Maternal exposure to essential and non-essential metals and thyroid hormone status in a Canadian pregnancy cohort

This study examined associations between maternal exposure to essential and non-essential metals and thyroid hormone status during pregnancy. Red blood cell concentrations of essential metals (i.e., copper, magnesium, manganese, molybdenum, selenium, zinc) and non-essential metals (i.e., arsenic, antimony, cadmium, lead, mercury) were measured in 436 pregnant individuals enrolled in the Alberta Pregnancy Outcomes and Nutrition (APrON) study. Maternal plasma concentrations of free triiodothyronine (FT3), free thyroxine (FT4), and thyroid stimulating hormone (TSH) were quantified and the FT3:FT4 ratio and thyrotropin T4 resistance index (TT4RI) were calculated. In single metal models, FT3 was negatively associated with lead and mercury, but positively associated with manganese, magnesium, and selenium. FT4 and TSH were negatively associated with magnesium and zinc. The FT3:FT4 ratio was negatively associated with lead, but positively associated with manganese, magnesium, and selenium, while TT4RI was negatively associated with magnesium and selenium. Bayesian kernel machine regression mixture models identified magnesium and selenium as the most influential metals for FT3, FT4, TSH, FT3:FT4 ratio and TT4RI, whereas mercury was the most important contributor to FT4 and the FT3:FT4 ratio. These findings suggest that essential and non-essential metals may influence thyroid function through distinct biological mechanisms, with essential metals potentially playing regulatory roles and non-essential metals exerting disruptive toxic effects.