Maternal Thyroid Dysfunction during Gestation, Preterm Delivery, and Birthweight. The Infancia y Medio Ambiente Cohort, Spain

Background Maternal clinical thyroid disorderscan cause reproductive complications. However, the effects of mild thyroid dysfunctions are not yet well established. The aim was to evaluate the association of maternal thyroid function during the first half of pregnancy with birthweight and preterm delivery. Methods We analysed data on 2170 pregnant women and their children from a prospective population-based cohort study in four Spanish areas. Mid-gestation maternal serum and urine samples were gathered to determine thyroid-stimulating hormone(TSH), free thyroxine (fT4), and urinary iodine concentration (UIC). Thyroid status was defined according to percentiledistribution as: euthyroid(TSH and fT4 >5th and 95th percentileand fT4 normal or  95th percentileand TSH normal), and hyperthyroidism (TSH  95th percentile). Response variables were birthweight, small and large for gestational age (SGA/LGA), and preterm delivery. Results An inverse association of fT4 and TSH with birthweight was found, the former remaining when restricted to euthyroidwomen. High fT4 levels were also associated with an increased risk of SGA [odds ratio, 95% confidence interval (CI) 1.28 (95% CI 1.08, 1.51)]. Mean birthweight was higher in the hypothyroxinaemic group (β = 109, P < 0.01). Iodine intake and UIC were not associated with birth outcomes. Conclusions High maternal fT4 levels during the first half of pregnancy were related to lower birthweight and increased risk of SGA newborns, suggesting that maternal thyroid function may affect fetal growth, even within the normal range.