Manganese and thyroid function in the national health and nutrition examination survey, 2011–2012

Manganese (Mn) exposure is prevalent, as it is found naturally as ionized trace elements and released into the environment as a byproduct of manufacturing and waste disposal. Animal and human studies have suggested variable effects on thyroid function, but the association of Mn exposure with thyroid function has not been evaluated in a national sample. To investigate the associations between serum and urinary Mn levels and serum thyroid hormone concentrations in a nationally representative sample. This was a cross-sectional analysis of data from the 2011–2012 National Health and Nutrition Examination Survey among 1360 participants. Serum thyroid stimulating hormone (TSH), total triiodothyronine (T3), total thyroxine (T4), free T3, and free T4. Serum Mn levels were positively associated with increasing total T4, free T3, and total T3 in the whole cohort (p < 0.01). Urinary Mn levels were not associated with thyroid hormone levels. When subgroup analyses were performed by gender, only males had total T4 associated with serum Mn [β = 0.01, p < 0.01, confidence interval (CI): 0.004–0.018]. In individuals under 22 years old, serum Mn was significantly associated with total T4 (β = 0.02, p = 0.002, CI: 0.008–0.029). Serum Mn was positively associated with Free T3 in both genders (β = 0.07, p < 0.001). While our findings do not suggest clinical thyroid dysfunction, there is an association between serum Mn and subclinical changes in thyroid function that warrant further studies. Regulatory action should be considered as Mn-based organometallic compounds are being considered as replacements for lead in gasoline and may pose future risks to human health.