A 4-year-old female Cocker spaniel weighing 7.74 kg was presented with a seven-month history of lethargy and skin problems. On the basis of clinical history, physical examination, laboratory tests, electrocardiogram, radiography, ultrasonography, and thyroid function tests, the dog was diagnosed as hypothyroidism. The dog was administered levothyroxine and presented a good response to the therapy. However, uncontrolled for about 6 months, the dog was re-admitted to hospital with depression and three seizure episodes. The dog was diagnosed as hypothyroidism with concurrent myxedema coma and gallbladder mucocele. With fluid therapy and administration of levothyroxine, the dog was successfully controlled for 3 months after starting treatment. This report describes the clinical features and successful treatment of a Cocker spaniel dog with concurrent hypothyroidism and myxedema coma.