Steenland et al. Overall human studies are insufficient for assessing whether PFOA is or is not related to immune suppression. Thyroid function. After experimental evi- dence of PFCs reducing T3 and T4 in rats and monkeys (Butenhoff et al. 2002; Lau et al. 2007), a number of occupational and com- munity studies have investigated these clinical markers of thyroid function, usually in cross- sectional studies. Emmett et al. (2006b) inves- tigated the association between PFOA and health outcomes in 371 community residents with high serum levels (median, 534 ng/mL) due to contaminated drinking water. No sig- nificant association was found between TSH and the serum level of PFOA. In a small com- munity study (n = 31) of New York State anglers, at much lower exposure levels (PFOA serum geometric mean = 1.33 ng/mL), poten- tial associations were investigated between serum concentrations of eight measured PFCs and levels of TSH and free T4 (Bloom et al. 2010). No associations were found, but study power was very low. Olsen et al. (2003) conducted a cross- sectional analysis that included two plants with 255 and 263 workers, respectively. Multivariate regression analysis by quartiles of PFOA or PFOS exposure showed no sig- nificant association of either compound with T3, T4, or TSH. A continuous regression sug- gested a slight positive association between log PFOA and log T3 (coefficient = 0.016, p-value = 0.01); however, the contribution of this association to the variance of the outcome 2 = 0.01). A longitu- was negligible (partial R dinal analysis of up to three examinations for 174 workers from the same company found no association between PFOA and thyroid hormones (Olsen et al. 2003). In a more recent occupational study, Olsen and Zobel (2007) reported on analyses of thyroid hormones in relation to PFOA among 552 employees in three plants (median serum level, 1,100 ng/mL). A log–log regres- sion adjusted for age, body mass index (BMI), and alcohol intake showed a negative slope for free T4 (p = 0.01) and positive slope for T3 (p = 0.05), with consistent direction of effect for each plant. For total T4 and TSH, correlations across plants and within plants were not significant. A recent cross-sectional analysis of self- reported thyroid disease in the NHANES in relation to either PFOA or PFOS reported a significant association for PFOA in females, and a similar but much less precise associa- tion in males, but no association for PFOS (Melzer et al. 2010). The OR for females of ever having had thyroid disease for the upper quartile of PFOA serum levels was 1.64 (95% CI, 1.09–2.46) compared with the first quartile, and for current thyroid disease plus thyroid hormone medication the equivalent 1104 OR was 1.86 (95% CI, 1.12–3.09). For men the respective findings were 1.58 (95% CI, 0.74–3.39) and 1.89 (95% CI, 0.60–5.90). These findings are striking given the very low exposures compared with the occupational studies, but the study suffers from using self-reported diagnoses, having no thyroid hormone data, having no exposure data at the time of diagnosis, and not distinguishing between hypo- and hyperthyroidism. In addi- tion, there is some concern that the disease or treatment of disease may affect serum levels of PFOA. Overall, the occupational data at high exposure levels show no consistent evidence of a reduction of thyroid hormone. In addition associations between T3 and T4 measured by the more common analog methods need to be treated with some caution because free T4 assay results have been reported to be affected by the presence of PFOS, which can lead to apparent positive associations (Chang et al. 2007). It is not known whether this is a problem with PFOA as well. Two community studies at lower exposure levels did not indicate an effect, but study sizes were small. The NHANES data show a strong effect on self-reported thyroid disease at very low levels, not easily interpreted in the context of the other studies. Sex hormones. Increases in estradiol and decreases in testosterone with PFOA expo- sure have been observed in rodents (Lau et al. 2007). However, there are very limited human data on hormones in relation to serum PFOA, found in three occupational cross- sectional studies. Olsen et al. (1998) studied 191 male workers divided into four exposure groups and measured estradiol, 17-hydroxy- progesterone, prolactin, and bound testos- terone. No significant associations between exposure and these hormones were found, but most of the population were in the two lowest exposure groups (< 1 and < 10 ng/mL), with only five subjects in the > 30 group. Costa et al. (2009) reported a multivariate regres- sion of estradiol and testosterone in relation to PFOA in a group of 56 workers followed in routine occupational surveillance, in which there were no associations of sex hormones with serum PFOA. Finally, Sakr et al. (2007a) found a significant association between serum PFOA and both estradiol and testosterone in men (n = 782) in linear regression models (regression coefficients = 22.3 and 0.6, respec- tively; p-values = 0.017 and 0.034, respec- tively). Apparently there were no significant findings for hormones in the 243 women. Liver function. Studies of the distribution of PFOA in rodents have found relatively high concentrations in the liver. Increased liver weight has been found in rodents and non- human primates exposed experimentally to PFOA (Lau et. at 2007). In rodents, but not necessarily in primates, liver toxicity is related to the PPARα mode of action. However, the nonhuman primate studies and the evidence of toxicity in PPAR-null mice suggest that liver toxicity via other modes of action may be relevant, including other nuclear receptors in the liver (Dewitt et al. 2009; Ren et al. 2009). The animal findings on liver toxicity have prompted a number of studies of liver enzymes in people exposed to PFOA. Emmett et al. (2006b) reported on the association between serum levels and several liver enzymes in serum among 371 commu- nity residents who were exposed to PFOA (median exposure, 354 ng/mL). Enzymes included alkaline phosphatase (ALP), aspar- tate aminotransferase (AST), alanine amino- transferase (also known as serum glutamic pyruvic transaminase), and gamma glutamyl transpeptidase (GGT). No correlations of serum PFOA and enzymes approached statis- tical significance. One of the comparisons for abnormal values (AST; abnormal not defined) was associated with PFOA, but the normal group had higher PFOA, suggesting a protec- tive rather than a harmful effect of PFOA. Costa et al. (2009) reported on a surveil- lance program of workers in a PFOA plant. Bilirubin and four liver enzymes showed no associations in cross-sectional analyses, but in a longitudinal analysis (n = 56) all four enzymes indicated a positive slope of increas- ing enzyme levels with respect to PFOA, three with p-values < 0.05 [alanine transaminase (ALT), GGT, and ALP] and one (AST) not significant. Results are presented in terms of regression coefficients for log-transformed variables, which are sometimes hard to inter- pret in terms of strength of effect. Total bili- rubin fell with increasing PFOA (p < 0.01). Sakr et al. (2007b), in a longitudinal study of liver enzymes in an occupational population (n = 454; mean serum PFOA = 1,130 ng/mL), found significant associations with AST (0.35 units increase per 1,000 ng/mL PFOA; 95% CI, 0.10–0.60). Total bilirubin declined with increasing PFOA (0.008 mg/L decline per 1,000 ng/mL; 95% CI, –0.0139 to –0.0021). In each of these cases, the change was small compared with the mean and standard devia- tion of these clinical markers. Sakr et al. (2007a) also conducted a cross- sectional study at the same plant of PFOA, lipids, and liver enzymes, in 1,025 workers with potential exposure to PFOA. AST, ALT, and total bilirubin were not significantly asso- ciated with the level of PFOA, but GGT was significantly positively associated with PFOA (p = 0.02). Observed increases were no longer significant when the analysis was restricted to those not on lipid-lowering medications. No notable change was found for the log of total bilirubin (Sakr et al. 2007a). Several reports by Olsen and colleagues have addressed the liver enzymes AST, GGT, ovlume 118 | number 8 | August 2010 • Environmental Health Perspectives