucleus in the hypothalamus. Oxytocin is secreted in both males and
females. In females, oxytocin acts on the mammary glands and uterus. In
males, oxytocin facilitates release of sperm into the urethra by causing
contraction of vas deferens.

The posterior lobe is composed of neural tissue \[neural ectoderm\] and
is derived from hypothalamus. Its function is to store oxytocin and
antidiuretic hormone. When the hypothalamic neurons fire these hormones
are release into the capillaries of the posterior lobe.

The posterior pituitary is, in effect, a projection of the hypothalamus.
It does not produce its own hormones, but only stores and releases the
hormones oxytocin and antidiuretic hormone. ADH is also known as
arginine vasopressin (AVP) or simply vasopressin.

#### Anterior pituitary

The anterior lobe is derived from oral ectoderm and is composed of
glandular epithelium. Communication between the hypothalamus and the
anterior pituitary (adenohypophysis) occurs through hormones (releasing
hormones and inhibiting hormones) produced by the hypothalamus and
delivered to the anterior pituitary via a portal network of capillaries.
It consists of three divisions: 1. pars distalis, 2. pars tuberalis, 3.
pars intermedia. The releasing and inhibiting hormones are produced by
specialized neurons of the hypothalamus called neurosecretory cells. The
hormones are released into a capillary network or primary plexus, and
transported through veins or hypophyseal portal veins, to a second
capillary network or secondary plexus that supplies the anterior
pituitary. The hormones then diffuse from the secondary plexus aunshine
into the anterior pituitary, where they initiate the production of
specific hormones by the anterior pituitary. Many of the hormones
produced by the anterior pituitary are tropic hormones or tropins, which
are hormones that stimulate other endocrine glands to secrete their
hormones.

The anterior pituitary lobe receives releasing hormones from the
hypothalamus via a portal vein system known as the
hypothalamic-hypophyseal portal system.

The anterior pituitary secretes:

-   thyroid-stimulating hormone (TSH)
-   adrenocorticotropic hormone (ACH)
-   prolactin
-   follicle-stimulating hormone (FSH)
-   luteinizing hormone (LH)
-   growth hormone (GH)
-   endorphins
-   and other hormones

It does this in response to a variety of chemical signals from the
hypothalamus, which travels to the anterior lobe by way of a special
capillary system from the hypothalamus, down the median eminence, to the
anterior lobe. These include:

-   thyrotropin-releasing hormone (TRH)
-   corticotropin-releasing hormone (CRH)
-   dopamine (DA), also called \'prolactin inhibiting factor\' (PIF)
-   gonadotropin-releasing hormone (GnRH)
-   growth hormone releasing hormone (GHRH)

These hormones from the hypothalamus cause release of the respective
hormone from the pituitary. The control of release of hormones from the
pituitary is via negative feedback from the target gland. For example
homeostasis of thyroid hormones is achieved by the following mechanism;
TRH from the hypothalamus stimulates the release of TSH from the
anterior pituitary. The TSH, in turn, stimulates the release of thyroid
hormones form the thyroid gland. The thyroid hormones then cause
negative feedback, suppressing the release of TRH and TSH.

The heart, gastrointestinal tract, the placenta, the kidneys and the
skin, whose major function is not the secretion of hormones, also
contain some specialized cells that produce hormones.

In addition, all cells, except red blood cells secrete a class of
hormones called eicosanoids. These hormones are paracrines, or local
hormones, that primarily affect neighboring cells. Two groups of
eicosanoids, the prostaglandins (PGs) and the leukotrienes (LTs), have a
wide range of varying effects that depend upon the nature of the target
cell. Eicosanoid activity, for example, may impact blood pressure, blood
clotting, immune and inflammatory responses, reproductive processes, and
the contraction of smooth muscles.

## Antagonistic Hormones

Maintaining homeostasis often requires conditions to be limited to a
narrow range. When conditions exceed the upper limit of homeostasis,
specific action, usually the production of a hormone is triggered. When
conditions return to normal, hormone production is discontinued. If
conditions exceed the lower limits of homeostasis, a different action,
usually the production of a second hormone is triggered. Hormones that
act to return body conditions to within acceptable limits from opposite
extremes are called **antagonistic hormones**. The two glands that are
the most responsible for homeostasis is the thyroid and the parathyroid.

The regulation of blood glucose concentration (through negative
feedback) illustrates how the endocrine system maintains homeostasis by
the action of antagonistic hormones. Bundles of cells in the pancreas
called the islets of Langerhans contain two kinds of cells, **alpha
cells** and **beta cells**. These cells control blood glucose
concentration by producing the antagonistic hormones insulin and
glucagon.

Beta cells secrete **insulin**. When the concentration of blood glucose
raises such in after eating, beta cells secret insulin into the blood.
Insulin stimulates the liver and most other body cells to absorb
glucose. Liver and muscle cells convert glucose to glycogen, for short
term storage, and adipose cells convert glucose to fat. In response,
glucose concentration decreases in the blood, and insulin secretion
discontinues through negative feedback from declining levels of glucose.

