Methimazole

 Action Inhibits synthesis of thyroid hormones.

 Indications Long-term therapy of hyperthyroidism; amelioration of hyperthyroidism in preparation for subtotal thyroidectomy or radioactive iodine therapy.

 Contraindications Use in nursing women.

 Route/Dosage

ADULTS: Initial dose: PO 15–60 mg/day in 3 equal doses at approximately 8-hr intervals. Maintenance: PO 5–15 mg/day. CHILDREN: Initial dose: PO 0.4 mg/kg/day. MAINTENANCE: PO Approximately ½ initial dose. Alternately, children may be given 0.5–0.7 mg/kg/day in 3 divided doses as initial therapy and 1/3–2/3 of initial dose for maintenance.

 Interactions

Anticoagulants: May decrease or increase anticoagulant action. Beta blockers: May increase effects of beta blockers, resulting in toxicity. Digoxin: May cause increase in effects of digitalis glycosides, including toxicity. Theophyllines: May alter theophylline clearance in hyperthyroid or hypothyroid patients.

 Lab Test Interferences None well documented.

 Adverse Reactions

CNS: Paresthesias; neuritis; headache; vertigo; drowsiness; neuropathies; NS stimulation; depression. DERM: Rash; urticaria; pruritus; erythema nodosum; skin pigmentation; xfoliative dermatitis; lupus-like syndrome including splenomegaly, hepatitis, periarteritis, hypoprothrombinemia and bleeding. EENT: Loss of taste; sialadenopathy. GI: Nausea; vomiting; epigastric distress. GU: Nephritis. HEPA: Jaundice; hepatitis. HEMA: Inhibition of myelopoiesis (eg, agranulocytosis, granulocytopenia, thrombocytopenia); aplastic anemia; hypoprothrombinemia; periarteritis. OTHER: Abnormal hair loss; arthralgia; myalgia; edema; lymphadenopathy; drug fever; interstitial pneumonitis; insulin autoimmune syndrome.

 Precautions

Pregnancy: Category D. Lactation: Avoid nursing. Agranulocytosis: Potentially most serious side effect. Discontinue drug in presence of agranulocytosis, aplastic anemia, hepatitis, fever or exfoliative dermatitis. Hemorrhage: May cause hypoprothrombinemia and bleeding. Monitor PT.

 Administration/Storage

• Administer around clock in 3 equal doses (ie, q 8 hr).
• Administer each dose with same amount of food to facilitate uniform absorption.
• Store in light-resistant container at room temperature.

 Assessment/Interventions

• Obtain patient history, including drug history and any known allergies.
• Assess for history of liver disease.
• Assess symptoms of hyperthyroidism that patient is experiencing.
• Obtain baseline vital signs and monitor during treatment.
• Obtain baseline hepatic blood work including bilirubin and liver enzymes.
• Obtain baseline CBC and monitor carefully during first 2 months of treatment.
• Obtain baseline thyroid levels and monitor monthly initially and q 2–3 mo for long-term therapy.
• Monitor drug's effects on hyperthyroid symptoms.
• Monitor PT, especially before surgery, to assess increased risk of bleeding.
• Observe for potential adverse reactions including rash or fever, and notify physician if any occur.

 Patient/Family Education

• Instruct patient in importance of taking proper dose exactly as scheduled.
• Advise patient not to stop, start or adjust dose of any medications, including otc, without discussing with physician.
• Explain to patient the importance of complying with follow-up appointments and lab work.
• Instruct patient to check pulse daily.
• Advise patient that iodine-restrictive diet may be necessary.
• Inform patient that response to therapy may take months.
• Instruct patient to report these symptoms to physician: rash, fever, sore throat, bruising or signs of infection or jaundice.
• Advise patient that drug may cause drowsiness and to use caution while driving or performing other tasks requiring mental alertness.