Sex-specific associations of blood mercury and iodine with thyroid hormones

Abstract

TPS 631: Metals and health 1, Johan Friso Foyer, Floor 1, August 27, 2019, 3:00 PM - 4:30 PM Thyroid hormones play an essential role in regulating many functions, including metabolism and cardiac function. Disruption of thyroid homeostasis may result in clinical or subclinical disease. The impact of low and moderate levels of mercury exposure on thyroid homeostasis in adults is controversial, and most prior investigations have not examined sex differences. A sufficient supply of iodide is required for thyroid hormone synthesis, but mercury can accumulate in the thyroid and reduce iodide uptake by binding to iodide. Thus, iodide levels may modify associations of mercury with thyroid hormones. A cross-sectional study with 3,369 participants, excluding those taking thyroid medications, in the National Health and Nutrition Examination Study 2009-2012 was conducted to investigate associations of total blood mercury with thyroid hormones - triiodothyronine (T3), total thyroxine (T4), free forms of T3 and T4 (FT3 and FT4), and thyroid-stimulating hormone (TSH). Blood mercury and urinary iodide were examined simultaneously using multivariable linear regression models stratified by sex, and interaction effects between mercury and iodine deficiency (<150 µg/L urine, the WHO recommendation) were investigated. The geometric means of blood mercury and creatinine-adjusted urinary iodine were 0.96 ug/L and 144.40 ug/L, respectively, and 59% of males and 50% of females had urinary iodide <150 µg/L. Associations between mercury and thyroid hormones varied by sex; significant inverse associations of mercury with and T3 and T4 were observed in females, but thyroid parameters were not associated with mercury in males. Negative associations between iodine and T3 and FT3 were detected in males and females. Interactions between mercury and iodide deficiency on thyroid hormones were not significant (P>0.05) in either sex. In conclusion the impacts of mercury and iodine on thyroid hormones were independent in this study. These findings illustrate the importance of conducting sex-stratified analyses in investigations of environmental exposures on thyroid hormone disruption.