Alpha cells secrete **glucagon**. When the concentration of blood
glucose drops such as during exercise, alpha cells secrete glucagon into
the blood. Glucagon stimulates the liver to release glucose. The glucose
in the liver originates from the breakdown of glycogen. Glucagon also
stimulates the production of ketone bodies from amino acids and fatty
acids. Ketone bodies are an alternative energy source to glucose for
some tissues. When blood glucose levels return to normal, glucagon
secretion discontinues through negative feedback.

Another example of antagonistic hormones occurs in the maintenance of
Ca^2+^ ion concentration in the blood. Parathyroid hormone (PTH) from
the parathyroid glands increases Ca^2+^ in the blood by increasing
Ca^2+^ absorption in the intestines and reabsorption in the kidneys and
stimulating Ca^2+^ release from bones. Calcitonin (CT) produces the
opposite effect by inhibiting the breakdown of bone matrix and
decreasing the release of calcium in the blood.

### Thyroid gland

The **Thyroid gland** is one of the largest endocrine glands in the
body. It is positioned on the neck just below the Larynx and has two
lobes with one on either side of the trachea. It is involved in the
production of the hormones T3 (triiodothyronine) and T4 (thyroxine).
These hormones increase the metabolic activity of the body's cells. The
thyroid also produces and releases the hormone calcitonin
(thyrocalcitonin) which contributes to the regulation of blood calcium
levels. Thyrocalcitonin or calcitonin decreases the concentration of
calcium in the blood. Most of the calcium removed from the blood is
stored in the bones.

The thyroid hormone consists of two components, thyroxine and iodine.
This hormone increases the metabolism of most body cells. A deficiency
of iodine in the diet leads to the enlargement of the thyroid gland,
known as a simple goiter. Hypothyroidism during early development leads
to cretinism. In adults, it produces myxedema, characterized by obesity
and lethargy. Hyperthyroidism leads to a condition known as exophthalmic
goiter, characterized by weight loss as well as hyperactive and
irritable behavior.

The thyroid gland is a two-lobed gland that manifests a remarkably
powerful active transport mechanism for up-taking iodide ions from the
blood. As blood flows through the gland, iodide is converted to an
active form of iodine. This iodine combines with an amino acid called
tyrosine. Two molecules of iodinated tyrosine then combine to form
thyroxine. Following its formation, the thyroxine becomes bound to a
polysaccharide-protein material called thyroglobulin. The normal thyroid
gland may store several weeks supply of thyroxine in this bound form. An
enzymatic splitting of the thyroxine from the thyroglobulin occurs when
a specific hormone is released into the blood. This hormone, produced by
the pituitary gland, is known as thyroid-stimulating hormone (TSH). TSH
stimulates certain major rate-limiting steps in thyroxine secretion, and
thereby alters its rate of release. A variety of bodily defects, either
dietary, hereditary, or disease induced, may decrease the amount of
thyroxine released into the blood. The most popular of these defects is
one that results from dietary iodine deficiency. The thyroid gland
enlarges, in the continued presence of TSH from the pituitary, to form a
goiter. This is a futile attempt to synthesize thyroid hormones, for
iodine levels that are too low. Normally, thyroid hormones act via a
negative feedback loop on the pituitary to decrease stimulation of the
thyroid. In goiter, the feedback loop cannot be in operation - hence
continual stimulation of the thyroid and the inevitable protuberance on
the neck. Formerly, the principal source of iodine came from seafood. As
a result, goiter was prevalent amongst inland areas far removed from the
sea. Today, the incidence of goiter has been drastically reduced by
adding iodine to table salt.

Thyroxine serves to stimulate oxidative metabolism in cells; it
increases the oxygen consumption and heat production of most body
tissues, a notable exception being the brain. Thyroxine is also
necessary for normal growth. The most likely explanation being that
thyroxine promotes the effects of growth hormone on protein synthesis.
The absence of thyroxine significantly reduces the ability of growth
hormone to stimulate amino acid uptake and RNA synthesis. Thyroxine also
plays a crucial role in the closely related area of organ development,
particularly that of the central nervous system.

If there is an insufficient amount of thyroxine, a condition referred to
as hypothyroidism results. Symptoms of hypothyroidism stem from the fact
that there is a reduction in the rate of oxidative energy-releasing
reactions within the body cells. Usually the patient shows puffy skin,
sluggishness, and lowered vitality. Other symptoms of hypothyroidism
include weight gain, decreased libido, inability to tolerate cold,
muscle pain and spasm, and brittle nails. Hypothyroidism in children, a
condition known as cretinism, can result in mental retardation,
dwarfism, and permanent sexual immaturity. Sometimes the thyroid gland
produces too much thyroxine, a condition known as hyperthyroidism. This
condition produces symptoms such as an abnormally high body temperature,
profuse sweating, high blood pressure, loss of weight, irritability,
insomnia and muscular pain and weakness. It also causes the
characteristic symptom of the eyeballs protruding from the skull called
exophthalmia. This is surprising because it is not a symptom usually
related to a fast metabolism. Hyperthyroidism has been treated by
partial removal or by partial radiation destruction of the gland. More
recently, several drugs that inhibit thyroid activity have been
discovered, and their use is replacing the former surgical procedures.
Unfortunately thyroid conditions require lifetime treatment and because
of the body\'s need for a sensitive balance of thyroid hormone both
supplementing and suppressing thyroid function can take months or even
years to regulate.

