<p><strong>Congenital Malformations</strong>: Methimazole readily crosses the placental membranes and can cause fetal harm, particularly when administered in the first trimester of pregnancy. If Methimazole is used during pregnancy or if the patient becomes pregnant while taking this drug, the patient should be warned of the potential hazard to the fetus. Since some congenital defects have been reported in offspring of patients treated with Methimazole, it may be appropriate to use other agents in pregnant women requiring treatment for hyperthyroidism, particularly in the first trimester of pregnancy. If Methimazole is used, the lowest possible dose to control the disease should be given.<br />
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<strong>Agranulocytosis</strong>: Agranulocytosis is potentially a life-threatening adverse reaction of Methimazole therapy. Patients should be instructed to immediately report to their physicians if any symptoms suggestive of agranulocytosis, such as fever or sore throat occur. Leukopenia, thrombocytopenia, and aplastic anemia (pancytopenia) may also occur. The drug should be discontinued in the presence of agranulocytosis, aplastic anemia (pancytopenia), ANCA-positive vasculitis, hepatitis, or exfoliative dermatitis and the patient&rsquo;s bone marrow indices should be monitored.<br />
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<strong>Liver Toxicity</strong>: Although there have been reports of hepatotoxicity associated with Methimazole, the risk of hepatotoxicity appears to be less with methimazole than with propylthiouracil, especially in the pediatric population. Drug treatment should be discontinued promptly in the event of clinically significant evidence of liver abnormality including hepatic transaminase values exceeding 3 times the upper limit of normal.<br />
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<strong>Hypothyroidism</strong>: Methimazole can cause hypothyroidism necessitating routine monitoring of TSH and free T4 levels with adjustments in dosing to maintain a euthyroid state. Because the drug readily crosses placental membranes, Methimazole can cause fetal goiter and cretinism when administered to a pregnant woman. For this reason, it is important that a sufficient, but not excessive, dose be given during pregnancy. Because Methimazole may cause hypoprothrombinemia and bleeding, prothrombin time should be monitored during therapy with the drug, especially before surgical procedures. Thyroid function tests should be monitored periodically during therapy. Once clinical evidence of hyperthyroidism has resolved, the finding of a rising serum TSH indicates that a lower maintenance dose of Methimazole should be employed.</p>