Thyroid tumors in dogs are relatively uncommon, representing around 1% to 4% of all neoplasias in the species, with thyrotoxicosis, or hyperactivity, being rare (<10% of cases). A female pinscher, aged 8 yrs, was taken to the Veterinary Hospital of Anhembi-Morumbi University, and presented with a large volume increase in the cervical region, a two-month evolution, dyspnea, coughing, and was eurhythmic, in good general health status. At radiological assessment, a decrease in the rostral third of the tracheal lumen was observed, adjacent or superposed in relation to the larynx, with soft tissue increase in the ventral region of the epiglottis and larynx. Considering the extension of the finding, a complete surgical resection was not possible, and an incisional biopsy was performed. The histopathological evaluation disclosed a small-cell carcinoma of the thyroid, with infiltrative growth that invaded the adjacent tissues. Laboratory assessment showed an increase of thyroid hormones, characterizing a functional follicular carcinoma of the thyroid [T4T = 4.59 μg/dL; T3T = 407.33 ng/dL; T4L= 4.8 ng/dL (radioimmunoassay, Coat-a-count®, DPC, USA; Laboratório Rhesus Veterinária, São Paulo)]. Chemotherapy with doxorubicin was started, 30mg/m²/IV every 3 weeks, and prednisone, 1mg/kg/PO/SID. There was a considerable initial improvement; however, after two cycles of chemotherapy, mass growth recurred, with progressive dyspnea and dysphagia. At the X-ray, an almost total occlusion of the trachea was observed in its rostral portion, and at this moment, a tracheotomy was performed. Despite the procedures, the evolution remained unfavorable, and the animal was sacrificed. The diagnosis of follicular carcinoma of thyroid was confirmed by a posterior anatomohistopathological assessment. It is noteworthy the fact that this is a report of a functional carcinoma, with consequent thyrotoxicosis.