#### T3 and T4 Function within the body

Iodine and T4 stimulate the spectacular apoptosis (programmed cell
death) of the cells of the larval gills, tail and fins Transforming the
aquatic, vegetarian tadpole into the terrestrial, carnivorous frog with
better neurological, visuospatial, olfactory and cognitive abilities for
hunting. Contrary to amphibian metamorphosis, thyroidectomy and
hypothyroidism in mammals may be considered a sort of phylogenetic and
metabolic regression to a former stage of reptilian life. Indeed, many
disorders that seem to afflict hypothyroid humans have reptilian-like
features, such as dry, hairless, scaly, cold skin and a general slowdown
of metabolism, digestion, heart rate and nervous reflexes, with
lethargic cerebration, hyperuricemia and hypothermia ( Venturi, 2000).
\]\] The Production of T3 and T4 are regulated by thyroid stimulating
hormone (TSH), released by the pituitary gland, a bean shape node in the
brain. TSH Production is increased when T3 and T4 levels are too low.
The thyroid hormones are released throughout the body to direct the
body\'s metabolism. They stimulate all cells within the body to work at
a better metabolic rate. Without these hormones the body\'s cells would
not be able to regulate the speed at which they performed chemical
actions. Their release will be increased under certain situations such
as cold temperatures when a higher metabolism is needed to generate
heat. When children are born with thyroid hormone deficiency they have
problems with physical growth and developmental problems. Brain
development can also be severely impaired.

#### The significance of iodine

Thyroid hormone cannot be produced without an abundant source of iodine.
The iodine concentration within the body, although significant, can be
as little as 1/25th the concentration within the thyroid itself. When
the thyroid is low on iodine the body will try harder to produce T3 and
T4 which will often result in a swelling of the thyroid gland, resulting
in a goiter.

## Extrathyroidal iodine

![Sequence of 123-iodide human scintiscans after an intravenous
injection, (from left) after 30 minutes, 20 hours, and 48 hours. A high
and rapid concentration of radio-iodide is evident in the periencephalic
and cerebrospinal fluid (left), salivary glands, oral mucosa and the
stomach. In the thyroid gland, I-concentration is more progressive, also
in the reservoir (from 1% after 30 minutes, to 5.8 % after 48 hours, of
the total injected dose. Highest iodide-concentration by the mammary
gland is evident only in pregnancy and lactation. High excretion of
radio-iodide is observed in the
urine.[^1]](Sequence_of_123-iodide_total_body_human_scintiscans.jpg "Sequence of 123-iodide human scintiscans after an intravenous injection, (from left) after 30 minutes, 20 hours, and 48 hours. A high and rapid concentration of radio-iodide is evident in the periencephalic and cerebrospinal fluid (left), salivary glands, oral mucosa and the stomach. In the thyroid gland, I-concentration is more progressive, also in the reservoir (from 1% after 30 minutes, to 5.8 % after 48 hours, of the total injected dose. Highest iodide-concentration by the mammary gland is evident only in pregnancy and lactation. High excretion of radio-iodide is observed in the urine.")
Iodine accounts for 65% of the molecular weight of T4 and 59% of the T3.
15--20 mg of iodine is concentrated in thyroid tissue and hormones, but
70% of the body\'s iodine is distributed in other tissues, including
mammary glands, eyes, gastric mucosa, the cervix, and salivary glands.
In the cells of these tissues iodide enters directly by sodium-iodide
symporter (NIS). Its role in
mammary tissue is related to fetal and neonatal development, but its
role in the other tissues is unknown. It has been shown to act as an
antioxidant in these tissues.

The US Food and Nutrition Board and Institute of Medicine recommended
daily allowance of iodine ranges from 150 micrograms /day for adult
humans to 290 micrograms /day for lactating mothers. However, the
thyroid gland needs no more than 70 micrograms /day to synthesize the
requisite daily amounts of T4 and T3. These higher recommended daily
allowance levels of iodine seem necessary for optimal function of a
number of body systems, including lactating breast, gastric mucosa,
salivary glands, oral mucosa, thymus, epidermis, choroid plexus,
etc.[^3][^4][^5] Moreover, iodine can add to double bonds of
docosahexaenoic acid and arachidonic acid of cellular membranes, making
them less reactive to free oxygen radicals.[^6]

#### Calcitonin

Calcitonin is a 32 amino acid polypeptide hormone. It is an additional
hormone produced by the thyroid, and contributes to the regulation of
blood calcium levels. Thyroid cells produce calcitonin in response to
high calcium levels in the blood. This hormone will stimulate movement
of calcium into the bone structure. It can also be used therapeutically
for the treatment of hypercalcemia or osteoporosis. Without this hormone
calcium will stay within the blood instead of moving into bones to keep
them strong and growing. Its importance in humans has not been as well
established as its importance in other animals.

