Associations of maternal urinary perchlorate, thiocyanate, and nitrate with maternal thyroid function during...

Perchlorate, thiocyanate, and nitrate are ubiquitous environmental contaminants that disrupt thyroid homeostasis primarily through inhibition of iodide uptake into the thyroid. Evidence linking prenatal exposure to these chemicals with maternal thyroid function remains limited and inconsistent.

We evaluated these associations in 192 pregnant women from the MARBLES cohort (2006-2020) who contributed repeated urine and serum samples. Urinary perchlorate, thiocyanate, nitrate, and iodide concentrations were quantified, and serum total triiodothyronine (TT3), total thyroxine (TT4), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) levels were assessed. The thyroid feedback quantile-based index (TFQI) was calculated from FT4 and TSH values. Single-pollutant associations were estimated using generalized estimating equation models, and mixture effects were evaluated using Bayesian kernel machine regression. Analyses were stratified by maternal urinary iodine concentration (UIC) group.

In the overall population, higher urinary thiocyanate concentrations were associated with lower FT4 levels (β = -0.17; 95% CI: -0.30, -0.04), with stronger effects observed at lower FT4 levels. Higher nitrate concentrations were associated with higher TT4 levels. Associations were generally stronger among women with UIC <150 μg/L. In this subgroup, higher thiocyanate was additionally associated with lower TFQI, while higher nitrate was associated with lower TSH levels. Mixture exposure was also associated with a decreasing trend in TFQI among women with UIC <150 μg/L and an increasing trend among women with UIC ≥150 μg/L.

These findings suggest that prenatal individual and mixture exposure to perchlorate, thiocyanate, and nitrate may alter maternal thyroid function, particularly among women with UIC <150 μg/L.