Thyroid hormones play an important role on growth, development and metabolism of all vertebrates. Serum concentration of thyroid hormones are controlled by TRH, somatostatin and TSH which determine the rate of hormone biosynthesis and release, as well as by deiodinases (mainly type I), enzymes which generate in peripheral tissues, approximately 75% of the T3 present in serum, from circulating T4; type II deiodinases, on the other hand, are supposed to generate T3 for the tissues in which they are expressed. The biological effects of thyroid hormones are elicited by means of their interaction with specific nuclear receptors present in specific regions of the DNA, mechanism by which they induce or repress important target genes, controlling, as a consequence, the synthesis of specific proteins. However, some thyroid hormone effects are elicited in seconds or minutes, which suggests the involvement of a non-genomic mechanism of action.
Thyroid hormones; Deiodinases; Target genes; Biological effects; Genomic actions; Non-genomic actions