Follicular-Compact-Cellular Carcinoma in the Thyroid Gland of a Dog

This case was determined accidentally in a 1.5-year-old, cross-breed male dog during a necropsy for an experimental study conducted in our laboratory, following a clinical examination as well as haematological, biochemical and urine analyses. The dog was clinically healthy. Haematological and biochemical parameters, except for a decrease in serum triiodothyronine (T3), and urine analysis findings were within the reference range. A mass 3 x 4 x 3 cm in size and 22 g in weight that was observed in the thyroid gland located in the left cranioventral cervical region at the level of the larynx was investigated macroscopically and microscopically. The cut surface of the mass was lobulated, hyperaemic and haemorrhagic and had necrotic areas. In the microscopical examination, this mass was diagnosed as follicular-compact-cellular carcinoma due to the presence of many incomplete follicular structures without colloid in their lumens, a few follicles filled with colloid in their lumens and solid areas surrounded by a fine connective tissue formed by pleomorphic, neoplastic cells resembling thyroid follicular epithelial cells and showing mitotic activity.