Little is known about the cardiac expression of different thyroid hormone receptor (TR) isoforms. The aim of the study was to investigate such patterns of TR expression at the protein level in different species and in some human tissues. Western blot analysis with specific polyclonal rabbit antibodies to each TR isoform was performed with samples from myocardium of the left ventricle from man, dog, guinea pig, rat and mouse, as well as with samples from several human tissues such as heart, skeletal muscle, brain, liver and thyroid. The TR alpha 1 isoform was present in all of the species examined. The TR alpha 2 was recognized in human, dog and guinea pig heart, while no such band was recognized in rat and mouse hearts. TR beta 1 was not detected in the human heart but in the other species. Similarly to TR alpha 1, TR beta 2 was detected in all of the species examined. In the human tissues studied, TR alpha 1 was detected in heart and skeletal muscle, whereas TR alpha 2 was found only in the heart. TR beta 1 was not detected in any of the examined human tissues, while TR beta 2 was found in all of them. These results revealed unique distributions of TR variants and they demonstrate common epitopes in TR in the different species. For the first time, the presence of a TR beta 2 isoform has been shown in human tissues. TR isoforms may have a tissue and species specific role in the regulation of gene expression and may in part explain variable tissue effects of thyroid hormones.