methimazole; thiamazole (Tapazole)
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Introduction
Tradename: Tapozole.
Indications
- 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:
- children:
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
- common (> 10%)
- maculopapular skin rash, mild transient leukopenia, fever rash less common at 15 mg QD vs 30 mg QD)[4]
- less common (1-10%)
- dizziness, nausea/vomiting, stomach pain, loss of taste, agranulocytosis, SLE-like syndrome
- uncommon (< 1%)
- other
- serious liver injury[5]
- hepatotoxicity < that of propylthiouracil[5]
Drug interactions
Test interactions
- interferes with thyroid scan, stop 1 week prior to scan
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
More general terms
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
- ↑ 4.0 4.1 4.2 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: https://www.ncbi.nlm.nih.gov/pubmed/17389704
- ↑ 5.0 5.1 5.2 5.3 5.4 Medical Knowledge Self Assessment Program (MKSAP) 17, 19, American College of Physicians, Philadelphia 2015, 2022
Medical Knowledge Self Assessment Program (MKSAP) 19 Board Basics. An Enhancement to MKSAP19. American College of Physicians, Philadelphia 2022 - ↑ 6.0 6.1 NEJM Knowledge+