methimazole

Action

Directly interferes with thyroid synthesis by preventing iodine from combining with thyroglobulin, leading to decreased thyroid hormone levels

Availability

Tablets: 5 mg, 10 mg

Indications and dosages

➣ Mild hyperthyroidism

Adults and adolescents: Initially, 15 mg P.O. daily in three equally divided doses at approximately 8-hour intervals. For maintenance, 5 to 15 mg/day in equally divided doses at approximately 8-hour intervals.

Children: Initially, 0.4 mg/kg/day in three divided doses at 8-hour intervals. For maintenance, approximately 0.2 mg/kg/day in three divided doses at 8-hour intervals.

➣ Moderate hyperthyroidism

Adults and adolescents: Initially, 30 to 40 mg P.O. daily in three equally divided doses at approximately 8-hour intervals. For maintenance, 5 to 15 mg/day in three equally divided doses at approximately 8-hour intervals.

Children: 0.4 mg/kg/day P.O. as a single dose or in divided doses at 8-hour intervals. For maintenance, approximately 0.2 mg/kg/day as a single dose or in three divided doses at 8-hour intervals.

➣ Severe hyperthyroidism

Adults and adolescents: Initially, 60 mg/day P.O. in three equally divided doses at approximately 8-hour intervals. For maintenance, 5 to 15 mg/day in three equally divided doses at approximately 8-hour intervals.

Children: Initially, 0.4 mg/kg/day P.O. as a single dose or in three divided doses at 8-hour intervals. For maintenance, approximately 0.2 mg/kg/day as a single dose or in three divided doses at 8-hour intervals.

Contraindications

• Hypersensitivity to drug

• Breastfeeding

Precautions

Use cautiously in:

• bone marrow depression

• patients older than age 40

• pregnant patients.

Administration

• Give with meals as needed to reduce GI upset.

Adverse reactions

CNS: headache, vertigo, paresthesia, neuritis, depression, neuropathy, CNS stimulation

GI: nausea, vomiting, constipation, epigastric distress, ileus, salivary gland enlargement, dry mouth, anorexia

GU: nephritis

Hematologic: thrombocytopenia, agranulocytosis, leukopenia, aplastic anemia

Hepatic: jaundice, hepatic dysfunction, hepatitis

Metabolic: hypothyroidism

Musculoskeletal: joint pain, myalgia

Skin: rash, urticaria, skin discoloration, pruritus, erythema nodosum, exfoliative dermatitis, abnormal hair loss

Other: fever, lymphadenopathy, lupuslike syndrome

Interactions

Drug-drug. Aminophylline, oxtriphylline, theophylline: decreased clearance of both drugs

Amiodarone, iodine, potassium iodide: decreased response to methimazole

Anticoagulants: altered requirements for both drugs

Beta-adrenergic blockers: altered beta blocker clearance

Digoxin: increased digoxin blood level

Drug-diagnostic tests. Granulocytes, hemoglobin, platelets, white blood cells: decreased values

Patient monitoring

• Check for agranulocytosis in patients older than age 40 and in those receiving more than 40 mg/day.

• Assess hematologic studies. Agranulocytosis usually occurs within first 2 months of therapy and is rare after 4 months.

• Monitor thyroid function tests periodically. Once hyperthyroidism is controlled, elevated thyroid-stimulating factor indicates need for dosage decrease.

• Assess liver function tests and check for signs and symptoms of hepatic dysfunction.

• Monitor patient for fever, sore throat, and other evidence of infection as well as for unusual bleeding or bruising.

• Assess patient for signs and symptoms of hypothyroidism, such as hard edema of subcutaneous tissue, drowsiness, slow mentation, dryness or loss of hair, decreased temperature, hoarseness, and muscle weakness.

Patient teaching

• Tell patient to take with meals if GI upset occurs.

• Advise patient to take exactly as prescribed to maintain constant blood level.

• Tell patient to report rash, fever, sore throat, unusual bleeding or bruising, headache, rash, yellowing of skin or eyes, abdominal pain, vomiting, or flulike symptoms.

• Caution female patient not to breastfeed while taking drug.

• As appropriate, review all other significant and life-threatening adverse reactions and interactions, especially those related to the drugs and tests mentioned above.