Intrapericardial ectopic thyroid carcinoma in a cat.

A 7-year-old spayed female domestic shorthair feline presented with tachycardia and was later euthanized due to a declining condition. On gross examination, the thoracic cavity contained an expansile, multiloculated mass that displaced the lungs dorsocaudally. The mass, within the pericardial sac, compressed adjacent myocardium. Cut surface revealed variably sized, fluid-filled spaces with multiple foci of hemorrhage and necrosis. Histologically, the mass was composed of solid foci of polygonal cells admixed with colloid-containing follicles. Immunohistochemical staining for thyroglobulin was positive, and staining for calcitonin was negative. Grossly, thyroid glands were normal, and serum thyroxine was within reference intervals.