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Methimazole

Class
Antithyroid agents
Subclass
Thyroid peroxidase inhibitors
Generic name
methIMAzole, thiamazole
Common formulations
Tablet
Dosage and administration
Adults patients
Symptomatic relief of hyperthyroidism • Mild, before thyroidectomy or radioactive iodine therapy
•
15 mg PO daily
Other off-label uses
•
Treatment of thyroid storm
•
Treatment of thyrotoxicosis in pregnant females with Graves' disease
Indications for use
Labeled indications
Adults
•
Treatment of Graves' disease in patients with hyperthyroidism (mild hyperthyroidism, not treatable with surgery or radioactive iodine)
•
Treatment of Graves' disease in patients with hyperthyroidism (moderate-to-severe hyperthyroidism, not treatable with surgery or radioactive iodine)
•
Treatment of Graves' disease in patients with hyperthyroidism (severe hyperthyroidism, not treatable with surgery or radioactive iodine)
•
Symptomatic relief of hyperthyroidism (mild, before thyroidectomy or radioactive iodine therapy)
Off-label indications
Adults
•
Treatment of thyroid storm
•
Treatment of thyrotoxicosis in pregnant females with Graves' disease
Safety risks
Contraindications
Hypersensitivity to Methimazole or any of its components
Warnings and precautions
Agranulocytosis
•
Use extreme caution in patients with a history of blood disorders or those taking other drugs that can cause bone marrow suppression.
Drug-induced liver injury
•
Use caution in all patients, especially in the pediatric population.
Fetal toxicity
•
Use caution in pregnant woman as, methimazole can cause fetal goiter and cretinism.
Hypothyroidism
•
Maintain a high level of suspicion as, methimazole can cause hypothyroidism necessitating routine monitoring of TSH and free T4 levels with adjustments in dosing to maintain a euthyroid state.
Vasculitis
•
Use caution in patients with a history of autoimmune diseases.
Specific populations
Renal impairment
eGFR 0-90 mL/min/1.73 m²
•
Use acceptable. No dose adjustment required.
Renal replacement therapy
Any modality
•
Use acceptable. No dose adjustment required.
Hepatic impairment
Any severity
•
Use with caution. Monitor serum aminotransferases. Monitor for toxicity.
Pregnancy and breastfeeding
Pregnancy
All trimesters
•
Avoid use. Evidence of fetal harm in humans. If a patient is, or becomes pregnant during therapy, she should be advised of the potential risks to the fetus; women of childbearing potential should discuss contraception with their healthcare provider.
•
Because congenital malformations have been associated with use of this drug, alternative anti-thyroid therapy is generally recommended during the first trimester; a switch to this drug is often recommended during the second and third trimester of pregnancy due to the potential of hepatotoxicity with propylthiouracil.
•
The lowest effective dose should be used; thyroid dysfunction often diminishes as pregnancy proceeds and dose reduction or discontinuation of therapy may be possible several weeks or months before delivery.
Breastfeeding
Acceptable for use during breastfeeding.
Low excretion in breastmilk (5-25%).
Low levels in breastfed infants (5-25%).
No overt adverse effects reported in breastfed infants.
Adverse reactions
Common 1-10%
•
Agranulocytosis, aplastic anemia, ↓ WBC count, ↓ platelet count
Unknown frequency
•
Acute liver failure, acute pancreatitis, arthritis, drug-induced liver injury, erythroderma, hypothyroidism, insulin autoimmune syndrome, jaundice, light stools, myopathy, nephrotic syndrome, parotitis, peripheral neuropathy, pharyngitis, abdominal pain, arthralgia, bloating, change in taste, dark urine, fatigue, fever, generalized pruritus, loss of appetite, nausea, skin rash, systemic lupus erythematosus, thyrotoxicosis, vasculitis
Interactions
Drug(s)
Check Interactions
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