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

BackgroundMaternal clinical thyroid disorders can 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. MethodsWe 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 (fT(4)), and urinary iodine concentration (UIC). Thyroid status was defined according to percentile distribution as: euthyroid (TSH and fT(4) >5th and <95th percentiles); hypothyroxinaemia (fT(4)<5th percentile and TSH normal), hypothyroidism (TSH > 95th percentile and fT(4) normal or <5th percentile), hyperthyroxinaemia (fT(4)>95th percentile and TSH normal), and hyperthyroidism (TSH<5th percentile and fT(4) normal or >95th percentile). Response variables were birthweight, small and large for gestational age (SGA/LGA), and preterm delivery. ResultsAn inverse association of fT(4) and TSH with birthweight was found, the former remaining when restricted to euthyroid women. High fT(4) 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 (=109, P<0.01). Iodine intake and UIC were not associated with birth outcomes. ConclusionsHigh maternal fT(4) 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.