Giant splenic myelolipoma in a cat with hyperthyroidism.

Case summary: A 15 shorthair cat presented after having fallen down the stairs. Examination by the referring veterinarian had demonstrated tachycardia and a large abdominal mass. The cat was referred for investigations. Blood tests demonstrated hyperthyroidism. A large, poorly vascularised abdominal mass was identified on ultrasonography. The mass was hyperechoic compared with the normal liver; however, the origin could not be determined. Fine-needle aspirate biopsies of the mass demonstrated extramedullary haematopoiesis. Surgical exploration revealed a 12 cm × 8 cm × 8 cm pale mass arising from the spleen. Histopathology determined this was a giant splenic myelolipoma. Relevance and novel information: Splenic myelolipoma is rarely reported in the domestic cat, with only five cases documented within the literature, and none of these having described giant myelolipoma. Indeed, giant myelolipomas are rarely reported in the human literature and are most commonly adrenal in origin. The pathogenesis of these masses is unclear; there have been several incidences in people with endocrine disorders, and it has been hypothesised that their occurrence may be related to endocrine stimulation. Here we report the first case of giant myelolipoma in a hyperthyroid cat.