Methima Use In Pregnancy & Lactation

Pregnancy: Pregnancy category: D.
Methimazole has been found to readily cross the placenta and may cause fetal harm, particularly when administered in the first trimester of pregnancy. The drug can also cause fetal goiter and hypothyroidism (cretinism) when administered to a pregnant woman. There was a distinct and consistent pattern of congenital malformations associated with the use of methimazole. Approximately 90% of the congenital malformations with methimazole were craniofacial malformations (e.g., scalp epidermal aplasia, facial dysmorphism, choanal atresia). Congenital anomalies, including esophageal atresia, choanal atresia, aplasia cutis, and iridic and retinal coloboma, have been observed in neonates born to mother taking methimazole during pregnancy. Nonteratogenic adverse effects, including fetal and neonatal hypothyroidism, have been observed following maternal methimazole use. Uncontrolled maternal hyperthyroidism may result in adverse neonatal outcomes (e.g. prematurity, low birth weight) and adverse maternal outcomes (e.g. preeclampsia, congestive heart failure, stillbirth, abortion). Antithyroid treatment is recommended for the control of hyperthyroidism during pregnancy. Due to an increased risk of congenital anomalies with methimazole, propylthiouracil is considered first-line therapy, especially during the first trimester. Patients receiving methimazole should be switched to propylthiouracil if pregnancy is confirmed in first trimester. Due to an increased risk of hepatotoxicity, use of methimazole may be preferred during the second and third trimester. If drug therapy is changed, maternal thyroid function should be monitored after 2 weeks and then every 2 to 4 weeks.
Lactation: Methimazole is distributed into milk. However, several studies found no effect on clinical status in nursing infants of women receiving methimazole. The American Thyroid Association considers doses of methimazole less than 30 mg/day to be safe during breast-feeding. Methimazole should be administered after breast-feeding and in divided doses.