Minimal methimazole exposure-induced fatal pancytopenia

This case highlights that refractory pancytopenia leading to death can occur with methimazole treatment even at a very low cumulative dose and after a very short duration of exposure. In addition, the standard treatments to correct the pancytopenia may not be effective and a bone marrow transplant may be required. Current American Thyroid Association guidelines do not recommend routine monitoring of the complete blood count in patients receiving thionamides because of the rapidity of the onset of agranulocytosis and the lack of positive evidence that such monitoring would be useful. Patient education concerning the clinical adverse effects of pancytopenia should be stressed before drug initiation. Patients should discontinue thionamides immediately and seek medical treatment if they develop anaemia, fever, pharyngitis, severe infection or easy bruising. Physicians need to be aware that pancytopenia from methimazole is not dose-related and be alert to any patient signs or symptoms of infection. Subspecialty consultation will be required.