Generic
Methimazole
5 brands available in the market
At a glance
Methimazole is an anti-thyroid drug used for the treatment of hyperthyroidism. It works by inhibiting the synthesis of thyroid hormones.
Description
Methimazole inhibits the synthesis of thyroid hormones and is effective in the treatment of hyperthyroidism. The drug does not inactivate existing thyroxine and triiodothyronine stored in the thyroid or circulating in the blood, nor does it interfere with the effectiveness of thyroid hormones given orally or by injection. Methimazole is readily absorbed in the gastrointestinal tract, metabolized in the liver, and excreted in the urine.
Indications
- Graves' disease or toxic multinodular goiter - when surgery or radioactive iodine therapy is not appropriate
- To ameliorate symptoms of hyperthyroidism in preparation for thyroidectomy or radioactive iodine therapy
Therapeutic class
Anti-thyroid Drugs
Pharmacological class
Thyroid Hormone Synthesis Inhibitor
Pharmacology
Methimazole inhibits thyroid peroxidase enzyme to prevent the synthesis of thyroid hormones (T3 and T4).
Mechanism of action
Controls hyperthyroidism by inhibiting thyroid hormone synthesis.
Dosage
Adult Dose (Oral):
- Mild hyperthyroidism: 15 mg/day (in 3 divided doses, 8-hour intervals)
- Moderate-severe: 30-40 mg/day
- Severe: 60 mg/day
- Maintenance: 5-15 mg/day
Pediatric Dose: 0.4 mg/kg/day (in 3 divided doses); Maintenance: approximately half of initial dose
Administration
- Swallow tablet whole; do not chew, crush or break
- May be taken with or without food (food may affect absorption)
- Take at the same time each day (relative to meals)
Missed dose
If you miss a dose, take it as soon as you remember; skip if it's almost time for next dose; do not double the dose.
Side effects
Common Side Effects:
- Skin rash, urticaria
- Nausea, vomiting, epigastric distress
- Arthralgia, paresthesia
- Loss of taste, hair loss
- Myalgia, headache
- Pruritus, drowsiness, neuritis
- Edema, vertigo, skin pigmentation
- Jaundice, sialadenopathy, lymphadenopathy
Precautions & warnings
- Report immediately if symptoms of agranulocytosis (fever, sore throat) occur
- Discontinue if liver toxicity (transaminases >3x ULN) occurs
- Monitor TSH and free T4 regularly for hypothyroidism
- Avoid use in first trimester of pregnancy (risk of fetal harm)
- Caution during lactation
Contraindications
- Hypersensitivity to Methimazole or any components
Drug interactions
- Oral anticoagulants (warfarin): May increase effect by inhibiting vitamin K activity; monitor PT/INR
- Beta-blockers: Dose reduction may be needed as hyperthyroidism improves
- Digitalis glycosides: Dose reduction may be needed when euthyroid
- Theophylline: Clearance decreases when euthyroid; dose reduction may be needed
Food interactions
Food may affect absorption; take at the same time relative to meals.
Use in pregnancy
Use in Pregnancy: Methimazole is unsafe during pregnancy (Category D); risk of congenital malformations, especially in the first trimester. Use only if needed and at the lowest effective dose.
Pregnancy Category: D — always consult a doctor before taking during pregnancy.
Use in lactation
Use in Lactation: Methimazole is excreted in breast milk; however, with regular thyroid function monitoring, it is considered safe. Caution is advised.
Pediatric use
Pediatric dose is established; Methimazole is preferred over propylthiouracil due to lower risk of liver injury.
Geriatric use
Elderly may require dose adjustment (based on renal function).
Renal / hepatic impairment
Renal Impairment: Dose adjustment may be needed; caution in severe renal insufficiency.
Hepatic Impairment: Monitor for hepatotoxicity; caution in severe hepatic impairment.
Overdose effects
Overdose may cause nausea, vomiting, epigastric distress, headache, fever, joint pain, pruritus, edema; agranulocytosis or aplastic anemia may occur; symptomatic treatment.
Storage conditions
Store below 30°C in a cool and dry place, away from light; keep out of reach of children.
Chemical structure
Methimazole