A 10 year old male castrated Bichon Frise was referred to the Cornell University Hospital for Animals’ Oncology Service in July 2015 for evaluation of a neck mass that had been detected two weeks prior to presentation. His owner reported a two-month history of weight loss, excessive panting, exercise intolerance, polydipsia, and hair loss. Physical examination revealed a large, firm, fixed mass with poorly defined margins on the cranial, ventral neck in the region of the thyroid gland. On cardiothoracic auscultation, a grade IV/VI left-sided systolic heart murmur was detected. Blood chemistry, thyroid hormone panel and urinalysis results were consistent with hyperthyroidism, supporting the diagnosis of a functional thyroid tumor. The functional nature of the tumor was further supported by significant uptake of technetium-99m on scintigraphy. Given these findings, radioactive iodine treatment was pursued at Michigan State University. This case report will discuss the clinical signs, diagnostic test selection, and prognosis associated with canine functional thyroid tumors as well as the physiology of hyperthyroidism and details of radioactive iodine therapy as a treatment modality in dogs.