Home Drug Guide Methimazole

Drug Guide

Generic Name

Methimazole

Brand Names Tapazole

Classification

Therapeutic: Antithyroid agent

Pharmacological: Thionamide

FDA Approved Indications

Treatment of hyperthyroidism, including Graves' disease

Mechanism of Action

Inhibits thyroid hormone synthesis by blocking the oxidation of iodide to iodine and blocking iodo-organic compound formation, leading to decreased thyroid hormone production.

Dosage and Administration

Adult: Initial dose typically 15-30 mg daily, adjusted based on thyroid function tests.

Pediatric: Dose varies; typically 0.5-1 mg/kg/day divided into 1-2 doses.

Geriatric: Use with caution, start at lower doses, and monitor thyroid function.

Renal Impairment: No specific dosage adjustment necessary.

Hepatic Impairment: Use with caution; monitor liver function.

Pharmacokinetics

Absorption: Well absorbed from the gastrointestinal tract.

Distribution: Bound minimally to plasma proteins.

Metabolism: Metabolized in the liver.

Excretion: Excreted primarily in the urine.

Half Life: Approximately 5-6 hours.

Contraindications

Hypersensitivity to methimazole or other thionamides.
Pregnancy (especially during first trimester) due to risk of fetal developmental issues

Precautions

Monitor for agranulocytosis, liver toxicity, and rash.
Use with caution in patients with hepatic or hematologic conditions.

Adverse Reactions - Common

Rash (Common)
Arthralgia (Common)
Gastrointestinal upset (Common)

Adverse Reactions - Serious

Agranulocytosis (Rare but serious)
Hepatotoxicity (Rare but serious)
Vasculitis (Rare)

Drug-Drug Interactions

Other antithyroid drugs, anticoagulants, beta-blockers

Drug-Food Interactions

N/A

Drug-Herb Interactions

N/A

Nursing Implications

Assessment: Monitor thyroid function tests regularly, especially in the first few months of therapy.

Diagnoses:

Risk for bleeding (due to agranulocytosis)
Impaired hepatic function

Implementation: Administer with food to reduce gastrointestinal upset, monitor for signs of agranulocytosis and hepatotoxicity.

Evaluation: Assess for clinical signs of hyperthyroidism resolution and monitor adverse effects.

Patient/Family Teaching

Instruct to report sore throat, fever, rash, jaundice immediately.
Warn about possible side effects including rash, fever, sore throat, or jaundice.
Advise adherence to scheduled blood tests.

Special Considerations

Black Box Warnings:

Severe liver injury and acute hepatic failure, often fatal, have been reported.
Agranulocytosis leading to severe infections is a serious adverse effect.

Genetic Factors: Not specifically tied to genetic testing.

Lab Test Interference: May alter the results of thyroid function tests, leading to a transient increase or decrease in hormone levels.

Overdose Management

Signs/Symptoms: Nausea, vomiting, abdominal pain, dizziness, weakness, potential signs of agranulocytosis or hepatotoxicity.

Treatment: Supportive care, monitor blood counts and liver function, activated charcoal if ingestion was recent, no specific antidote.

Storage and Handling

Storage: Store at room temperature away from moisture and light.

Stability: Stable under standard storage conditions.