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methimazole; thiamazole (Tapazole)
Tradename: Tapozole.
Indication:
- palliative treatment of hyperthyroidism
- to return the patient to a normal metabolic state prior to thyroid surgery
- control of thyrotoxic crisis that may accompany thyroidectomy
Contraindications:
- 1st trimester of pregnancy or breast-feeding [5]
- thyroiditis resulting in release of preformed thyroid hormone [5]
Dosage:
- start 5-20 mg PO TID, then adjust
- 15 mg QD - 15 mg BID [4]
- divide dose every 8 hours if GI intolerance occurs
- thyroid storm:
- 40-120 mg/day divided every 8 hours until euthyroid
- treat 1-2 years
- children:
- start: 0/4 mg/kg/day divided TID
- maintenance: 0.2 mg/kg/day divided TID
Tabs: 5, 10 mg.
Monitor:
- thyroid function every 3 months
- serum TSH*
- serum T4 &/or serum free T4
- prothrombin time every 3 months
- CBC if fever or pharyngitis to rule out agranulocytosis
* stop methimazole after 12-18 months if serum TSH & TSH receptor Ab are normal [6]
Adverse effects:
1) common (> 10%)
- maculopapular skin rash, mild transient leukopenia, fever rash less common at 15 mg QD vs 30 mg QD) [4]
2) less common (1-10%)
- dizziness, nausea/vomiting, stomach pain, loss of taste, agranulocytosis, SLE-like syndrome
3) uncommon (< 1%)
- thrombocytopenia, aplastic anemia, swollen salivary glands, cholestatic jaundice, nephrotic syndrome, constipation, goiter, weight gain, edema, paresthesia, drowsiness, vertigo, headache, urticaria, pruritus, hair loss, arthralgia
4) other
- serious liver injury [5]
- hepatotoxicity < that of propylthiouracil [5]
Drug interactions:
- warfarin, digoxin, insulin activity may be altered by changing thyroid status
Mechanism of action:
- inhibits synthesis of thyroid hormones by interfering with incorporation of iodine into tyrosine residues of thyroglobulin
- inhibits coupling of iodotyrosyl residues to form iodothyronine
- unlike propylthiouracil (PTU), it does not inhibit peripheral conversion of T4 to T3
- 10X more potent than PTU on a weight basis
Test interactions:
- interferes with thyroid scan, stop 1 week prior to scan
Interactions
drug adverse effects of antithyroid agent(s)
monitor with antithyroid agents
General
antithyroid agent
pyrrole
Properties
MISC-INFO: elimination route LIVER
pregnancy-category D
safety in lactation -
Database Correlations
PUBCHEM correlations
References
- The Pharmacological Basis of Therapeutics, 9th ed.
Gilman et al, eds. Permagon Press/McGraw Hill, 1996
- Drug Information & Medication Formulary, Veterans Affairs,
Central California Health Care System, 1st ed., Ravnan et al
eds, 1998
- Kaiser Permanente Northern California Regional Drug
Formulary, 1998
- Nakamura H, Noh JY, Itoh K, Fukata S, Miyauchi A, Hamada N.
Comparison of methimazole and propylthiouracil in patients
with hyperthyroidism caused by graves' disease.
J Clin Endocrinol Metab. 2007 Jun;92(6):2157-62. Epub 2007 Mar 27.
PMID: 17389704
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