Hypothyroidism is prevalent among dogs and cardiac effects of overt hypothyroidism have been described, but generally appear in long term disease. Pulsed wave tissue Doppler imaging (PWTDI) has been used to assess cardiac function in clinical and preclinical hypothyroid human patients, however there are no reports of PWTDI findings in hypothyroid dogs. This is a report of one dog, referred because of extreme tiredness during exercise. Clinical exam and a routine cardiological evaluation, including conventional echocardiography, had not identified any reason for this clinical sign. The dog was overweight and had pyoderma. Laboratory results indicated hyperlipidemia. On the first thyroid profile, he presented increased TSH (44mU/l; reference: 0-37mU/l), but normal thyroid hormone levels. He was put on a diet and exercise and was evaluated monthly. Six months later he developed alopecia and the area where the hair had been clipped was still not covered. Another thyroid profile was performed, confirming hypothyroidism (free T4 = 1pmol/l and TSH = 117mU/l). He was started on levothyroxine sodium supplementation and has been followed up. Conventional echo parameters were still within normal limits, but PWTDI showed a decrease in annular septum early diastolic velocity (Em) (0.087 to 0.053m/s vs. controls = 0.091±0.025 m/s) and early to late diastolic ratio (Em/Am) (1.331 to 0.702m/s vs. controls = 1.384±0.37); mid septum Em (0.067 to 0.050m/s vs. controls = 0.085±0.022m/s) and Em/Am (0.889 to 0.685 vs. controls = 1.395±0.504m/s); left ventricular free wall Em (0.140 to 0.090m/s vs. controls = 0.120±0.027m/s). Further investigation on hypothyroid dogs and during T4 supplementation must be undertaken.