Thyroid Hormones Increase the Transcription of Large Numbers of Genes
The general effect of thyroid hormone is to activate nuclear transcription of large numbers of genes (Figure 76–5). Therefore, in virtually all cells of the body, great numbers of protein enzymes, structural proteins, transport proteins, and other substances are synthesized. The net result is generalized increase in functional activity throughout the body.
Most of the Thyroxine Secreted by the Thyroid Is Converted to Triiodothyronine. Before acting on the genes to increasegenetic transcription, one iodide is removed from almost all the thyroxine, thus forming triiodothyro-nine. Intracellular thyroid hormone receptors have a very high affinity for triiodothyronine. Consequently, more than 90 per cent of the thyroid hormone mole-cules that bind with the receptors is triiodothyronine.
Thyroid Hormones Activate Nuclear Receptors. The thyroidhormone receptors are either attached to the DNA genetic strands or located in proximity to them. The thyroid hormone receptor usually forms a het-erodimer with retinoid X receptor (RXR) at specific thyroid hormone response elements on the DNA. Onbinding with thyroid hormone, the receptors become activated and initiate the transcription process. Then large numbers of different types of messenger RNA are formed, followed within another few minutes or hours by RNA translation on the cytoplasmic ribo-somes to form hundreds of new intracellular proteins. However, not all the proteins are increased by similar percentages—some only slightly, and others at least as much as sixfold. It is believed that most, if not all, of the actions of thyroid hormone result from the subse-quent enzymatic and other functions of these new proteins.