Methimazole

Antithyroid agent, Thionamide

Methimazole (thiamazole)

Tapazole; outside US Mercazole

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Summary

Classes

Antithyroid agent, Thionamide

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

Bioavailability

~93% (oral)

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.