### Parathyroid gland

There are four parathyroid glands. They are small, light-colored lumps
that stick out from the surface of the thyroid gland. All four glands
are located on the thyroid gland. They are butterfly-shaped and located
inside the neck, more specifically on both sides of the windpipe. One of
the parathyroid glands most important functions is to regulate the
body\'s calcium and phosphorus levels. Another function of the
parathyroid glands is to secrete parathyroid hormone, which causes the
release of the calcium present in bone to extracellular fluid. PTH does
this by depressing the production of osteoblasts, special cells of the
body involved in the production of bone and activating osteoclasts,
other specialized cells involved in the removal of bone.

There are two major types of cells that make up parathyroid tissue:

-   One of the major cells is called **oxyphil cells**. Their function
    is basically unknown.
-   The second type are called **chief cells**. Chief cells produce
    parathyroid hormone.

The structure of a parathyroid gland is very different from that of a
thyroid gland. The chief cells that produce parathyroid hormone are
arranged in tightly-packed nests around small blood vessels, quite
unlike the thyroid cells that produce thyroid hormones, which are
arranged in spheres called the thyroid follicles.

PTH or **Parathyroid Hormone** is secreted from these four glands. It is
released directly into the bloodstream and travels to its target cells
which are often quite far away. It then binds to a structure called a
receptor, that is found either inside or on the surface of the target
cells.

Receptors bind a specific hormone and the result is a specific
physiologic response, meaning a normal response of the body.

PTH finds its major target cells in bone, kidneys, and the
gastrointestinal system.

Calcitonin, a hormone produced by the thyroid gland that also regulates
ECF calcium levels and serves to counteract the calcium-producing
effects of PTH.

The adult body contains as much as 1 kg of calcium. Most of this calcium
is found in bone and teeth.

The four parathyroid glands secrete the parathyroid hormone (PTH). It
opposes the effect of thyrocalcitonin. It does this by removing calcium
from its storage sites in bones, releasing it into the bloodstream. It
also signals the kidneys to reabsorb more of this mineral, transporting
it into the blood. It also signals the small intestine to absorb more of
this mineral, transporting it from the diet into the blood.

Calcium is important for steps of body metabolism. Blood cannot clot
without sufficient calcium. Skeletal muscles require this mineral in
order to contract. A deficiency of PTH can lead to tetany, muscle
weakness due to lack of available calcium in the blood.

The parathyroid glands were long thought to be part of the thyroid or to
be functionally associated with it. We now know that their close
proximity to the thyroid is misleading: both developmentally and
functionally, they are totally distinct from the thyroid.

The parathyroid hormone, called parathormone, regulates the
calcium-phosphate balance between the blood and other tissues.
Production of this hormone is directly controlled by the calcium
concentration of the extracellular fluid bathing the cells of these
glands. Parathormone exerts at least the following five effects: (1) it
increases gastrointestinal absorption of calcium by stimulating the
active transport system and moves calcium from the gut lumen into the
blood; (2) it increases the movement of calcium and phosphate from bone
into extracellular fluid. This is accomplished by stimulating
osteoclasts to break down bone structure, thus liberating calcium
phosphate into the blood. In this way, the store of calcium contained in
bone is tapped; (3) it increases re-absorption of calcium by the renal
tubules, thereby decreasing urinary calcium excretion; (4) it reduces
the re-absorption of phosphate by the renal tubules (5)it stimulates the
synthesis of 1,25-dihydrixycholecalciferol by the kidney.

The first three effects result in a higher extracellular calcium
concentration. The adaptive value of the fourth is to prevent the
formation of kidney stones.

If parathyroid glands are removed accidentally during surgery on the
thyroid, there would be a rise in the phosphate concentration in the
blood. There would also be a drop in the calcium concentration as more
calcium is excreted by the kidneys and intestines, and more incorporated
into the bone. This can produce serious disturbances, particularly in
the muscles and nerves, which use calcium ions for normal functioning.
Over activity of the parathyroid glands, which can result from a tumor
on the glands, produces a weakening of the bones. This is a condition
that makes them much more vulnerable to fracturing because of excessive
withdrawal of calcium from the bones.

### Adrenal glands

**Adrenal glands** are a pair of ductless glands located above the
kidneys. Through hormonal secretions, the adrenal glands regulate many
essential functions in the body, including biochemical balances that
influence athletic training and general stress response. The
glucocorticoids include corticosterone, cortisone, and hydrocortisone or
cortisol. These hormones serve to stimulate the conversion of amino
acids into carbohydrates which is a process known as gluconeogenesis,
and the formation of glycogen by the liver. They also stimulate the
formation of reserve glycogen in the tissues, such as in the muscles.
The glucocorticoids also participate in lipid and protein metabolism.
The cortex of the adrenal gland is known to produce over 20 hormones,
but their study can be simplified by classifying them into three
categories: glucocorticoids, mineralocorticoids, and sex hormones.

