Treatment with antithyroid drugs may be associated with a range of minor side effects. Up to 5% of all patients started on treatment with methimazole or propylthiouracil may develop symptoms and signs consistent with allergic reactions that may include fevers, rashes, hives, and diffuse joint pains called arthralgias. These minor side effects are more likely to develop during the first few months of treatment in patients on high doses of thionamides. They usually subside over the course of a few weeks once treatment is discontinued. Patients who develop minor side effects when treated with one thionamide may not necessarily develop the same problems if they are started on treatment with another.
A more significant side effect that may be related to treatment with antithyroid drugs involves damage to the liver, a condition identified as hepatotoxicity. This condition encompasses a wide spectrum of abnormalities. At one end of the spectrum, some patients treated with propylthiouracil may present with increased levels of enzymes called aminotransferases that may reflect mild damage to the liver. Aminotransferases that are commonly measured on routine blood tests include aspartate aminotransferase and alanine aminotransferase. Some patients who are treated with methimazole may present with increased levels of an enzyme called alkaline phosphatase and a chemical compound called bilirubin that may reflect damage to the system of bile ducts that drain from the liver. Most patients who develop hepatotoxicity are completely asymptomatic, presenting with changes that may only be detected if blood tests are checked and followed over time. Mild hepatotoxicity usually resolves once antithyroid drugs are discontinued. At the other end of the spectrum, cases have been reported where patients treated with propylthiouracil have developed severe liver failure that progressed to the point where liver transplantation was necessary. Patients who are taking antithyroid drugs should be aware of the potential for the development of hepatotoxicity. Any patient who develops jaundice or pain in the upper right section of the abdomen while taking an antithyroid drug should be instructed to hold off on taking any further doses while arrangements are made to check appropriate blood tests.
The most serious side effect that may develop in patients treated with antithyroid drugs is a drop in the number of white blood cells present in the circulation called agranulocytosis. Neutrophils are a specific type of white blood cell that help to fight off bacterial infections. In the normal state, neutrophil counts in the blood usually range from 3,000,000 - 5,800,000 cells/liter. Up to 10% of all patients treated with antithyroid drugs may experience a temporary drop in the number of white blood cells present in the circulation with a neutrophil count that dips below 1,500,000 cells/liter. When followed over time, the neutrophil count gradually returns to normal in the majority of these patients. A very small fraction of these patients may progress to develop agranulocytosis with a neutrophil count that drops below 250 cells/liter. Patients who develop agranulocytosis may be at risk for contracting dangerous bacterial infections that may require hospitalization and treatment with potent antibiotics. Even though the overall risk is very slight, patients who are taking antithyroid drugs should be aware of the possibility of developing agranulocytosis. Any patient who develops a fever or sore throat while taking an antithyroid drug should be instructed to hold off on taking any further doses while arrangements are made to check appropriate blood tests.