Life-threatening haematological complication occurring in a cat after chronic carbimazole administration

CASE SUMMARY: An 11-year-old spayed female domestic shorthair cat with a history of hyperthyroidism treated with carbimazole for 7 months was presented for a check-up after a few episodes of vomiting. The cat had been receiving prednisolone at 0.5 mg/kg PO q12h for recent pancreatitis and concurrent inflammation of liver and small intestines confirmed by biopsies. Clinical examination revealed pale mucous membranes with a capillary refill time of <2 s. Haematology showed severely decreased packed cell volume (16%), and increased prothrombin time (42 s), partial thromboplastin time (>120 s) and fibrinogen serum concentration (3.5 g/l). Morphological changes of thrombocytes in the absence of thrombocytopenia were also noted. In-saline agglutination test was positive. Abdominal radiographic and ultrasonographic examinations excluded the presence of organ abnormalities and peritoneal effusion. Blood biochemistry was unremarkable. Feline leukaemia virus and feline immunodeficiency virus tests were negative. On the basis of these findings, immune-mediated anaemia secondary to chronic carbimazole administration was suspected. Prednisolone was increased to 2 mg/kg PO q24h and carbimazole tablets were stopped. Despite close monitoring and intensive care, the cat died the same evening of admission to the hospital. RELEVANCE AND NOVEL INFORMATION: This report suggests that severe haemotoxicity may occur as a sequel of chronic carbimazole administration in cats. Routine bloodwork and accurate follow-up of cats under treatment with thyrotoxic therapy may be advisable, in order to detect haematological changes before lethal complications occur.