From the FDA Drug Label
Agranulocytosis Agranulocytosis is potentially a life-threatening adverse reaction of methimazole therapy. Patients should be instructed to immediately report to their physicians any symptoms suggestive of agranulocytosis, such as fever or sore throat Leukopenia, thrombocytopenia, and aplastic anemia (pancytopenia) may also occur. The patient should be aware that low white count (leukopenia) is a potential adverse reaction of methimazole therapy, and it may be a sign of a more serious condition such as agranulocytosis. If a low white count occurs, the patient should immediately report to their physician and the drug should be discontinued if necessary 1.
From the Research
A low white blood cell count (neutropenia) is a known side effect of methimazole, which requires immediate medical attention, and according to the most recent study 2, if you're experiencing this, contact your doctor right away as they may need to reduce your dose or switch you to an alternative treatment regimen. Do not stop taking methimazole without medical guidance. Your doctor will likely order regular blood tests to monitor your counts, typically every 1-2 weeks initially, then less frequently as treatment continues. Severe neutropenia (counts below 1,000/μL) increases infection risk and may require temporary medication discontinuation. Methimazole causes neutropenia in about 0.2-0.5% of patients by triggering an immune response against white blood cells, as reported in earlier studies 3, 4. Watch for signs of infection such as fever, sore throat, or unusual fatigue. While taking methimazole, practice good hygiene, avoid sick contacts, and attend all follow-up appointments to ensure safe treatment of your thyroid condition. Some key points to consider include:
- Monitoring for cross-reactions between drugs should be performed to prevent further episodes of agranulocytosis 3
- Providing granulocyte colony-stimulating factor therapy to patients results in good prognosis 3, 5
- The management of methimazole-induced agranulocytosis and associated sequelae require subspecialty input and intensive evaluation and monitoring 2