Multifocal Myxedema and Mixed Thyroid Neoplasm in a Dog

An Old English Sheepdog developed multiple tumor-like masses bilaterally on the head, back, elbows, and hocks, and severe swelling of all digital pads. The gross lesions were the result of accumulation of myxedematous connective tissue in the dermis. Abundant glycosaminoglycan-rich ground substance was confirmed by colloidal iron, toluidine blue and alcian blue stains. The dog also had a mixed follicular-compact cellular carcinoma in the left thyroid gland. The right thyroid had a tumor composed of anaplastic mesenchymal cells forming myxomatous matrix and islands of abnormal cartilage closely integrated, and possibly contiguous, with a follicular-compact cellular carcinoma. The cellular atypism and numerous aberrant mitotic figures in the mesenchymal areas suggested malignancy, although pulmonary metastases derived only from the thyroid carcinoma. Clinically, the dog showed no signs of hypo- or hyperthyroidism, although a resting serum T-4 was slightly below normal.