They are triangular-shaped glands located on top of the kidneys. They
produce hormones such as estrogen, progesterone, steroids, cortisol, and
cortisone, and chemicals such as adrenalin (epinephrine),
norepinephrine, and dopamine. When the glands produce more or less
hormones than required by the body, disease conditions may occur.

The adrenal cortex secretes at least two families of hormones, the
**glucocorticoids** and **mineral corticoids**. The **adrenal medulla**
secretes the hormones **epinephrine** (adrenalin) and **norepinephrine**
(noradrenalin).

**Adrenal Cortex:** The hormones made by the Adrenal Cortex supply
long-term responses to stress. The two major hormones produced are
the**Mineral Corticoids** and the **Glucocorticoids.** The Mineral
Corticoids regulate the salt and water balance, leading to the increase
of blood volume and blood pressure. The **Glucocorticoids** are
monitoring the ACTH, in turn regulating carbohydrates, proteins, and fat
metabolism. This causes an increase in blood glucose. Glucocorticoids
also reduce the body\'s inflammatory response.

**Cortisol** is one of the most active glucocorticoids. It usually
reduces the effects of inflammation or swelling throughout the body. It
also stimulates the production of glucose from fats and proteins, which
is a process referred to as **gluconeogenesis**.

**Aldosterone** is one example of a mineralocorticoid. It signals the
tubules in the kidney nephrons to reabsorb sodium while secreting or
eliminating potassium. If sodium levels are low in the blood, the kidney
secretes more **renin**, which is an enzyme that stimulates the
formation of **angiotensin** from a molecule made from the liver.
Angiotensin stimulates aldosterone secretion. As a result, more sodium
is reabsorbed as it enters the blood.

Aldosterone, the major mineralocorticoid, stimulates the cells of the
distal convoluted tubules of the kidneys to decrease re-absorption of
potassium and increase re-absorption of sodium. This in turn leads to an
increased re-absorption of chloride and water. These hormones, together
with such hormones as insulin and glucagon, are important regulators of
the ionic environment of the internal fluid.

The renin-angiotensin-aldosterone mechanism can raise blood pressure if
it tends to drop. It does this in two ways. Angiotensin is a
vasoconstrictor, decreasing the diameter of blood vessels. As vessels
constrict, blood pressure increases. In addition, as sodium is
reabsorbed, the blood passing through the kidney becomes more
hypertonic. Water follows the sodium into the hypertonic blood by
osmosis. This increases the amount of volume in the blood and also
increases the blood pressure.

**Adrenal Medulla** The hypothalamus starts nerve impulses that travel
the path from the bloodstream, spinal cord, and sympathetic nerve fibers
to the Adrenal Medulla, which then releases hormones. The effects of
these hormones provide a short-term response to stress.

Excessive secretion of the glucocorticoids causes **Cushing\'s
syndrome**, characterized by muscle atrophy or degeneration and
hypertension or high blood pressure. Under secretion of these substances
produces **Addison\'s disease**, characterized by low blood pressure and
stress.

Epinephrine and norepinephrine produce the \"fight or flight\" response,
similar to the effect from the sympathetic nervous system. Therefore,
they increase heart rate, breathing rate, blood flow to most skeletal
muscles, and the concentration of glucose in the blood. They decrease
blood flow to the digestive organs and diminish most digestive
processes.

left\|framed\|300px\|Suprarenal glands viewed from the
front.

right\|framed\|300px\|Suprarenal glands viewed from
behind.

The adrenal sex hormones consist mainly of male sex hormones (androgens)
and lesser amounts of female sex hormones (estrogens and progesterone).
Normally, the sex hormones released from the adrenal cortex are
insignificant due to the low concentration of secretion. However, in
cases of excess secretion, masculine or feminine effects appear. The
most common syndrome of this sort is \"virilism\" of the female.

Should there be an insufficient supply of cortical hormones, a condition
known as Addison\'s disease would result. This disease is characterized
by an excessive excretion of sodium ions, and hence water, due to lack
of mineralocorticoids. Accompanying this is a decreased blood glucose
level due to a deficient supply of glucocorticoids. The effect of a
decreased androgen supply cannot be observed immediately. Injections of
adrenal cortical hormones promptly relieve these symptoms.

Hormonal production in the adrenal cortex is directly controlled by the
anterior pituitary hormone called adrenocorticotropic hormone (ACTH).

The two adrenal glands lie very close to the kidneys. Each adrenal gland
is actually a double gland, composed of an inner core like medulla and
an outer cortex. Each of these is functionally unrelated.

