Maternal use of thyroid hormones in pregnancy and neonatal outcome

To describe neonatal outcome including the presence of congenital malformations in infants born to women substituted with thyroid hormones, and the maternal characteristics of these women.

Register study based on prospectively collected data in relation to delivery.

Swedish Health Registers.

All pregnant women (n=848,468) and all infants born (n=861,989) in Sweden from 1 July 1995 to 31 December 2004.

Women who reported the use of thyroid hormones in early pregnancy or obtained a prescription for thyroid hormones later in pregnancy (n=9,866), as well as their infants (n=10,055) were identified from the Swedish Medical Birth Register. The reference population consisted of all women giving birth and their offspring during the same time interval.

Neonatal outcome, malformations and maternal characteristics. Data were analyzed with adjustments for identified confounders.

Women using thyroxine had an increased rate of pre-eclampsia, diabetes (pre-existing or gestational), cesarean sections and inductions of labour compared to women in the reference population. The risk for preterm birth was marginally increased (OR 1.13, 95% CI 1.03-1.25). Neonatal thyroid disease was found in eight infants (seven with thyreotoxicosis and one unspecified), the expected number was 0.2. No further anomalies in neonatal diagnoses were found. A small but statistically significant risk for congenital malformations (OR =1.14, 95% CI 1.05-1.26) was found.

Women on thyroid substitution during pregnancy had an increased risk for some pregnancy complications, but their infants were only slightly affected.