Thyroid follicular adenocarcinoma in a ferret.

A 5-year-old male castrated ferret was presented to the Washington State University College of Veterinary Medicine for evaluation of progressive hair loss and a large, rapidly growing ventral neck mass. The patient had been diagnosed previously with an insulinoma, which was managed medically. Fine-needle aspirates of the neck mass were performed. The cytologic results were most consistent with epithelial neoplasia, likely a carcinoma; thyroid origin was considered likely based on tumor location and cell morphology. The tumor grew rapidly, and the owners elected euthanasia 1 week after examination. At necropsy, a circumscribed, ovoid mass disrupted the right cervical musculature next to the right lobe of the thyroid gland. Histopathologic evaluation revealed an infiltrative mass consisting of cuboidal cells arranged in solid sheets and irregular follicles enclosing colloid. The cells were large, with prominent nucleoli, and had a high mitotic rate. The histopathologic diagnosis was consistent with thyroid follicular adenocarcinoma. Immunochemical findings confirmed thyroglobulin production by neoplastic cells, but to a lesser extent than in normal ferret thyroid tissue. To our knowledge, this is the first case of thyroid follicular adenocarcinoma to be reported in a ferret, with only 1 other case of thyroid carcinoma, a C-cell carcinoma, described previously.