Desensitization to Methimazole

Objective Thionamides (methimazole and propylthiouracil) have been associated with common side effects, such as rash and pruritus, and rare but serious adverse effects, such as agranulocytosis and hepatotoxicity. Methimazole is usually the preferred thionamide for the treatment of hyperthyroidism if the patient is not planning to conceive or not in the first trimester of pregnancy, given the less frequent dosing and lower risk of hepatotoxicity. In patients who experience rash or itching when treated with methimazole, switching them to propylthiouracil is one treatment option. Here we report our experience regarding desensitization to methimazole to allow continued treatment with methimazole as an alternative management option. Methods We conducted a retrospective chart review of patients at a single institution who had side effects to methimazole and who were desensitized to methimazole under the supervision of an allergist. A total of 7 patients were included who experienced side effects to methimazole that did not include agranulocytosis or hepatotoxicity. Results All 7 patients were able to take methimazole for treatment of their hyperthyroidism, either for continued medical therapy or as a bridge to definitive therapy, with either surgery or radioactive iodine treatment. Conclusion Under the supervision of an allergist, desensitization to methimazole is an option for treating patients who experience side effects to methimazole (excluding agranulocytosis and hepatotoxicity).