Methimazole is indicated:

• In patients with Graves’ disease associated with hyperthyroidism or toxic multinodular goiter, when surgery or radioactive iodine therapy is not an appropriate treatment option.
• To reduce symptoms of hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy.

Methimazole inhibits the synthesis of thyroid hormones and is therefore effective in the treatment of hyperthyroidism. It does not inactivate preformed thyroxine (T4) or triiodothyronine (T3) stored in the thyroid gland or circulating in the blood, nor does it interfere with the action of thyroid hormones administered orally or by injection. Methimazole is well absorbed from the gastrointestinal tract, metabolized in the liver, and excreted in the urine.

Methimazole is administered orally. The total daily dosage is usually given in 3 divided doses at approximately 8 hour intervals.

Adults: The initial daily dosage is 15 mg for mild hyperthyroidism, 30 to 40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, divided into 3 doses at 8 hour intervals. The maintenance dosage is 5 to 15 mg daily.

Pediatric: Initially, the daily dosage is 0.4 mg/kg of body weight divided into 3 doses and given at 8 hour intervals. The maintenance dosage is approximately 1/2 of the initial dose.

Use in children & adolescents: Because of post-marketing reports of severe liver injury in pediatric patients treated with propylthiouracil, Methimazole is the preferred choice when an antithyroid drug is required for a pediatric patient.

With medicine: Oral anticoagulants (e.g., warfarin) — Methimazole may inhibit vitamin K activity, thereby increasing the effect of oral anticoagulants. Additional monitoring of PT/INR is recommended, especially before surgical procedures.
β-adrenergic blocking agents — Hyperthyroidism may increase the clearance of beta-blockers with a high extraction ratio. Dose reduction may be required when a hyperthyroid patient becomes euthyroid.
Digitalis glycosides — Serum digitalis levels may increase when hyperthyroid patients on stable therapy become euthyroid; dose reduction may be required.
Theophylline — Clearance may decrease when hyperthyroid patients become euthyroid; dose reduction may be necessary.

With food: Should be taken consistently in relation to meals, as food may unpredictably affect absorption.

Methimazole is contraindicated in patients with hypersensitivity to the drug or any of its components.

Common: Skin rash, urticaria, nausea, vomiting, epigastric discomfort, arthralgia, paresthesia, loss of taste, hair loss, myalgia, headache, pruritus, drowsiness, neuritis, edema, vertigo, skin pigmentation, jaundice, sialadenopathy, lymphadenopathy.

Rare: Agranulocytosis, granulocytopenia, thrombocytopenia, aplastic anemia, drug fever, lupus-like syndrome, insulin autoimmune syndrome (may cause hypoglycemic coma), hepatitis (jaundice may persist after discontinuation), periarteritis, hypoprothrombinemia, and very rare nephritis.

Pregnancy Category D. Due to rare reports of congenital malformations, alternative agents may be preferred during pregnancy, especially in the first trimester. Methimazole is excreted in breast milk; however, studies have shown no adverse effects in nursing infants when thyroid function is regularly monitored (weekly or biweekly).

Congenital malformations: Methimazole crosses the placenta and may cause fetal harm, especially in the first trimester. Patients should be informed about fetal risks. Other agents may be preferred in pregnancy. If used, the lowest effective dose should be given.

Agranulocytosis: A potentially life-threatening adverse effect. Patients should immediately report fever or sore throat. Other hematologic effects include leukopenia, thrombocytopenia, and aplastic anemia. The drug should be discontinued if agranulocytosis, aplastic anemia, ANCA-positive vasculitis, hepatitis, or exfoliative dermatitis occurs.

Liver toxicity: Risk is lower than propylthiouracil but hepatotoxicity may occur. Treatment should be stopped if liver enzymes exceed 3 times the upper normal limit.

Hypothyroidism: Methimazole may cause hypothyroidism; regular monitoring of TSH and free T4 is required. It may also cause fetal goiter and cretinism, so careful dosing is required in pregnancy.

Bleeding risk: May cause hypoprothrombinemia; PT should be monitored, especially before surgery. Thyroid function tests should be monitored regularly. Rising TSH indicates need for dose reduction.

Symptoms may include nausea, vomiting, epigastric discomfort, headache, fever, joint pain, pruritus, and edema. Aplastic anemia or agranulocytosis may develop within hours to days. Less frequent effects include hepatitis, nephrotic syndrome, exfoliative dermatitis, neuropathies, and CNS stimulation or depression. Treatment is supportive based on clinical condition.

Anti-thyroid drugs

Store in a cool (below 30°C), dry place, protected from light and out of reach of children.