Action

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

Availability

Tablets: 50 mg

Indications and dosages

➣ Hyperthyroidism

Adults: Initially, 300 to 450 mg P.O. daily in equally divided doses q 8 hours; for maintenance, 100 to 150 mg P.O. daily.

➣ Thyrotoxic crisis

Adults: 200 mg P.O. q 4 to 6 hours during first 24 hours, then a maintenance dosage of 100 to 150 mg P.O. daily

Contraindications

• Hypersensitivity to drug

Precautions

Use cautiously in:

• decreased bone marrow reserve

• pregnancy and breastfeeding patients

• children.

Administration

• Give with meals to reduce GI upset.

• Be aware that drug shouldn't be used in children unless patient is allergic to or intolerant of methimazole, and there are no other treatment options available.

Adverse reactions

CNS: drowsiness, headache, vertigo, neuritis, paresthesia

GI: nausea, vomiting, diarrhea, epigastric distress

Hematologic: agranulocytosis, leukopenia, thrombocytopenia

Hepatic: jaundice, hepatic necrosis, liver failure

Metabolic: hypothyroidism

Musculoskeletal: joint pain, myalgia

Skin: rash, urticaria, pruritus, skin discoloration, alopecia, cutaneous vasculitis

Other: taste loss, fever, lymphadenopathy, parotitis, edema

Interactions

Drug-drug. Anticoagulants: potentiation of anticoagulant effect

Drug-diagnostic tests. Alanine aminotransferase, alkaline phosphatase, aspartate aminotransferase, bilirubin, lactate dehydrogenase: increased levels

Granulocytes, platelets: decreased levels Prothrombin time: prolonged

Patient monitoring

• Monitor CBC and liver and thyroid function tests.

• Assess for signs and symptoms of hypothyroidism (cold intolerance, nonpitting edema, fatigue, weight gain, and depression).

See Monitor for severe rash, fever, jaundice, or enlarged cervical lymph nodes. If present, stop therapy and notify prescriber.

Patient teaching

• Instruct patient to take with meals to reduce GI upset.

• Teach patient to recognize and report signs and symptoms of hypothyroidism and jaundice.

• Advise patient to discuss iodine intake (as in iodized salt and shellfish) with prescriber.

• Tell patient to avoid over-the-counter cold remedies that contain iodine.

• Caution patient to avoid driving and other hazardous activities until he knows how drug affects concentration and alertness.

• Advise female patient of childbearing age to discuss pregnancy or breastfeeding with prescriber before taking.

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