In this study we aimed to determine whether increased serum PFOA or PFOS concentrations were associated with thyroid disease in a general adult U.S. population sample. The prevalence of thyroid disease is markedly higher in women than in men, so we estimated sex-specific associations. We found that, across all the available data from NHANES, thyroid disease associations with serum PFOA concentrations are present in women and are strongest for those currently being treated for thyroid disease. In men, we also found a near significant association between PFOA and treated thyroid disease. An interaction term analysis suggests that the PFOA trends in men and women are not significantly different, despite the relative rarity of thyroid disease in men. In addition, we found a nominally significant association between PFOS concentrations and treated thyroid disease in men but not in women.
The presence of associations with both PFOA and PFOS raises the issue of how best to perform risk assessments for combinations of perfluorochemicals. The somewhat divergent risk patterns for the two compounds support their separate risk assessment (Scialli et al. 2007
), given that current legislative advice (Minnesota Department of Health 2008
) is to consider the combined effects of chemicals only when two or more chemicals in a mixture affect the same tissue, organ, or organ system.
Our results are important because PFAAs are detectable in virtually everyone in society (Kannan et al. 2004
), with ubiquitous presence across global populations (Calafat et al. 2006
). Occupational exposure to PFOA reported in 2003 showed mean serum values of 1,780 ng/mL (range, 40–10,060 ng/mL) (Olsen et al. 2003a
) and 899 ng/mL (range, 722–1,120 ng/mL) (Olsen et al. 2003c
). Production of PFOS was halted in 2002 in the United States by its principal producer, due largely to concerns over bioaccumulation and toxicity. Since then, voluntary industry reductions in production and use of other perfluorinated compounds, such as the U.S. EPA–initiated PFOA Stewardship Program (U.S. EPA 2006
), have contributed to a decreasing trend in human exposure for all perfluorinated compounds (with the notable exception of perfluorononanoic acid) (Calafat et al. 2007
; Olsen et al. 2007
). In May 2009, PFOS was listed under the Stockholm Convention on Persistent Organic Pollutants (2008)
Our results can be compared with previous studies of human populations and of nonhuman primates. A 6-month study of cynomolgus monkeys chronically exposed to PFOA showed no associations between PFOA and thyroid parameters, at mean serum PFOA concentrations higher than those reported in NHANES, although only male monkeys were involved (Butenhoff et al. 2002
). The largest human study of PFOA centers on an industrial facility in Washington, West Virginia, from which PFOA spread to the population through air, water, occupational, and domestic exposure in a point-source contamination. The C8 Health Project (Steenland et al. 2009
) has measured PFOA concentrations in > 69,000 residents. Markedly high concentrations were found, with an arithmetic mean of 83 ng/mL and a median concentration in serum of 28 ng/mL (C8 Science Panel 2008
), far higher than the NHANES concentrations in the general population. Preliminary analyses report associations between PFOA and total cholesterol, low-density lipoproteins, and triglyceride concentrations in multivariate models adjusting for age, BMI, sex, education, smoking, alcohol, and regular exercise. Comprehensive cross-sectional and follow-on analyses of associations with thyroid disease have not yet been reported but are expected to be released in 2010–2011 (C8 Science Panel 2009
Importantly, disruption to thyroid hormone balance was not found in other studies of populations exposed to PFOA, despite the considerably higher levels reported in some studies (Emmett et al. 2006
; Olsen et al. 2003b
). Emmett et al. (2006)
studied 371 residents of a community with long-standing environmental exposure to PFOA and found a median serum PFOA concentration of 181–571 ng/mL but no association between serum PFOA and a history of thyroid disease. A study that included thyroid hormone levels reported a positive association between serum PFOA concentration and T3
levels in occupationally exposed workers, although there were no changes in other thyroid hormones (Olsen et al. 2001
). Modest associations between PFOA and thyroid hormones (negative for free T4
and positive for T3
) were reported in 506 PFOA production workers across three production facilities (Olsen and Zobel 2007
); there were no associations between TSH or T4
and PFOA, and the free hormone levels were within the normal reference range.
