Palliative treatment of hyperthyroidism. Therapy maintains patient in euthyroid state for a period of several (generally 1–2) years until spontaneous remission occurs; however, spontaneous remission does not occur in all patients, and most patients eventually require ablative therapy (i.e., surgery, radioactive iodine). Because methimazole does not affect underlying cause of hyperthyroidism, generally avoid long-term use; minimum duration of therapy necessary before assessing whether spontaneous remission has occurred not clearly established.
May be used in juvenile hyperthyroidism to delay ablative therapy; if remission does not occur, may continue methimazole for several years to postpone ablation until child is older.
Amelioration of hyperthyroidism in preparation for surgical treatment (e.g., subtotal thyroidectomy). Therapy with methimazole returns the hyperthyroid patient to a normal metabolic state prior to thyroidectomy and controls the thyrotoxic crisis that may accompany thyroidectomy. Also used when thyroidectomy is contraindicated or not advisable.
Amelioration of hyperthyroidism in preparation for radioactive iodine therapy (e.g., in geriatric patients, in patients with cardiac disease). Therapy with methimazole controls symptoms of hyperthyroidism before and during radioactive iodine therapy until the ablative effects of iodine occur. However, pretreatment with thioamides may lower the cure rate and increase the need for subsequent doses of radioactive iodine.
May be used for management of thyrotoxic crisis, although not considered antithyroid agent of first choice. Usually initiated before iodide (e.g., potassium iodide, strong iodine solution) therapy.
Because of ease of administration and better adherence, methimazole generally is preferred over propylthiouracil for management of most hyperthyroid situations (except during pregnancy or lactation or for management of thyrotoxic crisis).
Dosage and Administration
General
May use a β-adrenergic blocking agent (e.g., propranolol) concomitantly to manage peripheral signs and symptoms of hyperthyroidism, particularly cardiovascular effects (e.g., tachycardia).
Administration
Administer orally. May administer rectally† as extemporaneously prepared suppositories.
Oral Administration
Some manufacturers recommend administering daily dosage orally in 3 equally divided doses at approximately 8-hour intervals. In adults, some clinicians suggest administering as a single daily dose or, alternatively, in divided doses.
Dosage
Pediatric Patients
Hyperthyroidism
Palliative Treatment to Achieve Spontaneous Remission
Oral
Initially, 0.4 mg/kg daily given in 3 equally divided doses at approximately 8-hour intervals. Continue therapy at initial dosage for about 4–8 weeks until symptoms resolve and euthyroidism is achieved. Then gradually taper to a dosage that maintains euthyroidism.
Maintenance dosage: Approximately half of initial dosage.
Optimum duration of therapy not clearly established. If relapse occurs following discontinuance of therapy, initially reinstitute methimazole, then consider alternate forms of therapy.
Adults
Hyperthyroidism
Palliative Treatment to Achieve Spontaneous Remission
Oral
Initially, some manufacturers recommend 15 mg daily for mild hyperthyroidism, 30–40 mg daily for moderately severe hyperthyroidism, or 60 mg daily for severe hyperthyroidism. Continue therapy at initial dosage for about 4–8 weeks until symptoms resolve and euthyroidism is achieved. Then gradually taper to a dosage that maintains euthyroidism.
Usual maintenance dosage: 5–30 mg daily.
Optimum duration of therapy not clearly established; most data support duration of 12–18 months. If relapse occurs following discontinuance of therapy, initially reinstitute methimazole, then consider alternate forms of therapy. According to some clinicians, may continue thioamide therapy indefinitely if well tolerated and if alternative therapies (e.g., surgery, radioactive iodine) are not desired.
Preparation for Surgical Treatment
Oral
Initially, some manufacturers recommend 15 mg daily for mild hyperthyroidism, 30–40 mg daily for moderately severe hyperthyroidism, or 60 mg daily for severe hyperthyroidism.
Some clinicians recommend continuing therapy at initial dosage for about 6–8 weeks until euthyroidism is achieved; then add iodide therapy for 10–14 days (to decrease vascularity of thyroid gland) before surgery.
Preparation for Radioactive Iodine Therapy
Oral
Initially, some manufacturers recommend 15 mg daily for mild hyperthyroidism, 30–40 mg daily for moderately severe hyperthyroidism, or 60 mg daily for severe hyperthyroidism.
Some clinicians recommend continuing therapy at initial dosage until euthyroidism is achieved; then discontinue methimazole 4–6 days before radioactive iodine therapy. May reinstitute methimazole 4 days after radioactive iodine therapy as needed (e.g., in patients with cardiac disease).
Prescribing Limits
Adults
Hyperthyroidism
Oral
Some clinicians recommend avoiding dosages >40 mg daily if possible. (See Hematologic Effects under Cautions.)
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