Air pollution exposure in association with maternal thyroid function during early pregnancy.

Association of prenatal air pollution exposure with maternal thyroid hormone (TH) levels remains unclear, especially during early pregnancy when even small changes in maternal TH could affect fetal neurodevelopment. We examined the effect of air pollution exposure on maternal TH levels in the second trimester of pregnancy. Serum concentrations of free thyroxine (FT4) and thyroid-stimulating hormone (TSH) in 8077 pregnant women were measured by fluorescence and chemiluminescence immunoassays. Prenatal exposure to fine particulate matter (PM2.5) and nitrogen dioxide (NO2) was estimated using land use regression models. FT4 levels were significantly inversely associated with both PM2.5 and NO2 exposure. A 10 μg/m3 increase in NO2 exposure in first trimester and PM2.5 exposure in second trimester was associated with 0.61% (95% CI, -0.88% to -0.35%) and 0.73% (95% CI, -1.25% to -0.20%) decrease in FT4 levels, respectively. PM2.5 exposure was also associated with elevated odds of maternal hypothyroxinemia. A 10 μg/m3 increase in PM2.5 exposure in both first and second trimester was associated with 28% (OR = 1.28, 95% CI, 1.05-1.57) and 23% (OR = 1.23, 95% CI, 1.00-1.51) increase in the odds of maternal hypothyroxinemia, respectively. Our findings suggest that air pollution may interfere with maternal thyroid function during early pregnancy.