Lymphocytic thyroiditis with an oncocytic alteration in a laboratory beagle.

Histopathological, immunohistochemical, and ultrastructural characteristics of lymphocytic thyroiditis in an untreated four-year-old male beagle were described. Histopathologically, the thyroid glands were composed of two distinct cell types: round to oval cells with eosinophilic granular cytoplasm (Type A), which is consistent with the features of oncocytic oxyphils, and larger round cells with amphophilic or pale cytoplasm (Type B). These cell types extensively and diffusely infiltrated with a mixture of lymphocytes and plasma cells, destroying the follicular structure. Immunohistochemistry revealed that Type A cells were positive for thyroglobulin and cytochrome C, and that Type B cells were positive for calcitonin, synaptophysin, and cytochrome C. These results indicate that Type A and B cells stem from follicular and C cells, respectively. Ultrastructural investigation showed that microfollicles and microvilli were evident in the cytoplasm and along the luminal surface of Type A cells. Thus, the lymphocytic thyroiditis observed in the beagle exhibited a morphology similar to that of Hashimoto thyroiditis in humans, particularly in view of an oncocytic alteration of follicular cells.