Risk of adverse perinatal outcomes with antithyroid treatment during pregnancy: a nationwide population-based study

Objective To compare, using two large nationwide population-based data sets, the risk of adverse pregnancy outcomes (low birthweight [LBW], preterm birth, small for gestational age [SGA] and congenital anomalies) among pregnant women with hyperthyroidism classified into three groups: receiving propylthiouracil (PTU) treatment during pregnancy, receiving methimazole/carbimazole (MMI) treatment, and no antithyroid treatment during pregnancy. Design A matched case-control study. Setting Taiwan. Sample A total of 2830 mothers with hyperthyroidism and 14 150 age-matched randomly selected mothers without hyperthyroidism were included. Methods Conditional logistic regression analyses were performed to examine the risk of adverse pregnancy outcomes (LBW, preterm birth, SGA and major congenital anomalies) among these three groups. Main outcome measures LBW, preterm birth, SGA and major congenital anomalies. Results Women receiving PTU treatment during pregnancy had a higher risk of giving birth to LBW infants than those not receiving antithyroid treatment (odds ratio = 1.40; 95% CI 1.00-1.96), after adjusting for maternal education, anaemia, hyperlipidaemia, pregestational diabetes, pregestational hypertension, hyperemesis gravidarum and infant's gender and birth order. However, children of women receiving MMI treatment did not have increased risks of any adverse fetal outcome relative to mothers not receiving antithyroid treatment. Conclusions Our study finds an increased risk of LBW among babies of mothers with hyperthyroidism receiving PTU treatment during pregnancy relative to untreated mothers with hyperthyroidism.