Methimazole: Mechanism, Clinical Use, and Dosing Recommendations

Primary Mechanism of Action

Methimazole irreversibly inhibits thyroid peroxidase (TPO), the heme‑containing enzyme that catalyzes iodination of thyroglobulin and coupling of iodotyrosines to form T₃ and T₄, thereby blocking thyroid hormone synthesis at the glandular level. 1

Unlike propylthiouracil, methimazole does not inhibit the peripheral conversion of T₄ to the more active T₃, so it does not affect deiodinase activity in peripheral tissues. 1

Treatment with methimazole is associated with a reduction or complete disappearance of TSH‑receptor antibodies in the majority of patients, suggesting a possible immunomodulatory contribution to disease remission. 1

In patients with subclinical hyperthyroidism, the decision to treat with methimazole should be based on cardiovascular and bone‑health risk rather than the presence of overt thyrotoxic symptoms. 2
For individuals older than 60 years who have a suppressed TSH < 0.1 mIU/L, treatment is recommended because of the increased risk of atrial fibrillation and bone loss, even when free T₄ and T₃ are within normal limits. 2
When TSH is modestly suppressed (0.1–0.45 mIU/L), routine methimazole therapy is not advised unless the patient has documented cardiovascular disease, osteoporosis risk factors, or symptomatic hyperthyroidism. 2

Methimazole is the preferred antithyroid medication for most clinical scenarios; however, it should be avoided during the first trimester of pregnancy, in thyroid storm, or in patients with documented methimazole intolerance, where alternative therapy is indicated. (American College of Endocrinology & American Association of Clinical Endocrinologists) [3][4]

Methimazole’s long intrathyroidal half‑life allows once‑daily dosing, providing a practical advantage over propylthiouracil, which requires three‑times‑daily administration. (American College of Endocrinology & American Association of Clinical Endocrinologists) [1][3]