From the FDA Drug Label
Patients who receive methimazole should be under close surveillance and should be cautioned to report immediately any evidence of illness, particularly sore throat, skin eruptions, fever, headache, or general malaise. Inform patients that cases of vasculitis resulting in severe complications have occurred with Methimazole Inform patients to promptly report symptoms that may be associated with vasculitis including new rash, hematuria or decreased urine output, dyspnea or hemoptysis Patients should be advised that if they become pregnant or intend to become pregnant while taking an antithyroid drug, they should contact their physician immediately about their therapy Because methimazole may cause hypoprothrombinemia and bleeding, prothrombin time should be monitored during therapy with the drug, especially before surgical procedures. Thyroid function tests should be monitored periodically during therapy
The precautions to be taken when starting Methimazole (MMI) include:
- Close surveillance of patients for any evidence of illness
- Monitoring of white-blood-cell and differential counts if symptoms of agranulocytosis occur
- Reporting of symptoms associated with vasculitis, such as new rash, hematuria, or decreased urine output
- Monitoring of prothrombin time during therapy, especially before surgical procedures
- Periodic monitoring of thyroid function tests during therapy
- Warning patients about the potential hazard to the fetus if they become pregnant or intend to become pregnant while taking the drug 1, 1
From the Research
When starting methimazole, it is crucial to inform your healthcare provider about all medications you're taking and any allergies you have, especially to thionamide drugs, and to closely monitor for signs of serious side effects, as the most recent and highest quality study suggests that long-term continuation of low-dose methimazole therapy is effective and safe in preventing recurrent hyperthyroidism 2.
Key Considerations
- Watch for signs of serious side effects including fever, sore throat, unusual bleeding or bruising, yellowing of skin/eyes, dark urine, or severe fatigue, which could indicate rare but serious complications like agranulocytosis or liver problems, as reported in studies 3, 4.
- Regular blood tests will be necessary to monitor your thyroid function and blood cell counts, especially during the initial treatment period, to minimize the risk of adverse effects 5, 6.
- Take methimazole at the same time each day with or without food, and don't stop the medication without consulting your doctor even if you feel better, as consistent treatment is essential for managing hyperthyroidism 2.
- Pregnant women should discuss risks with their doctor as dosage adjustments may be needed, and patients with a history of adverse reactions to methimazole may require alternative treatments or desensitization 3, 4.
Important Precautions
- Methimazole can cause rare but serious adverse effects, such as agranulocytosis and hepatotoxicity, which require immediate medical attention 3, 4.
- Patients who experience side effects to methimazole, excluding agranulocytosis and hepatotoxicity, may be desensitized to the medication under the supervision of an allergist, allowing for continued treatment with methimazole 3.
- The long-term continuation of low-dose methimazole therapy has been shown to be effective and safe in preventing recurrent hyperthyroidism, with a significant reduction in the risk of recurrence compared to discontinuation of the medication 2.