Horner's syndrome associated with a functional thyroid carcinoma in a dog.

A seven-year-old entire male Irish setter was presented because of a neck mass, prolapse of the third eyelid and apparent drooping of the upper eyelid. Historical findings included increased appetite as well as polyuria and polydipsia for about two weeks. The most remarkable findings on physical examination were right-sided Horner's syndrome, pre-scapular lymphadenopathy and a large, ventral cervical mass. Lateral cervical radiographs showed a large, soft tissue opacity surrounding the trachea and retropharyngeal area which was causing displacement and narrowing of the cervical trachea and oesophagus. Results of thyroid testing suggested hyperthyroidism. At necropsy, a large, invasive tumour was identified in the ventral cervical region and multiple metastases of various sizes were detected in the lungs. Histopathological examination of the tumour revealed follicular thyroid carcinoma and confirmed widespread pulmonary metastasis.