The adrenal medulla secretes two hormones, adrenalin or epinephrine and
noradrenalin or norepinephrine, whose functions are very similar but not
identical. The adrenal medulla is derived embryologically from neural
tissue. It has been likened to an overgrown sympathetic ganglion whose
cell bodies do not send out nerve fibers, but release their active
substances directly into the blood, thereby fulfilling the criteria for
an endocrine gland. In controlling epinephrine secretion, the adrenal
medulla behaves just like any sympathetic ganglion, and is dependent
upon stimulation by sympathetic preganglionic fibers.

Epinephrine promotes several responses, all of which are helpful in
coping with emergencies: the blood pressure rises, the heart rate
increases, the glucose content of the blood rises because of glycogen
breakdown, the spleen contracts and squeezes out a reserve supply of
blood, the clotting time decreases, the pupils dilate, the blood flow to
skeletal muscles increase, the blood supply to intestinal smooth muscle
decreases and hairs become erect. These adrenal functions, which
mobilize the resources of the body in emergencies, have been called the
fight-or-flight response. Norepinephrine stimulates reactions similar to
those produced by epinephrine, but is less effective in conversion of
glycogen to glucose.

The significance of the adrenal medulla may seem questionable since the
complete removal of the gland causes few noticeable changes; humans can
still exhibit the flight-or-fight response. This occurs because the
sympathetic nervous system complements the adrenal medulla in
stimulating the fight-or-flight response, and the absence of the
hormonal control will be compensated for by the nervous system.

### Pancreas

The **pancreas** is very important organ in the digestion system and the
circulatory system because it helps to maintain our blood sugar levels.
The pancreas is considered to be part of the gastrointestinal system. It
produces digestive enzymes to be released into the small intestine to
aid in reducing food particles to basic elements that can be absorbed by
the intestine and used by the body. It has another very different
function in that it forms insulin, glucagon and other hormones to be
sent into the bloodstream to regulate blood sugar levels and other
activities throughout the body.

It has a pear-shape to it and is approximately 6 inches long. It is
located in the middle and back portion of the abdomen. The pancreas is
connected to the first part of the small intestine, the duodenum, and
lies behind the stomach. The pancreas is made up of glandular tissue:
any substance secreted by the cells of the pancreas will be secreted
outside of the organ.

The digestive juices produced by the pancreas are secreted into the
duodenum via a Y-shaped duct, at the point where the common bile duct
from the liver and the pancreatic duct join just before entering the
duodenum. The digestive enzymes carried into the duodenum are
representative of the exocrine function of the pancreas, in which
specific substances are made to be passed directly into another organ.

The pancreas is unusual among the body\'s glands in that it also has a
very important endocrine function. Small groups of special cells called
**islet cells** throughout the organ make the hormones of insulin and
glucagon. These, of course, are hormones that are critical in regulating
blood sugar levels. These hormones are secreted directly into the
bloodstream to affect organs all over the body.

**Insulin** acts to lower blood sugar levels by allowing the sugar to
flow into cells. Glucagon acts to raise blood sugar levels by causing
glucose to be released into the circulation from its storage sites.
Insulin and glucagon act in an opposite but balanced fashion to keep
blood sugar levels stable.

A healthy working pancreas in the human body is important for
maintaining good health by preventing malnutrition, and maintaining
normal levels of blood sugar. The digestive tract needs the help of the
enzymes produced by the pancreas to reduce food particles to their
simplest elements, or the nutrients cannot be absorbed. Carbohydrates
must be broken down into individual sugar molecules. Proteins must be
reduced to simple amino acids. Fats must be broken down into fatty
acids. The pancreatic enzymes are important in all these
transformations. The basic particles can then easily be transported into
the cells that line the intestine, and from there they can be further
altered and transported to different tissues in the body as fuel sources
and construction materials. Similarly, the body cannot maintain normal
blood sugar levels without the balanced action of insulin and glucagon.

The pancreas contains exocrine and endocrine cells. Groups of endocrine
cells, the **islets of Langerhans**, secrete two hormones. The beta
cells secrete **insulin**; the alpha cells secrete **glucagon**. The
level of sugar in the blood depends on the opposing action of these two
hormones.

Insulin decreases the concentration of glucose in the blood. Most of the
glucose enters the cells of the liver and skeletal muscles. In these
cells, this **monosaccharide** is converted to the polysaccharide
glycogen. Therefore, insulin promotes **glycogenesis** or glycogen
synthesis, in which glucose molecules are added to chains of glycogen.
Excess glucose is also stored as fat in adipose tissue cells in response
to insulin.

Insulin deficiency leads to the development of **diabetes mellitus**,
specifically **type I**, juvenile diabetes. As the pancreas does not
produce sufficient insulin, it is treated by insulin injections. In
**type II** or maturity onset diabetes, the pancreas does produce enough
insulin, but the target cells do not respond to it.

As already stated, the pancreas is a mixed gland having both endocrine
and exocrine functions. The exocrine portion secretes digestive enzymes
into the duodenum via the pancreatic duct. The endocrine portion
secretes two hormones, insulin and glucagon, into the blood.

