Associations Between Thyroid Hormone and Iodine Levels and Biomarkers of Exposure to Environmental Chemicals in the U.S.

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P-136 Introduction: Exposure to environmental chemicals has been associated with a range of adverse health effects, including effects on thyroid hormone levels in animals and humans. PCBs, dioxins, and organochlorine pesticides are among the chemicals associated with hormonal effects, including changes in the levels of thyroxin, thyroid stimulating hormone, and iodine. Thyroid hormone is critical for normal growth and development, and alterations in thyroid hormone levels may have significant health implications. We analyzed data from a statistical sample of approximately 4,500 persons in the United States to study the associations between exposure to PCBs and organochlorine compounds, and levels of serum thyroxin and thyroid stimulating hormone, and urinary iodine in the U.S. population and selected subpopulations defined by demographic characteristics, socioeconomic status, and geographic region. Methods: Between 1999 and 2002, the U.S. National Health and Nutrition Examination Survey (NHANES) collected data on the levels of environmental contaminants in blood and on the levels of serum thyroxin and thyroid stimulating hormone in the general U.S. population. Urinary iodine was collected in 2001–2002. We statistically analyzed these data to study the distributions of the chemicals and hormones across the U.S. subpopulations and to examine the correlations between exposure to the environmental chemicals and the hormone levels. Results: The geometric mean thyroxin level in the U.S. is 7.64 μg/dL (95% confidence limits 7.49–7.80). As expected, women have significantly higher levels than men. The total concentration of PCBs 138, 153, and 180 in blood varied signficantly with age, race, and country of origin, but not with gender or income. For example, the geometric mean concentration among the U.S. born was 73.8 ng/g (69.2–78.6) while it was 38.1 ng/g (33.2–43.6) among those born in Mexico. Similar results were obtained for other PCBs and organochlorine pesticides. Pearson and Spearman correlations between the environmental chemical levels and thyroid hormone levels tended to be low in magnitude, although many were statistically significant. Discussion and Conclusions: While most environmental chemicals compounds were observed at relatively low levels, resulting in limited associations with the hormone levels, there were specific compounds and subpopulations that exhibited higher levels and stronger associations. These included PCBs and metals in older adults, and PCBs in those classified as “other race” (Asians, Native Americans, Pacific Islanders). The findings and conclusions in this presentation have not been formally disseminated by the U.S. Environmental Protection Agency and should not be construed to represent any Agency determination of policy.