Methimazole (Tapazole) During Pregnancy: First Trimester Risk

What does the evidence say about methimazole during pregnancy?

1. MotherToBaby describes methimazole exposure in pregnancy and advises discussing medication changes with healthcare providers rather than stopping abruptly. 1
2. The StatPearls methimazole chapter notes rare serious adverse effects, placental transfer, and pregnancy-specific concern about first-trimester exposure and fetal effects. 2
3. A meta-analysis of 13 observational articles found congenital anomalies were more frequent with methimazole than propylthiouracil exposure (reported odds ratio 0.80 for propylthiouracil versus methimazole); miscarriage and liver-toxicity comparisons were not statistically different, and switching did not clearly improve anomaly risk versus propylthiouracil alone. 3
4. ACOG explains that untreated thyroid disease can pose serious risks to a pregnant person and fetus and that most pregnant patients with thyroid disease can have healthy babies with appropriate care. 4
5. The American Thyroid Association guideline states that propylthiouracil is generally favored in the first trimester and methimazole may be resumed later to reduce liver toxicity concerns, with the lowest effective dose and frequent testing. 5

Is methimazole safe in each trimester?

• First trimester. Methimazole exposure in early pregnancy is associated with a characteristic defect pattern in human reports. Contact the prescriber promptly because propylthiouracil is generally favored when an antithyroid medicine is needed during the first trimester.
• Second trimester. Guidelines commonly consider switching from propylthiouracil to methimazole after the first trimester to reduce maternal liver risk. The dose should remain the lowest effective dose with regular thyroid testing.
• Third trimester. Continue coordinated thyroid monitoring. Excess antithyroid medicine can affect fetal thyroid function, while poor control of maternal hyperthyroidism also carries serious risks.