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Medication Card: Methimazole

Medication Card: Methimazole

Class

  • Antithyroid agent

Actions

  • Inhibits the synthesis of thyroid hormones by interfering with the incorporation of iodine into tyrosine residues of thyroglobulin. Does not inactivate existing thyroid hormones.

Indications

  • - Hyperthyroidism (especially Graves' disease)

  • - Preoperative preparation for thyroidectomy

  • - Thyrotoxic crisis (in combination with other therapies)

Contraindications

  • - Hypersensitivity to methimazole

  • - Pregnancy (especially 1st trimester – PTU preferred)

  • - Lactation (use with caution)

Caution

  • - Hepatic impairment

  • - Bone marrow suppression

  • - Patients at risk for infection

  • - Geriatric population

Adverse Effects

  • - Agranulocytosis (potentially fatal)

  • - Rash, pruritus

  • - Fever

  • - Arthralgia

  • - Hepatotoxicity

  • - Drowsiness

  • - GI upset (nausea, vomiting)

Drug-Drug Interactions

  • - May increase effects of anticoagulants (e.g., warfarin)

  • - Additive bone marrow suppression with other myelosuppressive agents

  • - Decreased effect of thyroid hormones (when transitioning)

Assess

  • - Monitor CBC with differential (watch for leukopenia or agranulocytosis)

  • - Monitor liver function tests

  • - Assess for signs of infection (e.g., sore throat, fever)

  • - Thyroid function tests (T3, T4, TSH)

  • - Baseline and periodic physical exam

Teach

  • - Report sore throat, fever, or malaise immediately (signs of agranulocytosis)

  • - Take medication consistently at the same time each day

  • - Do not stop medication abruptly

  • - Avoid crowds or people who are sick (due to infection risk)

  • - Inform provider of any unusual bleeding or bruising

  • - Monitor for signs of hypothyroidism (e.g., fatigue, weight gain, cold intolerance)