Remnant thymic tissue as a cause of hypercalcaemia in a cat

A 2‐year‐old female spayed British Shorthair cat presented for investigation of chronic small intestinal diarrhoea, which was determined to be attributable to ionised hypercalcaemia. A cranial mediastinal mass was identified on computed tomography; median sternotomy was performed, and the mediastinal mass was excised. Histopathology, including immunohistochemistry, diagnosed the mediastinal mass as being remnant thymic tissue. Subsequent follow‐up 10 months postoperatively showed restoration of normocalcaemia and concurrent resolution of gastrointestinal symptoms. To the authors' knowledge, this is the first case to report remnant thymic tissue as a cause of hypercalcaemia. This case report aims to describe the clinical presentation, computed tomography description, histopathological findings and treatment outcome of a thymic remnant causing hypercalcaemia in a cat.