Thyroglobulin, Calcitonin, Ki 67 and CEA Immunohistochemical Staining of Dog Thyroid Carcinomas

Immunohistochemical studies were conducted in 20 cases of canine thyroid tumours. On histological grounds the tumours included four well-differentiated follicular and four well-differentiated medullary carcinomas. Four tumours were suspected to be of possible follicular origin but were poorly-differentiated or anaplastic (undifferentiated); similarly six tumours that were poorly-differentiated or anaplastic were suspected to represent medullary tumours. Two tumours were so poorly differentiated that they could not be readily classified. Fifteen of the tumours stained positively for thyroglobulin. Eleven of the tumours stained for thyroglobulin but not calcitonin; they included eight that were of follicular origin, two tumours that had been thought to be of medullary origin and one that was a poorly differentiated tumour not thought to be of medullary origin; immunohistochemical staining allowed their reclassification as follicular carcinoma. Four other tumours that were stained with thyroglobulin had been thought to be of medullary origin and were also stained with calcitonin; immunohistochemical staining allowed their reclassification as mixed tumours. Immunohistochemistry is essential for accurate diagnosis of canine thyroid tumours. The occurrence of dual staining is consistent with a diagnosis of mixed medullary-follicular carcinoma that is recognised in humans but has not been described previously in animals but should be added to the classification of canine thyroid tumours. In addition to medullary tumours that were stained with both calcitonin and thyroglobulin, two medullary tumours stained with calcitonin alone. Two poorly-differentiated tumours, possibly of medullary origin, and another poorly-differentiated tumour, were not stained with either thyroglobulin or calcitonin. All tumours contained Ki-67 positively stained cells, regardless of histological appearance or calcitonin/thyroglobulin staining; the results of Ki-67 staining did not correlate with the subjective classification of well or poorly differentiated tumours. In the present study only one of the cases, a poorly differentiated tumour, that was thyroglobulin-positive, stained positively for CEA.