Environ Health Perspect

Background Thyroid hormone homeostasis can be disrupted by exposure to ubiquitous and bioaccumulative organochlorines such as polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs). Whereas investigations of health effects have generally focused on human populations with relatively high exposures through occupation, accident, or high fish consumption, general population exposures may also carry risk. Methods We studied associations of total thyroxine (T4) and thyroid-stimulating hormone (TSH) with PCBs, dioxin-like toxic equivalents (TEQs), and p,p′-diphenyldichloroethene (DDE) in adult participants without thyroid disease who participated in the 1999–2002 National Health and Nutrition Examination Survey, a cross-sectional survey examining a random sample representative of the U.S. population. Results We found inverse associations of total T4 with exposure to TEQs in both sexes, with stronger associations in females. In women, mean T4 was 8.2 μg/dL, and levels were on average 0.75 μg/dL lower (95% confidence interval, 0.04–1.46) in women in the highest quintile of TEQ exposure compared with the lowest two quintiles. Effects were stronger in people > 60 years of age, with negative associations of T4 with PCBs and TEQs, and positive associations of TSH with PCBs and TEQs in older women, and a negative association of TSH with PCBs in older men. Conclusions The data show a dose-dependent decrease in total T4 with exposure to TEQs at levels similar to those found in the general U.S. population. The effects were stronger in women. The results suggest that older adults, who have a high risk of thyroid disease, may be more at risk for disruption of thyroid hormone homeostasis by dioxin-like organochlorines than younger adults.

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