Insulin is a hormone that acts directly or indirectly on most tissues of
the body, with the exception of the brain. The most important action of
insulin is the stimulation of the uptake of glucose by many tissues,
particularly the liver, muscle and fat. The uptake of glucose by the
cells decreases blood glucose and increases the availability of glucose
for the cellular reactions in which glucose participates. Thus, glucose
oxidation, fat synthesis, and glycogen synthesis are all accentuated by
an uptake of glucose. It is important to note that insulin does not
alter glucose uptake by the brain, nor does it influence the active
transport of glucose across the renal tubules and gastrointestinal
epithelium.

As stated, insulin stimulates glycogen synthesis. It also increases the
activity of the enzyme that catalyzes the rate-limiting step in glycogen
synthesis. Insulin also increases triglyceride levels by inhibiting
triglyceride breakdown, and by stimulating production of triglyceride
through fatty acid and glycerophosphate synthesis. The net protein
synthesis is also increased by insulin, which stimulates the active
membrane transport of amino acids, particularly into muscle cells.
Insulin also has effects on other liver enzymes, but the precise
mechanisms by which insulin induces these changes are not well
understood.

Insulin is secreted by beta cells, which are located in the part of the
pancreas known as the islets of Langerhans. These groups of cells, which
are located randomly throughout the pancreas, also consist of other
secretory cells called alpha cells. It is these alpha cells that secrete
glucagon. Glucagon is a hormone that has the following major effects: it
increases hepatic synthesis of glucose from pyruvate, lactate, glycerol,
and amino acids (a process called gluconeogenesis, which also raises the
plasma glucose level); and it increases the breakdown of adipose tissue
triglyceride, thereby raising the plasma levels of fatty acids and
glycerol. The glucagon secreting alpha cells in the pancreas, like the
beta cells, respond to changes in the concentration of glucose in the
blood flowing through the pancreas; no other nerves or hormones are
involved.

It should be noted that glucagon has the opposite effects of insulin.
Glucagon elevates the plasma glucose, whereas insulin stimulates its
uptake and thereby reduces plasma glucose levels; glucagon elevates
fatty acid concentrations, whereas insulin converts fatty acids and
glycerol into triglycerides, thereby inhibiting triglyceride breakdown.

The alpha and beta cells of the pancreas make up a push-pull system for
regulating the plasma glucose level.

### Sex organs

The Sex organs (Gonads) are the testes in the male, and the ovaries in
the female. Both of these organs produce and secrete hormones that are
balanced by the hypothalamus and pituitary glands.

The main hormones from the reproductive organs are:

**Testosterone** is more prominent in males. It belongs to the family of
androgens, which are steroid hormones producing masculine effects.
Testosterone stimulates the development and functioning of the primary
sex organs. It also stimulates the development and maintenance of
secondary male characteristics, such as hair growth on the face and the
deep pitch of the voice.

**Estrogen** In females, this hormone stimulates the development of the
uterus and vagina. It is also responsible for the development and
maintenance of secondary female characteristics, such as fat
distribution throughout the body and the width of the pelvis.

#### Male

right\|framed

The **testes** produce **androgens** (i.e., \"testosterone\").
**Testosterone** is classified as a steroid and is responsible for many
of the physical characteristics in males like.

-   Broad shoulders
-   Muscular body
-   Hair

Testosterone increases protein production. Hormones that build up
protein are called **anabolic steroids**. Anabolic steroids are
available commercially and are being used by athletes because they help
improve their physical ability, however, they do have major side effects
such as:

-   Liver and kidney disorders
-   Hypertension (high blood pressure)
-   Decreased sperm count and impotency
-   Aggressive behavior (\"roid rage\")
-   Balding
-   Acne

{{-}}

#### Female

!Schematic frontal view of female
anatomy.{width="240"}

The **ovaries** produce **estrogen** and **progesterone**. Estrogen
increases at the time of puberty and causes the growth of the uterus and
vagina. Without estrogen egg maturation would not occur. Estrogen is
also responsible for secondary sex characteristics such as female body
hair and fat distribution. Estrogen and Progesterone are responsible for
the development of the breast and for the uterine cycle. Progesterone is
a female hormone secreted by the corpus luteum after ovulation during
the second half of the menstrual cycle. It prepares the lining of the
uterus for implantation of a fertilized egg and allows for complete
shedding of the endometrium at the time of menstruation. In the event of
pregnancy, the progesterone level remains stable beginning a week or so
after conception.

{{-}}

### Pineal gland

The pineal gland (also called the pineal body or epiphysis) is a small
endocrine gland in the brain. It is located near the center of the
brain, between the two hemispheres, tucked in a groove where the two
rounded thalamic bodies join. It consists of two types of cells 1.
parenchymal cells 2. neuroglial cells.

The pineal gland is a reddish-gray body about the size of a pea (8 mm in
humans) located just rostro-dorsal to the superior colliculus and behind
and beneath the stria medullaris, between the laterally positioned
thalamic bodies. It is part of the epithalamus.

