Methimazole
Methimazole (thiamazole)
Tapazole; outside US Mercazole
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Summary
Classes
Common uses
Hyperthyroidism (Graves disease, toxic nodular goiter)0, Thyroid storm (with PTU often preferred acutely)0, Preoperative thyroid preparation0, Pre-radioiodine treatment0
Pharmacy
Starting dose
10-40 mg PO daily depending on hyperthyroidism severity; titrate by clinical and biochemical response (target TSH/free T4)
Preparations
5, 10 mg tablets
US FDA Max
60 mg/d typical
Pharmacology
Routes
Oral
Onset
Biochemical improvement within 2-4 weeks; full euthyroid state 6-12 weeks
Duration
24 hours
Half-life
~4-6 hours (plasma); intrathyroidal accumulation gives a much longer functional duration[1]
Bioavailability
~93% (oral)[1]
Pregnancy
Avoid in first trimester (aplasia cutis, choanal/esophageal atresia association); switch to PTU in first trimester, then back to methimazole in second/third for lower PTU hepatotoxicity risk.[citation needed]
Legal status
Rx-only in US
Purported mechanism
Methimazole inhibits thyroid peroxidase (TPO), blocking iodination of thyroglobulin tyrosine residues and the coupling of mono- and diiodotyrosine to form T3 and T4; the result is reduced new hormone synthesis. Unlike propylthiouracil, methimazole does not inhibit peripheral T4-to-T3 conversion.0 Agranulocytosis is the most-feared adverse effect (~0.3%, usually first 90 days of treatment; warn patients to seek urgent CBC for fever or severe sore throat). Hepatotoxicity is class-recognized but more often associated with PTU[1].
References
- ↑ 1.0 1.1 1.2 FDA Prescribing Information, Tapazole (methimazole), Pfizer/King, current revision. https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/007599s029lbl.pdf