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. In such cases, white-blood-cell and differential counts should be obtained to determine whether agranulocytosis has developed
- The effects of Methimazole (MMI) on Complete Blood Count (CBC) include the risk of agranulocytosis, which may be indicated by a decrease in white-blood-cell count.
- Patients taking MMI should be monitored for signs of illness and have their white-blood-cell and differential counts checked if they report any symptoms. 1
From the Research
Methimazole can cause significant side effects on complete blood count (CBC) results, including agranulocytosis, neutropenia, thrombocytopenia, and aplastic anemia, and patients should be closely monitored for these effects, particularly during the first three months of treatment. The most serious CBC abnormality is agranulocytosis, characterized by a severe decrease in neutrophils, which occurs in approximately 0.1-0.5% of patients 2. More commonly, methimazole can cause mild to moderate neutropenia, with neutrophil counts dropping below normal ranges. Thrombocytopenia (low platelet count) and aplastic anemia (affecting all blood cell lines) are rarer but serious potential effects.
Key Points to Consider
- These hematologic changes typically occur within the first few months of treatment, with the highest risk during the initial 90 days 3.
- Patients on methimazole should have baseline CBC testing before starting treatment and regular monitoring, particularly during the first three months 2.
- Patients should be educated to report symptoms like fever, sore throat, unusual bleeding, or bruising immediately, as these may indicate blood dyscrasias 2.
- The mechanism behind these effects involves direct toxicity to bone marrow precursor cells and possible immune-mediated destruction of mature blood cells 4.
- If severe CBC abnormalities develop, methimazole should be discontinued and alternative treatments for hyperthyroidism considered 5.
Monitoring and Management
- Regular CBC monitoring is crucial to detect potential hematologic effects early 2.
- Granulocyte colony-stimulating factor (G-CSF) therapy may be beneficial in patients with severe neutropenia or agranulocytosis 2, 4.
- Patients should be closely monitored for signs of infection, and empirical antibiotic therapy should be initiated promptly if suspected 2.