The pineal gland is a midline structure, and is often seen in plain
skull X-rays, as it is often calcified. The main hormone produced and
secreted by the pineal gland is melatonin. Secretion is highest at night
and between the ages of 0-5. Melatonin acts mainly on gonads.

{{-}}

## Glossary

**Adrenal Gland:** endocrine gland that is located on top of each kidney

**Amino Acid-derived:** hormones that are modified amino acids

**Antagonistic Hormones:** hormones that act to return body conditions
to within acceptable limits from opposite extremes

**Calcitonin:** hormone produced by the thyroid; contributes to the
regulation of blood calcium levels

**Eicosanoids:** lipids that are synthesized from the fatty acid chains
of phospholipids found in plasma membrane

**Endocrine Glands:** glands that have no duct and release their
secretions directly into the intercellular fluid or into the blood

**Endocrine System:** a control system of ductless glands that secrete
chemical messengers called hormones

**Estrogen:** hormone in females; stimulates the development of the
uterus and vagina

**Exocrine Glands:** glands that release their cellular secretions
through a duct which empties to the outside or into the lumen (empty
internal space) of an organ

**Hormone:** a specific chemical substance produced by certain cells
that control, or help to control, cellular processes elsewhere in an
organism

**Insulin:** hormone that acts to lower blood sugar levels by allowing
the sugar to flow into cells

**Iodine:** chemical in the body; Thyroid hormone can not be produced
with out it

**Lipid-soluble Hormones:** diffuse through the cell membranes of target
cells

**Parathyroid:** four masses of tissue, two embedded posterior in each
lateral mass of the thyroid gland

**Pancreas:** organ involved with the digestion system and the
circulatory system; helps to maintain blood sugar levels

**Pineal Gland:** small endocrine gland in the brain located near the
center of the brain, between the two hemispheres, tucked in a groove
where the two rounded thalamic bodies join

**Pituitary Gland:** endocrine gland that is attached to the
hypothalamus of the lower forebrain

**Polypeptide and Proteins:** hormones that are chains of amino acids of
less than or more than about 100 amino acids

**Steroids:** hormones that are lipids that are synthesized from
cholesterol; characterized by four interlocking carbohydrate rings

**Testosterone:** hormone more prominent in males; belongs to the family
of androgens, which are steroid hormones producing masculinizing effects

**Thyroid Gland:** endocrine gland that consists of two lateral masses
that are attached to the trachea

**Thyroxine:** serves to stimulate oxidative metabolism in cells;
increases the oxygen consumption and heat production of most body
tissues

**Water-soluble Hormones:** bind to a receptor protein on the plasma
membrane of the cell

## Chapter Review Questions

Answers for these questions can be found here

1\. My child just fell and was hurt, the anxious feeling that I feel is
caused by:

:   A\) glucagon
:   B\) insulin
:   C\) epinephrine
:   D\) adrenocorticotropic
:   E\) None of these

2\. All of Bob's life he has had to take insulin shots, this is caused
because

:   A\) his beta cells don't function correctly
:   B\) his alpha cells don't function correctly
:   C\) his DA hormone isn't functioning correctly
:   D\) his GHRH hormone isn't functioning correctly

3\. The reason iodine is in salt is

:   A\) to prevent diabetes
:   B\) to prevent simple goiters
:   C\) to prevent Addison's disease
:   D\) to prevent Cushing\'s syndrome

4\. All hormones react to a negative feedback except

:   A\) progesterone
:   B\) estrogen
:   C\) prolactin
:   D\) oxytocin
:   E\) none of these

5\. If I have a high blood calcium level it may be due to

:   A\) calcitonin
:   B\) parathyroid
:   C\) glucocorticoids
:   D\) glucagon

6\. Hormones that are lipids that are synthesized from cholesterol

:   A\) protein
:   B\) amino acid-derived
:   C\) polypeptide
:   D\) steroids
:   E\) eicosanoids

7\. This type of hormone must bind to a receptor protein on the plasma
membrane of the cell

:   A\) water soluble
:   B\) lipid soluble
:   C\) steroid
:   D\) polypeptide
:   E\) a and d
:   F\) b and c

8\. Endocrine glands release hormones in response to

:   A\) Hormones from other endocrine glands
:   B\) Chemical characteristics of the blood
:   C\) Neural stimulation
:   D\) All of the above

9\. The anterior pituitary secretes

:   A\) oxytocin
:   B\) endorphins
:   C\) ADH
:   D\) TRH

10\. Chief cells produce

:   A\) epinephrine
:   B\) glucagon
:   C\) insulin
:   D\) mineralocorticoids
:   E\) parathyroid hormone

11\. Name the eight major endocrine glands.

12\. Name the four major groups hormones can be chemically classified
into.

## References

[^1]:

[^2]:

[^3]:

[^4]:

[^5]:

[^6]: Cocchi, M. and Venturi, S. Iodide, antioxidant function and
    Omega-6 and Omega-3 fatty acids: a new hypothesis of a biochemical
    cooperation? Progress in Nutrition, 2000, 2, 15-19