Thyroid hormone regulation of the rat thyrotropin beta-subunit gene.

In order to determine the molecular mechanisms by which thyroid hormone and other factors regulate the biosynthesis of thyrotropin (TSH), we have isolated and characterized the gene encoding the beta-subunit of rat TSH. The transcriptional unit is 4.9 Kb in size and contains 3 exons and two introns of 3.9 and 0.4 Kb. The gene possesses two promoters and hence two associated transcriptional start sites. Several lines of evidence indicate that there is differential regulation of the two TSH beta transcripts by thyroid hormone. In particular, rat TSH beta mRNA-2, the transcript associated with the downstream promoter, is regulated 6- to 8-fold by thyroid hormone. Further studies involving in vitro mutagenesis and transfection of the TSH beta gene into pituitary cells will hopefully provide important information concerning structure-function relations of the regulatory regions of the gene and thyroid hormone action.