It is a known fact that thyroid hormones have an influence on the heart function. Opinions on hypothyroidism and its effect on dilated cardiomyopathy (DCM) development are controversial and often contradictory. This prospective study examined the potential association between hypothyroidism and DCM in dogs. A total of 35 dogs with DCM were included in the study. Preclinical DCM was diagnosed in 18 patients, and clinical DCM was diagnosed in 17 patients, four of which were also diagnosed with hypothyroidism. There was a positive correlation between the thyroid-stimulating hormone and body weight, left atrium size and the serum NT-proBNP concentration in patients with preclinical DCM. Euthyroid dogs with DCM had higher total thyroxine values and more ventricular premature complexes than hypothyroid dogs. Although our study did not confirm a direct relationship between hypothyroidism and DCM as a possible cause, other correlations were detected that had not yet been described in veterinary medicine.