Can methimazole (antithyroid medication) cause bone marrow failure?

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Last updated: June 10, 2025 • View editorial policy

From the FDA Drug Label

Agranulocytosis is potentially a life-threatening adverse reaction of methimazole therapy. ... Leukopenia, thrombocytopenia, and aplastic anemia (pancytopenia) may also occur. The drug should be discontinued in the presence of agranulocytosis or aplastic anemia (pancytopenia), and the patient’s bone marrow indices should be monitored

Methimazole can cause bone marrow failure, as evidenced by the potential for agranulocytosis, leukopenia, thrombocytopenia, and aplastic anemia (pancytopenia). Patients should be monitored for these conditions, and the drug should be discontinued if they occur 1.

From the Research

Methimazole can cause bone marrow failure, although this is a rare but serious adverse effect. Methimazole, a common antithyroid medication used to treat hyperthyroidism, typically works well with manageable side effects for most patients. However, it can occasionally cause agranulocytosis (severe reduction in white blood cells) or pancytopenia (reduction in all blood cell types), which can lead to bone marrow failure. This typically occurs within the first 3 months of treatment, with the highest risk in the first 90 days, as reported in a case study published in the Thyroid journal 2. The risk is estimated at 0.2-0.5% of patients taking the medication.

Some key points to consider:

  • Patients taking methimazole should be aware of warning signs including fever, sore throat, mouth ulcers, unusual bleeding or bruising, and fatigue, which warrant immediate medical attention and blood count testing.
  • Regular blood monitoring is often recommended during the initial treatment period.
  • The mechanism involves an immune-mediated reaction where the drug or its metabolites trigger an immune response against bone marrow cells, as suggested by in vitro studies of CFU-C inhibition of peripheral mononuclear cells 3.
  • If bone marrow suppression occurs, the medication must be discontinued immediately and alternative treatments for hyperthyroidism considered, such as the use of recombinant human granulocyte/monocyte colony stimulating factor (rHu GM-CSF) to restore normal granulocyte count 4.
  • The prognosis of aplastic anemia during the treatment of antithyroid therapy is rather good, especially with prompt treatment and discontinuation of the offending medication, as reported in a case review published in Recenti progressi in medicina 5.
  • Strategies for monitoring, early detection, diagnostic confirmation, and appropriate supportive care are well developed for cytotoxic therapy and can be applied to idiosyncratic drug-induced myelosuppression, as discussed in a review article published in Drug safety 6.

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.