Background: Perchlorate, thiocyanate, and nitrate are ubiquitous environmental contaminants known to disrupt thyroid homeostasis by inhibiting iodide uptake into the thyroid gland. However, evidence linking prenatal exposure to these chemicals with alterations in maternal thyroid function remains limited and inconsistent. This study investigated these associations and evaluated potential effect modification by maternal iodine status.

Methods: We included 192 pregnant women (2007–2020) from MARBLES (Markers of Autism Risk in Babies; Learning Early Signs), a prospective pregnancy study enriched for familial autism risk, who provided repeated urine and serum samples during pregnancy. Urinary perchlorate, thiocyanate, and nitrate concentrations were measured and adjusted for creatinine. Serum total triiodothyronine, total thyroxine (TT4), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) levels were quantified. Single-pollutant associations were estimated using generalized estimating equation models and linear quantile mixed models. Mixture effects were evaluated using Bayesian kernel machine regression. Analyses were stratified by maternal iodine status (urinary iodine concentration [UIC] <150 vs. ≥150 µg/L).

Results: In the full sample, higher urinary thiocyanate concentrations were associated with lower FT4 levels (β = -0.17, 95% CI: -0.30, -0.04), and no overall mixture effects were observed. Among women with UIC <150 µg/L, associations were stronger: higher thiocyanate was associated with lower FT4, higher nitrate was linked to higher TT4 and lower TSH, and mixture exposure showed a decreasing trend in FT4. No statistically significant associations were observed among women with UIC ³150 µg/L.

Conclusions: These findings suggest that prenatal exposure to thiocyanate and nitrate may be associated with alterations in maternal thyroid function, particularly among women with insufficient iodine.