Evaluation the Safety of Methimazole and Propylthiouracil in Pregnancy and Lactation

OBJECTIVE To investigate the safety of methimazole(MMI) and propylthiouracil(PTU) in pregnancy and lactation. METHODS The literatures were reviewed and summarized based on the large-scale representative studies and guidelines in recent years. RESULTS Both MMI/carbimazole(CMZ) and PTU or shift between MMI/CMZ and PTU in early pregnancy were associated with an increased ration of birth defects. The prevalence of birth defects was high in children before 2 years old (PTU, 8.0%; MMI/CMZ, 9.1%; MMI/CMZ and PTU, 10.1%; nonexposed, 5.7%). MMI/CMZ and PTU were associated with urinary system malformation, and PTU with malformations in the face and neck region. Choanal atresia, esophageal atresia, omphalocele, omphalomesenteric duct anomalies, and aplasia cutis were common in MMI/CMZ-exposed children. The relative high risk is confined to gestational weeks 6-10. MMI at doses up to 20-30 mg·d^-1 was safe for lactating mothers and their infants. PTU at doses up to 300 mg·d^-1 was a second-line agent due to concerns about severe hepato-toxicity. CONCLUSION Both MMI/CMZ and PTU are associated with birth defects, but the spectrum of malformations is different. Antithyroid drugs should be administrated following a feeding and in divided doses.