Maternal use of thyroid hormones in pregnancy and neonatal outcome.

OBJECTIVE: 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.

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

SETTING: Swedish Health Registers.

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

METHODS: 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.

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

RESULTS: 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.

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