Maternal Use of Thyroid Hormones in Pregnancy and Neonatal Outcome

Primary hypothyroidism is not uncommon in women of reproductive age and, without hormone replacement, it may be difficult to conceive a child or to maintain a pregnancy. Whether obstetrical complications are more frequent in women receiving a thyroxine substitute remains uncertain. The study population, based on prospectively collected data, included 9,866 women from the Swedish Health Registers who reported using thyroid hormones early in pregnancy or received a prescription for thyroid hormones later in pregnancy, along with their 10,055 infants. The findings were analyzed by adjusting for identified possible confounding factors. Compared to women in a reference population, those who used thyroxine while pregnant had increased rates of preeclampsia, preexisting or gestational diabetes, cesarean section, and induction of labor. The risk of preterm birth in women taking thyroid hormones also was increased, but only marginally. Seven newborn infants had thyrotoxicosis and one had unspecified thyroid disease; the expected number was 0.2. The risk of congenital malformations was small but nevertheless significant, with an odds ratio of 1.14 and a 95% confidence interval of 1.05-1.26. Maternal use of thyroid hormones was not associated with low birth weight, but it was associated with a marginally increased risk of being large for gestational age. Comparison of women taking thyroid hormones with the reference population disclosed no increased risk of low Apgar scores, hypoglycemia, respiratory diagnoses, jaundice, or central nervous system disorders. Women taking substitute thyroid hormones during pregnancy are at increased risk of some pregnancy complications, but their infants are affected to only a slight degree.