New advances in understanding the molecular mechanisms of thyroid hormone action.

Thyroid hormone regulation o f gene transcription is a complex process. There are multiple thyroid hormone receptors (TRs) encoded on separate genes that bind to thyroid hormone-response elements (TREs) of target genes containing different orientation and spacing of half-sites. Additionally, there are multiple TR complexes-monomers, homodimers, and heterodimers with other related nuclear proteins-which bind to TREs and may play important roles in gene transcription. Recently, it has been shown that DNA binding of these TR complexes can be differentially regulated by either ligand or TR phosphorylation. Diversity among TR complexes and TREs, as well as mechanisms for regulating TR binding to TREs, may enable sensitive and precise transcriptional control of target genes.