Boxed Warning: PTU Preferred for Patients in Early Pregnancy


Severe liver injuries have been associated with use of the antithyroid drug propylthiouracil, and the Food and Drug Administration has added a boxed warning to the product's label conveying this risk, the agency announced.

The warning for propylthiouracil (PTU) says that there have been reports of severe liver injury and acute liver failure—including fatalities—in adults and children who've been treated with the drug.

The warning also includes a statement concerning preferential prescribing of the drug for patients in early pregnancy. The warning notes that because birth defects have been associated with use of the antithyroid drug methimazole during the first trimester, “propylthiouracil may be the treatment of choice during and just before the first trimester of pregnancy.”

Information about PTU use during early pregnancy was based on a review of postmarketing data on PTU and methimazole. The review indicated that reports of congenital malformations were about threefold greater with methimazole than PTU, and there was a “distinct and consistent” pattern of congenital malformations associated with methimazole but not PTU.

Serious adverse events associated with PTU should be reported to the FDA at

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