Dosing and uses of Northyx, Tapazole (methimazole)
Adult dosage forms and strengths
tablet
- 5mg
- 10mg
Hyperthyroidism
Mild: 15 mg/day PO divided q8hr initially
Moderate: 30-40 mg/day PO divided q8hr initially
Severe: 60 mg/day PO divided q8hr initially
Maintenance: 5-15 mg/day PO divided q8hr
Graves Disease
10-20 mg/day PO; after euthyroidism is achieved, reduce dosage by 50% and administer for 12-18 months
Thyrotoxic Crisis
20-30 mg q6-12hr for short term, then reduce dosage to maintenance (5-15 mg/day) or reduce frequency to q12hr or q24hr
Pediatric dosage forms and strengths
tablet
- 5mg
- 10mg
Hyperthyroidism
Initial: 0.4-0.7 mg/kg/day PO divided q8hr
Maintenance: 1/3-2/3 of initial PO dosage divided q8hr PO
Not to exceed 30 mg/day
Graves Disease
0.2-0.5 mg/kg/day PO; after euthyroidism is achieved, reduce dosage by 50% and administer for 1-2 years
Northyx, Tapazole (methimazole) adverse (side) effects
Frequency not defined
Agranulocytosis
Alopecia
Aplastic anemia
Dermatologic reactions (eg, rash, pruritus, skin pigmentation)
Hepatotoxicity
Leukocytoclastic vasculitis
Neuritis
Polyarthritis
Pruritus
Salivary gland swelling
Skin pigmentation
Thrombocytopenia
Urticaria
Vertigo
Warnings
Contraindications
Hypersensitivity
Breastfeeding
Cautions
Liver disease
Hepatic reactions may occur
Pregnancy
Lupuslike syndrome reported
Antineutrophil cytoplasmic autoantibody (ANCA)-positive vasculitis reported
Dosage >40 mg/day
Patient age >40 years
Concurrent administration of other agranulocytosis-causing drugs
Risk of rare but serious agranulocytosis, cholestastic jaundice, and rare congenital abnormalities (propylthiouracil [PTU] preferred in pregnancy)
Prolonged treatment may cause hypothyroidism
Discontinue if agranulocytosis, pancytopenia, hepatitis, fever, or exfoliative dermatitis occurs
10 times more potent than PTU, but action may be less consistent
High rate of relapse (more likely in smokers)
Pregnancy and lactation
Pregnancy category: D (drug crosses placenta)
Lactation: Drug distributed in breast milk; use contraindicated by manufacturer (American Academy of Pediatrics [AAP] Committee states that drug is compatible with nursing; American Academy of Family Physicians [AAFP] states that it is safe for nursing)
Pregnancy categories
A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.
B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.
C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.
D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.
X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.
NA: Information not available.
Pharmacology of Northyx, Tapazole (methimazole)
Mechanism of action
Inhibits synthesis of thyroid hormone by blocking oxidation of iodine in thyroid gland; blocks synthesis of thyroxine (T4) and triiodothyronine (T3)
Absorption
Bioavailability: 80-95%
Onset: PO, 12-18 hr
Duration: 36-72 hr
Peak plasma time: 1-2 hr
Distribution
Protein bound: None
Metabolism
Metabolized by liver
Elimination
Half-life: 4-6 hr
Excretion: Urine
