Thyroid hormone action

The range of effects of thyroid hormone is possibly the most varied of all vertebrate hormones, as is evidenced by the gross alterations in normal physiology observed in hypo- and hyperthyroidism. Thyroid hormone plays essential roles in the development and differentiation of practically all tissues. In adult life it sets the capacity and rate of many cellular processes. Thyroid hormone acts primarily by modulating the expression of a host of cellular components. Research conducted during the past two decades has revealed an unexpected complexity in the regulation of gene expression and has greatly increased our understanding of the action of thyroid hormone. Thyroid hormone (TH) plays essential roles in the development, differentiation, and maintenance of metabolism in all tissues. TH (T₃ or T₄) is initially transported into the cell by specific transporters. Cytoplasmic conversion of inactive T₄ to T₃ the preceeds TH transport into the nucleus where T₃ binds to the nuclear TH receptor. There are known nongenomic effects as well. Cofactors then modulate the activated TH receptor which activates or inhibits transcription of a TH responsive gene.