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

Abstract

Background: Maternal clinical thyroid disorders can cause reproductive complications. However, the effects ofmild thyroid dysfunctions are not yet well established. The aim was to evaluate the association of maternal thyroidfunction 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-basedcohort study in four Spanish areas. Mid-gestation maternal serum and urine samples were gathered to determinethyroid-stimulating hormone (TSH), free thyroxine (fT4), and urinary iodine concentration (UIC). Thyroid statuswas defined according to percentile distribution as: euthyroid (TSH and fT4>5th and <95th percentiles);hypothyroxinaemia (fT4< 5th percentile and TSH normal), hypothyroidism (TSH > 95th percentile and fT4normalor <5th percentile), hyperthyroxinaemia (fT4> 95th percentile and TSH normal), and hyperthyroidism (TSH < 5thpercentile and fT4normal or >95th percentile). Response variables were birthweight, small and large for gesta-tional age (SGA/LGA), and preterm delivery.Results: An inverse association of fT4and TSH with birthweight was found, the former remaining when restrictedto euthyroid women. High fT4levels were also associated with an increased risk of SGA [odds ratio, 95% confi-dence 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 fT4levels during the first half of pregnancy were related to lower birthweight andincreased risk of SGA newborns, suggesting that maternal thyroid function may affect fetal growth, even withinthe normal range.