A 7-year-old female American cocker spaniel presented with chronic regurgitation and megaesophagus onradiography at a local hospital. Dry and coarse hair coat, tail alopecia, and bradycardia were observed during physicalexamination. Mild, non-regenerative anemia was indicated by complete blood count (CBC) analysis. Concurrenthypercholesterolemia and hypertriglycemia were indicated by biochemistry tests. Decreased levels of total and freethyroxine (T4) and increased canine thyroid stimulating hormone (cTSH) levels were also detected. Megaesophaguswas confirmed by radiographic examination. Based on the results, a diagnosis of hypothyroidism with megaesophaguswas made. The patient was initially treated with levothyroxine sodium at a dosage of 0.02 mg/kg twice a day. Aftertwo weeks of treatment for hypothyroidism, the dilated esophagus returned to its original size and no further regurgitationwas observed. No relapse was observed during two months of follow-up.