Spontaneous bilateral thyroid follicular cell carcinoma (subtype: compact cellular carcinoma) with C-cell complexes in a male beagle.

We report the features of spontaneous bilateral thyroid follicular cell carcinoma in a 10-year-old male beagle. Necropsy revealed bilateral masses on the trachea, corresponding to the left and right sides of the thyroid gland. The masses were elastic, encapsulated, and distinct, with no connecting tumor tissues between them. Histologically, the tumor cells exhibited a predominant sheet-like growth pattern in both masses, and small follicular structures containing colloids were observed. Immunohistochemically, >50% of the tumor cells were positive for thyroglobulin. In the sheet-like growth area, all tumor cells were positive for cytokeratin and approximately 50% of them were positive for vimentin. The tumor cells were negative for calcitonin and parathormone. Electron microscopy of the tumor cells revealed colloid droplets and lysosomes in the cytoplasm, which are characteristics of follicular cells of the thyroid gland, although they were abnormally shaped and smaller in size compared to the normal cells. Many calcitonin-positive C cells were observed in the nodule area without a capsule in the left mass and were scattered within the tumor in the right mass. C cells were found individually and were negative for Ki-67 expression. Therefore, each of these cells was deemed to be derived from an individual C-cell complex. Based on these morphological features, the tumor was diagnosed as spontaneous bilateral thyroid follicular cell carcinoma of the compact cellular carcinoma subtype. This is the first report of electron microscopic findings and co-expression of cytokeratin and vimentin in thyroid follicular cell carcinoma in beagles.