Adverse side effects, including agranulocytosis and anemia, from methimazole treatment of a hyperthyroid cat.

A 13-year-old spayed female domestic shorthair cat was brought to a veterinary clinic because of lethargy and anorexia. The cat had a history of hyperthyroidism and had been treated with methimazole for 40 d before the appointment. Hematologic examination revealed a severe non-regenerative anemia and leukopenia characterized by severe neutropenia along with low basophil and eosinophil numbers. Discontinuation of methimazole resulted in the complete return to normal values of the white blood cell numbers and a marked increase in red blood cell numbers. This case demonstrated the need for follow-up veterinary examinations after prescription of methimazole treatment.