Methimazole Pregnancy and Breastfeeding Warnings
Methimazole Pregnancy Warnings
Methimazole has been assigned to pregnancy category D by the FDA. Methimazole readily crosses the placental membranes and can induce goiter and cretinism in the developing fetus. In addition, rare instances of congenital defects: aplasia cutitis, as manifested by scalp defects; esophageal atresia with tracheoesophageal fistula; and choanal atresia with absent/hypoplastic nipples, have occurred in infants born to mothers receiving methimazole therapy during pregnancy. The risk is especially high during the first trimester (during organogenesis). The manufacturer recommends using the lowest dose possible if methimazole is used during pregnancy. If the patient becomes pregnant while taking the drug, the patient should be warned of the potential hazard to the fetus. Propylthiouracil is typically the preferred agent in pregnancy as is crosses the placenta to a lesser degree.
Methimazole Breastfeeding Warnings
Methimazole is excreted into human milk. Studies found no adverse effects on nursing infants of mothers taking methimazole, especially if thyroid function is monitored at weekly or biweekly intervals.
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