FDA Issues Warning About Thyroid Drug Propylthiouracil / PTU Used to Treat Hyperthyroidism

By Mary Shomon, About.com GuideMay 10, 2010

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The US Food and Drug Administration (FDA) has added what's known as a "black box" warning to the label for the drug propylthiouracil -- commonly referred to as PTU. PTU and methimazole -- which also goes by the brand name Tapazole -- are the two antithyroid drugs available in the US to treat hyperthyroidism. The FDA's warning states that severe liver injury and acute liver failure, in some cases fatal, is associated with use of PTU both in adults and children.

The new warning says, however, that PTU may be appropriate to use in two key situations:

When a patient cannot tolerate the other antithyroid drug treatment such as methimazole (Tapazole), or other treatments for hyperthyroidism, such as radioactive iodine (RAI) or surgery
When a woman is in the first trimester of pregnancy-- because there is an increased risk of birth defects in babies whose mothers have taken the thyroid drug methimazole during the first trimester of pregnancy

Patients who are taking PTU should not panic, however, as the overall risk of problems is quite small. The FDA's warning is based on the fact that since 1969, when the FDA's Adverse Events Reporting System database started tracking adverse events related to medications, there have been 34 cases of severe liver injury associated with PTU. Of the 23 adults affected, 13 deaths and 5 liver transplants were reported, and among the 11 children, 2 children died and 7 required liver transplantation. The PTU findings compare to 5 cases of severe liver injury -- and 3 deaths -- that have been reported with methimazole.

According to the FDA, however: "Although the number of identified reports of postmarketing cases of severe liver injury with propylthiouracil use between 1969 to 2009 is 34, FDA included information about this adverse event in the Boxed Warning due to the severity of the cases, some of which have been fatal, and to ensure that healthcare professionals are aware of this risk and are vigilant for the signs and symptoms of hepatic toxicity."

So, as you can see, given that hundreds of thousands of people take antithyroid drugs every year, the overall risk is quite small. But based on these findings and the medical literature, the FDA wants physicians to be aware that PTU is associated with a higher risk for clinically serious or fatal liver injury, compared with methimazole.

The FDA recommends that physicians monitor patients who are taking PTU for signs and symptoms of liver injury, particularly during the first 6 months after beginning treatment. And the FDA says that PTU should not be administered to children unless the child is allergic to or unable to tolerate methimazole, and other options are not available.

The following are the FDA's recommendations for healthcare professionals:

Be aware that severe liver injury and acute liver failure, including fatal cases, have been reported in adult and pediatric patients taking propylthiouracil.
Propylthiouracil should be reserved for patients who cannot tolerate methimazole or for patients for whom radioactive iodine therapy or surgery is not appropriate treatment when initiating treatment.
Propylthiouracil may be the treatment of choice during and just before the first trimester of pregnancy, as fetal abnormalities have been observed with methimazole in early pregnancy.
Propylthiouracil is not recommended for use in pediatric patients except in rare instances in which other alternative treatments are not appropriate.
Encourage patients to read the medication guide when picking up their prescription for propylthiouracil.
Review the newly revised label for complete information on the use of propylthiouracil.

_{Source: FDA Drug Safety Communication: New Boxed Warning on severe liver injury with propylthiouracil, 04-21-2010}

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