Ectopic thyroid carcinoma in the left atrium in a horse

Summary A 25‐year‐old Irish sport horse was presented for investigation of a 7‐day history of apathy and lethargy. Clinical examination revealed tachycardia, tachypnoea, distended, pulsating jugular veins and ventral oedema. Serum biochemistry showed azotaemia. The ultrasonographic examination of the abdomen and the thorax revealed peritoneal, pleural and pericardial effusion. Following the assessment, the mare was euthanised and a full post‐mortem examination was performed. The main findings included a ruptured mass in the left atrium of the heart with subsequent haemopericardium. By histological examination and immunoreactivity for thyroglobulin‐1, the cardiac mass could be further classified as a thyroid carcinoma. As there was no evidence of thyroid neoplasia in the rest of the carcass, it is likely that the neoplasia originated from ectopic thyroid tissue in the heart. Additional findings included multiple intestinal haemangiomas and perianal melanomas. The case report indicates that cardiac thyroid carcinoma may be a possible differential diagnosis in horses diagnosed with congestive heart failure.