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.