Names and pronunciation
(meth-im′a-zole) Tapazole
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Drug reference
(meth-im′a-zole) Tapazole
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Func. class.: Thyroid hormone antagonist (antithyroid)
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Chem. class.: Thioamide
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Do not confuse: methimazole/metoprolol/minoxidil
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ACTION: Inhibits synthesis of thy- roid hormones by decreasing iodine use in manufacture of thyroglobulin and iodothyronine; does not affect circula- tory T4, T3
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USES: Hyperthyroidism, preparation for thyroidectomy
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Unlabeled uses: Thyroid storm
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CONTRAINDICATIONS: Preg- nancy, breastfeeding, hypersensitivity Precautions: Infection, bone marrow suppression, hepatic disease, bleeding disorders
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SIDE EFFECTS CNS: Drowsiness, headache, vertigo, fever, paresthesias, neuritis ENDO: Enlarged thyroid GI: Nausea, diarrhea, vomiting, jaun- dice, hepatitis, loss of taste GU: Nephritis HEMA: Agranulocytosis, leukopenia, thrombocytopenia, hypothrombinemia, lymphadenopathy, bleeding, vasculitis INTEG: Rash, urticaria, pruritus, alo- pecia, hyperpigmentation, lupuslike syndrome MS: Myalgia, arthralgia, nocturnal mus- cle cramps
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INTERACTIONS Increase: bone marrow depression— radiation, antineoplastic agents Increase: response to digoxin; monitor digoxin level, reduce dose if needed Decrease: effectiveness—amiodarone, potassium iodide; methIMAzole dose may need to be increased Decrease: anticoagulant effect—warfa- rin; monitor PT, INR Drug/Lab Test Increase: PT, AST, ALT, alk phos
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DOSAGE AND ROUTES Hyperthyroidism • Adult: PO 15 mg/day (mild hyper- thyroidism); 30-40 mg/day (moder- ate to severe); 60 mg/day (severe); maintenance 5-15 mg/day; may be divided • Child: PO 0.4 mg/kg/day in divided doses q8hr; continue until euthyroid; maintenance dose 0.2 mg/kg/day in di- vided doses q8hr, max 30 mg/24 hr; may be divided Thyroid storm (unlabeled) Adult: PO 60-80 mg/day
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Available forms: Tabs 5, 10 mg
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Administer: • With meals to decrease GI upset • At same time each day to maintain product level • Lowest dose that relieves symptoms; discontinue before RAIU
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Assess: • Hyperthyroidism: palpitations, ner- vousness, loss of hair, insomnia, heat in- tolerance, weight loss, diarrhea; product adjustment may be needed • Hypothyroidism: constipation, dry skin, weakness, fatigue, headache, intolerance to cold, weight gain; adjustment may be needed; check for edema: puffy hands, feet, periorbits; these indicate hypothyroidism • Pulse, B/P, temperature • Weight daily; same clothing, scale, time of day; weight increase or decrease is a sign of need to adjust product dose • Monitor lab work: T3, T4, which are increased; serum TSH, which is de- creased; free thyroxine index, which is increased if dosage too low; discontinue product 3-4 wk before RAIU • Blood dyscrasias: CBC; monitor leu- kopenia, thrombocytopenia, agranulocy- tosis; if these occur, product should be discontinued and other treatment initi- ated; may occur at doses >40 mg/day • Hypersensitivity: rash, enlarged cervi- cal lymph nodes; product may have to be discontinued • Hypoprothrombinemia: bleeding, pe- techiae, ecchymosis M • Clinical response: after 3 wk should include increased weight; decreased T4, pulse • Bone marrow suppression: sore throat, fever, fatigue • Pregnancy/breastfeeding: may cause fetal harm; do not use in preg- nancy; avoid use in breastfeeding
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Evaluate: • Therapeutic response: weight gain, de- creased pulse, decreased T4, B/P
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CLINICAL CONSIDERATIONS
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Patient and caregiver information: • Not to breastfeed • To take pulse daily • To report redness, swelling, sore throat, mouth lesions, fever, which indi- cate blood dyscrasias • To keep graph of weight, pulse, mood • To avoid OTC products, seafood that contain iodine, other iodine products • Not to discontinue product abruptly because thyroid crisis may occur; stress patient response • That response may take several months if thyroid is large • Symptoms and signs of overdose: periorbital edema, cold intolerance, mental depression • Symptoms of inadequate dose: tachy- cardia, diarrhea, fever, irritability • To take medication as prescribed; not to skip or double dose • To report yellowing of skin/eyes, dark urine, anorexia, right upper abdominal pain; may indicate hepatic dysfunction HIGH ALERT
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PHARMACOKINETICS Onset rapid; peak 1-2 hr; half-life 4-6 hr; excreted in urine, breast milk; crosses placenta
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• Adult: PO 15 mg/day (mild hyper- thyroidism); 30-40 mg/day (moder- ate to severe); 60 mg/day (severe); maintenance 5-15 mg/day; may be divided
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• Child: PO 0.4 mg/kg/day in divided doses q8hr; continue until euthyroid; maintenance dose 0.2 mg/kg/day in di- vided doses q8hr, max 30 mg/24 hr; may be divided Thyroid storm (unlabeled) Adult: PO 60-80 mg/day
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