Metals in blood and urine, and thyroid function among adults in the United States 2007-2008

The thyroid is integral to regulation of development and metabolism. Certain metals have been shown to affect thyroid function in occupationally exposed persons, but few studies have been conducted in the general population. This study evaluates the association between biomarkers of metal exposure and thyroid hormones in the US population. Analyses included adults participating in the 2007-2008 National Health and Nutrition Examination Survey, with no history of thyroid disease or use of thyroid medications, and with data on metals in blood (lead, cadmium and mercury) and urine (lead, cadmium, mercury, barium, cobalt, cesium, molybdenum, antimony, thallium, tungsten and uranium), and thyroid hormones (TSH, free and total T3 and T4) in serum (N=1587). Multivariate linear regression was used to model the association between thyroid hormone levels, and metals in either urine (creatinine-adjusted) or blood. Metal concentrations were considered as both continuous and categorical variables. Models were adjusted for: age, sex, race, BMI, serum lipids, serum cotinine, pregnancy and menopausal status, and use of selected medications. Few participants (<5%) had free T3, free T4, or TSH levels outside the reference range. However, 9.2% (SE=1.2%) had low T3 and 9.4% (SE=1.1%) had low T4. Metals were detected in nearly all blood and urine samples, with the highest levels seen for urinary molybdenum (median 42.5μg/L). When including all blood metals, mercury was associated with decreases in T3 and T4, while cadmium was associated with decreased TSH. Urinary cadmium was associated with increases in both T3 and T4 (models including all metals measured in urine). Urinary thallium and barium were associated with decreased T4 (both) and T3 (barium). For TSH, cesium was associated with decreased, and tungsten with increased levels. Given the high prevalence of exposure to metals, associations of the size reported here could indicate an appreciable contribution of metals exposure to the prevalence of thyroid disorders. These findings indicate the importance of further research to further examine these relationships.