A linear extrapolation of the findings reported here would be expected to lead to associations being more evident at higher exposure levels, yet this is not supported by the literature. Nonlinearity of response is not uncommon for receptor-mediated systems such as endocrine-signaling pathways that act to amplify the original signal. Large changes in cell function can occur in response to extremely low concentrations, but which may become saturated and hence unresponsive at higher concentrations (vom Saal and Hughes 2005
; Welshons et al. 2003
The mechanisms involved in thyroid homeostasis are numerous and complex, and there are multiple potential targets for disruption of thyroid hormone homeostasis (Schmutzler et al. 2007
). These include thyrotropin receptor (Santini et al. 2003
), iodine uptake by the sodium iodide transporter (Schröder van der Elst et al. 2003
), type 1 5′-deiodinase (Ferreira et al. 2002
), transthyretin (Kohrle et al. 1988
), thyroid hormone receptor (Moriyama et al. 2002
), and the thyroid hormone–dependent growth of pituitary cells (Ghisari and Bonefeld-Jorgensen 2005
). Depression of serum T4
has been reported by several authors in PFOS-exposed rats (Lau et al. 2003
; Luebker et al. 2005
; Seacat et al. 2003
). One mechanism by which PFAAs may deplete T4
is through induction of the hepatic uridine diphosphoglucuronosyl transferase (UGT) system, which is involved in hepatic metabolism of thyroid hormone and biliary clearance of T4
-glucuronide (Barter and Klaassen 1994
). Because PFOA is an agonist for PPARα, it is plausible that induction of hepatic UGT in PFAA-exposed rats (Yu et al. 2009
) could represent a PPARα-mediated response. The involvement of another PPARα agonist, WY 14643, in enhancing the hepatic degradation of thyroid hormone has recently been shown (Weineke et al. 2009
A growing body of data describes the in vitro
binding affinity of PFOA to human serum-binding proteins (Chen and Guo 2009
) and to PPARα, -β, and -γ and other nuclear receptors (Vanden Huevel et al. 2006
), but the contribution of these mechanisms to PFOA’s thyroid-mediating effects in humans remains to be established. Many cellular and metabolic processes, including lipid metabolism, energy homeostasis, and cell differentiation, are controlled by PPARα. Early studies of the effects of PFAAs in rodents showed that a single dose lowered heart rate and body temperature and depressed T4
. Replacement of T4
did not reverse the clinical symptoms of hypothermia (Gutshall et al. 1988
; Langley and Pilcher 1985
). Although circulating thyroid hormone levels were low, liver enzymes responsive to thyroid hormone levels were elevated, suggesting that thyroidal homeostasis was not functionally compromised. Chang et al. (2007)
found that exposure to PFOS for up to 3 weeks did not affect functional thyroid status, because free T4
, TSH, and various thyroid-responsive liver enzymes were all unaffected. These findings and later results have led to proposals that displacement of circulating thyroid hormones from plasma protein-binding sites and a reduced responsiveness of the HPT axis contribute significantly towards PFOA’s hypothyroid-inducing effects (Lau et al. 2007
). Whatever the mechanisms involved, it is clear that more research is merited to clarify the pathways involved.
The feedback mechanism by which the rate of release of TSH and the circulating levels of T3
are regulated tends to show a low level of individual variation (Felt-Rasmussen et al. 1980
). Therefore, subtle disruption of the HPT axis within normal reference ranges may have negative health consequences for the individual while remaining within normal reference values, highlighting the importance of including both clinical and laboratory end points in such studies. The NHANES data do not allow specification of the precise type of thyroid disease present, because NHANES does not report on individual hormone levels. PFOA concentration was positively associated with free T4
and negatively associated with T3
levels in a cohort of 506 exposed workers, with a near significant association with TSH levels (Olsen et al. 2007
), although all effects were regarded as modest.
The limitations of these analyses should be noted. We based the PFOA and PFOS measures on a single serum sample. Although PFOA has a half-life of 4 years (Olsen et al. 2007
), so a single sample is likely to represent medium-term internal dose, samples taken at several time points might be more accurate in classifying exposure. Any misclassification from single measures would tend to decrease power and underestimate the real strengths of association. Second, the PFOA concentrations were measured at the same time as disease status, making attribution of causal direction difficult. This raises the possibility of reverse causation. One might hypothesize that after onset of thyroid disease, changes in the nature of exposure or in the pharmacokinetics of PFOA might occur [including patterns of absorption, distribution (including protein binding) or excretion]. Because the associations we report were present in people who were on thyroid hormone replacements, which effectively mimic normal thyroid function, a mechanism for reverse causation through changes in pharmacokinetics is difficult to imagine. Confounding by unmeasured factors is also possible, but it is unlikely that confounding could explain similar findings reported from some of the diverse experimental and observational studies discussed above.
Post hoc association testing with other common diseases (necessarily involving multiple statistical testing) did not identify other robust associations of higher PFC concentration with increased disease prevalence, suggesting specificity of our findings for thyroid disease. An apparent association between higher PFOS concentrations and lower prevalence of COPD requires replication, to exclude a false-positive result from multiple testing.
In addition to the limitations of our analyses, the strengths should also be noted: This is the first large-scale analysis in a nationally representative general adult population of directly measured serum concentrations of PFOA and PFOS. In addition, the associations present are strongest for the most specific identification of thyroid disease, based on reported diagnosis with current use of thyroid-specific medication. The NHANES study also supported adjustment of models for a range of potential confounding factors, which in fact made relatively minor differences to the key estimates, suggesting that the associations are robust.
Further work is clearly needed to characterize the PFOA and PFOS associations with specific thyroid diagnoses and thyroid hormone levels in the general population and to clarify whether the associations reflect pathology, changes in exposure, or altered pharmacokinetics. Longitudinal analyses are also needed to establish whether high exposures predict future onsets of thyroid disease, although concurrent alteration of thyroid functioning would still be a cause for concern.