Methimazole and Blood dyscrasias

The use of thioamide antithyroid agents may infrequently be associated with the development of blood dyscrasias, primarily agranulocytosis but also leukopenia, thrombocytopenia, and aplastic anemia. Most cases of agranulocytosis occur within the first two months of therapy, with incidence declining gradually thereafter. Patients older than 40 years of age and those receiving more than 40 mg/day of methimazole appear to be at substantially increased risk (a dosage association has not been reported for propylthiouracil). Therapy with thioamide derivatives should be administered cautiously in patients with or predisposed to blood dyscrasias and/or bone marrow depression. Clinical monitoring of hematopoietic function is recommended, with particular focus on leukocyte and differential counts. Because leukopenia (i.e. leukocyte count < 4000/mm3) may also occur in 10% of patients with untreated hyperthyroidism and is often associated with relative granulocytopenia, baseline count should be determined prior to initiating antithyroid medication. Patients should be instructed to immediately report during therapy any signs or symptoms suggestive of myelosuppression such as fever, sore throat, local infection, easy bruising, or bleeding. Cessation of antithyroid drug and institution of appropriate supportive measures are necessary if agranulocytosis or aplastic anemia develops.