These studies have demonstrated that the murine TSH subunit genes are sensitively regulated by thyroid hormone. The T3 receptor complex interacts with both the TSH beta and alpha-subunit gene either in or near the 5' flanking region in close proximity to the transcriptional start sites. This interaction interrupts transcription from the start sites, thus decreasing transcription of the two genes. As a result steady state mRNA levels of both TSH beta and alpha-subunit genes are decreased in the cytoplasm of thyrotropic cells. This series of interactions explains most of the effects of T3 on TSH biosynthesis.