Regulation of gene expression by thyroid hormones

This chapter describes the importance of thyroid hormones in the regulation of gene expression. Thyroid hormones have remarkable effect on differentiation and development in vertebrates. In man, for example, the lack of thyroid hormones can result in severe developmental deficiencies in the central nervous and skeletal systems. Thyroid hormones also have noticeable influence on metabolism in adults. The studies so far specify that hormones influence a number of gene products; in fact, changes in the levels of certain proteins and enzymes are known to account at least in part, for particular physiological responses to the hormone. Additionally, the finding of intranuclear receptors whose characteristics are suggestive of their involvement in thyroid responses has also directed attention to the nucleus as a potential site of thyroid hormone regulation. The chapter also describes thyroid hormone binding components in other cellular fractions. High-affinity and limited-capacity thyroid hormone binding components have also been identified in the cytosol, mitochondria, and membrane fractions. The properties of the cytosol binders have shown considerable variation as reported in different tissues. For example, the cytosol binding proteins in cultured pituitary cells have a higher affinity for thyroxine than for triiodothyronine