Methimazole
Pronunciation: (meth-IMM-uh-zole)Class: Antithyroid agent
Trade Names:
Tapazole
- Tablets 5 mg
- Tablets 10 mg
Pharmacology
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Inhibits synthesis of thyroid hormones.
Pharmacokinetics
Absorption
Methimazole bioavailability is 80% to 95%.
Distribution
A high amount of methimazole is excreted in breast milk, has high transplacental passage, and is 0% protein bound.
Metabolism
Methimazole is rapidly metabolized.
Elimination
Less than 10% is excreted in urine. The t ½ is 6 to 13 h.
Indications and Usage
Long-term therapy of hyperthyroidism; amelioration of hyperthyroidism in preparation for subtotal thyroidectomy or radioactive iodine therapy.
Contraindications
Use in nursing women.
Dosage and Administration
Adults Initial dosePO 15 to 60 mg/day in 3 equal doses at approximately 8-h intervals.
MaintenancePO 5 to 15 mg/day.
Children Initial dosePO 0.4 mg/kg/day.
MaintenancePO Approximately ½ initial dose. Alternately, children may be given 0.5 to 0.7 mg/kg/day in 3 divided doses as initial therapy and 1/ 3 to 2/ 3 of initial dose for maintenance.
Storage/Stability
Store in light-resistant container at room temperature (59° to 86°F).
Drug Interactions
AnticoagulantsMay decrease or increase anticoagulant action.
Beta blockersMay increase effects of beta blockers, resulting in toxicity.
DigoxinMay cause increase in effects of digitalis glycosides, including toxicity.
TheophyllinesMay alter theophylline clearance in hyperthyroid or hypothyroid patients.
Laboratory Test Interactions
None well documented.
Adverse Reactions
CNS
Paresthesias; neuritis; headache; vertigo; drowsiness; neuropathies; CNS stimulation; depression.
Dermatologic
Rash; urticaria; pruritus; erythema nodosum; skin pigmentation; exfoliative dermatitis; lupus-like syndrome including splenomegaly, hepatitis, periarteritis, hypoprothrombinemia and bleeding.
EENT
Loss of taste; sialadenopathy.
GI
Nausea; vomiting; epigastric distress.
Genitourinary
Nephritis.
Hematologic
Inhibition of myelopoiesis (eg, agranulocytosis, granulocytopenia, thrombocytopenia); aplastic anemia; hypoprothrombinemia; periarteritis.
Hepatic
Jaundice; hepatitis.
Miscellaneous
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.
Overdosage
Symptoms
Nausea, vomiting, epigastric distress, headache, fever, arthralgia, pruritus, edema, pancytopenia, agranulocytosis, exfoliative dermatitis, hepatitis, neuropathies, CNS stimulation or depression.
Patient Information
- 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 health care provider.
- 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 the following symptoms to health care provider: 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.
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methimazole - Includes detailed dosage instructions.
