Abstract
This report describes the gross, histological, and immunohistochemical features of medullary thyroid carcinoma (MTC) with pulmonary metastases in a young dog. Sheets of pleomorphic cells supported by fibrous stroma characterized the primary mass, while metastatic nodules had a neuroendocrine pattern. Despite differing histologic features, all masses showed marked immunoreactivity against calcitonin and multiple neuroendocrine markers consistent with MTC. Although MTC is a well-recognized entity, it may be difficult to distinguish this mass from other thyroid neoplasms, necessitating immunohistochemical characterization.