Association of exposure to ambient particulate matter with maternal thyroid function in early pregnancy

Background: It is known that maternal thyroid dysfunction during early pregnancy can cause adverse pregnancy complications and birth outcomes. This study was designed to examine the association between ambient par-ticulate matter with aerodynamic diameters <= 2.5 mu m (PM2.5) and particulate matter with aerodynamic diameters <= 10 mu m (PM10) exposure and maternal thyroid function during early pregnancy.Methods: This study was based on data from a birth cohort study of 921 pregnant women in China. We estimated associations between ambient PM2.5 and PM10 exposure during the first trimester of pregnancy (estimated with land-use regression models) and maternal thyroid hormone concentrations (free thyroxine (FT4), free tri-iodothyronine (FT3), and thyroid-stimulating hormone (TSH)) collected between weeks 10 and 17 of gesta-tion using linear regression models adjusting for potential confounders. Ambient PM2.5 and PM10 concentrations were modeled per interquartile range (IQR) increment and as tertiles based on the distribution of the exposure levels. Results: An IQR increment (68 mu g/m3) in PM2.5 exposure was associated with a significant decrease in maternal FT4 levels (8 =-0.60, 95% CI:-1.07,-0.12); and a significant decrease in FT4/FT3 ratio (8 =-0.13, 95% CI:-0.25,-0.02). Further analyses showed that, relative to the lowest tertile, women in both the middle and highest tertiles of PM2.5 had significantly lower concentrations of maternal FT4 and FT4/FT3 ratio. No significant associations were found between PM2.5 and FT3 or TSH levels. PM10 exposure was not significantly associated with maternal thyroid function. Conclusions: Our study suggested that higher ambient PM2.5, not PM10, exposed during the first trimester of pregnancy were associated with a significant decrease in maternal serum FT4 concentrations and FT4/FT3 ratio. Studies in populations with different exposure levels are needed to replicate our study results.