Most Common Side Effect of Methimazole
The most common side effect of methimazole is gastrointestinal disturbances, particularly nausea and diarrhea. 1
Common Side Effects of Methimazole
Methimazole is an antithyroid medication commonly used to treat hyperthyroidism. While it is effective in managing thyroid disorders, it is associated with several side effects that clinicians should be aware of:
- Gastrointestinal effects: Nausea and diarrhea are the most commonly reported side effects
- Skin reactions: Rash and pruritus (itching) occur frequently
- Upper respiratory tract infections: These can develop during treatment
Serious Adverse Effects
While not common, methimazole can cause several serious adverse effects that require immediate attention:
Agranulocytosis
- Potentially life-threatening reaction occurring in approximately 1-2% of patients
- Typically develops within the first 1-4 months of treatment, but can occur as early as 6 days after starting medication 2
- Can rarely develop even after discontinuation of methimazole 3
- Symptoms include fever, sore throat, and other signs of infection
- Patients should be instructed to immediately report these symptoms to their physician
- Requires immediate discontinuation of the medication and monitoring of bone marrow indices
Hepatotoxicity
- Less common with methimazole than with propylthiouracil, especially in pediatric populations 1
- Symptoms include anorexia, pruritus, right upper quadrant pain, jaundice
- Monitoring of liver function tests is recommended
- Drug should be discontinued promptly if significant liver abnormalities develop
Vasculitis
- Severe complications including leukocytoclastic cutaneous vasculitis, acute kidney injury, glomerulonephritis, and pulmonary hemorrhage have been reported
- Most cases associated with ANCA-positive vasculitis
- May require treatment with corticosteroids, immunosuppressants, or plasmapheresis
First Trimester Use and Congenital Malformations
Methimazole crosses the placental membrane and can cause fetal harm when administered in the first trimester of pregnancy. Rare instances of congenital defects have been reported, including:
- Aplasia cutis
- Craniofacial malformations
- Gastrointestinal malformations
- Omphalocele
Due to these risks, alternative treatments may be more appropriate during pregnancy, particularly in the first trimester 1.
Monitoring Recommendations
To minimize the risk of serious adverse effects:
- Monitor complete blood count with differential, especially during the first few months of therapy
- Perform liver function tests periodically
- Instruct patients to report immediately any symptoms suggestive of agranulocytosis (fever, sore throat)
- Monitor thyroid function tests to avoid hypothyroidism
- Consider prothrombin time monitoring, especially before surgical procedures
Drug Interactions
Methimazole may interact with several medications:
- Oral anticoagulants (increased effect due to vitamin K activity inhibition)
- Beta-adrenergic blocking agents (may require dose reduction when patient becomes euthyroid)
- Digitalis glycosides (serum levels may increase when patient becomes euthyroid)
- Theophylline (clearance may decrease when patient becomes euthyroid)
Conclusion
While gastrointestinal effects represent the most common side effects of methimazole, clinicians must remain vigilant for the potentially life-threatening complications such as agranulocytosis and hepatotoxicity. Patient education regarding warning signs and regular monitoring are essential components of safe methimazole therapy.