Thyrotropin receptor (TSHR) mRNA expression has previously been detected in human heart, suggesting a possible role for the receptor in cardiac function and pathophysiology. In the present study we examined the regional distribution of TSHR mRNA in pig heart to map potential cardiac sites of TSH action. Polyadenylated mRNA extracted from thyroid, atria, ventricles, aorta, coronary arteries, epicardial fat, and purified preparations of atrial and ventricular cardiomyocytes was subjected to reverse-transcriptase polymerase chain reaction (RT-PCR) using primers designed to amplify a 311 base pair (bp) DNA segment of the human TSHR. After reverse transcription of 100 ng mRNA, cDNA was amplified by PCR using TSHR primers and compared by electrophoresis on 2% agarose gels. Relative levels of TSHR cDNA (normalized to glyceraldehyde 3-phosphate dehydrogenase [GAPDH]) were as follows: Coronary arteries, epicardial fat > right atrium > left atrium > right ventricle, aorta > left ventricle, ventricular cardiocytes. In contrast to ventricular cardiocytes, purified atrial cardiocytes expressed levels of TSHR mRNA readily detectable with RT-PCR. These findings demonstrate that TSHR mRNA expression in porcine heart varies regionally, and furthermore suggest that areas of highest expression (coronary arteries, adipose tissue, right atrium) are potential sites for a functional or pathologic role of the TSHR.