What is Methimazole?
Methimazole is an antithyroid medication (thioamide) that inhibits thyroid hormone synthesis and is used to treat hyperthyroidism, including during pregnancy and in pediatric patients. 1
Mechanism of Action
Methimazole works by inhibiting the synthesis of thyroid hormones within the thyroid gland. 1 Specifically:
- Blocks thyroid hormone production by interfering with iodine organification in the thyroid gland 2
- Does not inactivate existing thyroid hormones already stored in the thyroid or circulating in the bloodstream 1
- Does not interfere with exogenous thyroid hormone administration (oral or injectable) 1
- Has additional antioxidant and immunomodulatory effects by scavenging hydrogen peroxide and modulating interferon-gamma signaling pathways in thyroid cells 3
Pharmacokinetics
- Readily absorbed from the gastrointestinal tract with bioavailability of 80-95% 1, 4
- Virtually non-protein-bound in circulation (unlike propylthiouracil which is 80% protein-bound) 4
- Volume of distribution approximately 40L 4
- Half-life of 3-5 hours with total clearance around 200 mL/minute 4
- Metabolized in the liver and excreted in the urine, with less than 10% excreted unchanged 1, 4
- Concentrated in thyroid tissue, where it exerts effects for periods exceeding measurable serum concentrations 4
Clinical Uses
Primary Indication
Special Populations
Pregnancy:
- Effective for treating hyperthyroidism in pregnant women as a thioamide option 5
- Crosses the placenta and can potentially cause fetal goiter and cretinism if dosed excessively 1
- Recent evidence shows no significant differences between propylthiouracil and methimazole in newborn cord-blood thyroid function tests, aplasia cutis rates, or fetal anomalies 5
- Safe for breastfeeding mothers when used appropriately, with monitoring of infant thyroid function recommended 5, 1
- May be preferable to switch to methimazole for second and third trimesters due to propylthiouracil's hepatotoxicity risk 1
Pediatric Patients:
- Preferred antithyroid drug for children due to severe liver injury reports with propylthiouracil 1
Dosing Strategy
- Goal is to maintain free T4 or free thyroxine index in the high-normal range using the lowest possible dose 5
- Monitor free T4 or FTI every 2-4 weeks during initial treatment 5
- Initial doses typically 40-60 mg daily, then reduced to maintenance doses of 5-10 mg daily 2
- Beta blockers (e.g., propranolol) can be added to control symptoms until thyroid hormone levels decrease 5
Adverse Effects and Monitoring
Serious Adverse Effects
- Agranulocytosis - typically presents with sore throat and fever; requires immediate discontinuation and CBC 5
- Pancytopenia - can occur even with minimal exposure and very low cumulative doses; may be fatal and refractory to standard treatments 6
- Hepatitis 5
- Vasculitis 5
- Thrombocytopenia 5
Minor Adverse Effects
- Urticaria - can be managed with gradual reintroduction protocol using antihistamines 7
Important Monitoring Considerations
- Routine CBC monitoring is not recommended by current guidelines due to rapid onset of agranulocytosis and lack of evidence for monitoring effectiveness 6
- Patient education is critical - instruct patients to discontinue immediately and seek medical attention if they develop fever, sore throat, severe infection, easy bruising, or signs of anemia 6
- Adverse effects are not dose-related for serious hematologic complications 6
Drug Interactions
- Digitalis glycosides: Serum levels may increase as patients become euthyroid; reduced digitalis dosing may be needed 1
- Theophylline: Clearance may decrease as patients become euthyroid; reduced theophylline dosing may be needed 1
Comparison to Propylthiouracil
- Methimazole does not inhibit peripheral T4 to T3 conversion (unlike propylthiouracil) 4
- Higher placental and breast milk transfer due to greater lipid solubility compared to propylthiouracil 4
- Generally preferred over propylthiouracil except in first trimester pregnancy, due to propylthiouracil's hepatotoxicity risk 1, 8