Effects of Maternal Trace Elements Exposure in Early Pregnancy on Maternal Thyroid Hormones and Birth Outcomes

While heavy metals are established contributors to adverse health outcomes, evidence regarding trace element effects on maternal thyroid hormones and birth outcomes remains limited. This prospective study investigated effects of first-trimester exposure to seven trace elements, including vanadium(V), chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), arsenic (As), and Se (selenium), on maternal third-trimester thyroid hormone levels and birth outcomes in Shanghai, China. Among 2069 enrolled pregnant women, 1351 with complete data were analyzed. Linear regression model, restricted cubic spline (RCS), and quantile g computation (QGC) were employed to assess element-outcome relationships. QGC analysis revealed that the increase in the element's mixture was associated with the elevation of total triiodothyronine (TT3) (β = 0.026, 95%CI: 0.004, 0.048) and the infant's birth length (β = 1.120, 95%CI: 1.000, 1.250), with Cr being the primary contributor. Linear regression showed that elevated Cr concentrations were positively associated with the infant's birth length and head circumference, with estimated changes of 0.143 g and 1.61 mm, respectively. Each ln-unit elevated Se and As levels resulting in a 1.19 mm increase in the infant's head circumference and 0.13 mU/L increase in thyroid-stimulating hormone (TSH) concentration, respectively. Additionally, RCS analyses showed U-shaped relationships between As, Cr, and free triiodothyronine (FT3), as well as V and TT3. Our QGC results demonstrated that maternal exposure to element mixtures in early pregnancy was associated with altered thyroid hormones and accelerated fetal growth, predominantly driven by Cr. The U-shaped relationships for V, Cr, and As with maternal TT3/FT3 levels underscore the need for biphasic clinical safety thresholds for prenatal exposure.