Associations of metal mixtures with thyroid function and potential interactions with iodine status: results from a cross-sectional study in MEWHC.

Few studies are available on associations between metal mixture exposures and disrupted thyroid hormone homeostasis; particularly, the role of iodine status was ignored. Here, we aimed to explore the cross-sectional relationship of blood cell metals with thyroid homeostasis and explore the potential modifying effect of iodine status. Among 328 workers from the manganese-exposed workers healthy cohort (MEWHC), we detected thyroid function parameters: thyroid stimulating hormone (TSH), total triiodothyronine (TT3), free triiodothyronine (FT3), total tetraiodothyronine (TT4), free tetraiodothyronine (FT4) as well as calculated sum activity of peripheral deiodinases (G) and thyroid's secretory capacity (G). Inductively coupled plasma mass spectrometry (ICP-MS) was used to measure 22 metal concentrations in blood cells. Based on the consistent results of least absolute shrinkage and selection operator (LASSO) and Bayesian kernel machine regression (BKMR) analyses, there were significant positive associations between copper and TSH (β = 2.016), iron and FT4 (β = 0.403), titanium and G (β = 0.142), nickel and G (β = 0.057), and negative associations between copper and FT4 (β =  - 0.226), selenium and G (β =  - 0.332), among the participants. Interestingly, we observed an inverted-U shape relationship between magnesium and FT4. Furthermore, we found a synergistic effect between arsenic and copper on the TSH level, while antagonistic effects between nickel and copper as well as nickel and selenium on the TSH level. We observed a modified effect of iodine status on association between strontium and G (P = 0.026). It suggests metal mixture exposures can alter thyroid homeostasis among the occupational population, and deiodinase activity had a modified effect on association between strontium and G. Validation of these associations and elucidation of underlying mechanisms require further researches in the future.