The relationship between hypothyroidism and cardiovascular diseases in dogs is frequently associated with clinical signs like sinusal bradicardia, myocardial alterations and ECG findings such as low voltage QRS and inverted T wave. Echocardiographic studies in hypothyroid dogs have not identified pericardial effusion. Hypothyroidism in human patients is a well-known cause of pericardial effusion, but cardiac tamponade is not a frequent clinical sign. A Golden Retriever was presented with a clinical history of progressive lethargy, exercise intolerance, cold intolerance and increased respiratory effort for 3 weeks. At the clinical examination the dog presented dyspnea, muffled heart sound and hypertension. Echocardiography showed fluid in the pericardial sac and cardiac tamponade and pericardiocentesis was performed. Analyses of the fluid showed a non-neoplastic etiology. The information given by the owners, the alopecia zones detected in tail and abdomen and the clinical response, led to analise the thyroid hormones, and hypothyroidism was confirmed. Following two pericardiocentesis and levothyroxine supplementation, the effusion resolved. Although the etiology of pericardial effusion was not clear, it is possible that the pericardial effusion had been caused by hypothyroidism or the dog had idiophatic pericardial effusion and coincidental hypothyroidism. However, according to the clinical signs and the evolution of the dog, an evaluation of thyroid hormones in dogs with pericardial effusion must be considered even though pericardial effusion is a non-described sign in canine hypothyroidism.