Thyroid Hormones

Iodine is incorporated into the tyrosine residues of thyroglobulin. Tyrosine is first iodized to monoiodotyrosine and then to diiodotyrosine, which later enzy­matically couple together and form thyi roxine and triiodothyronine. The synthe­sis and rel ease of thyroid hormones are controlled by the thyrotropin-releasing hormone—thyrotropin axis. Secretion of the thyrotropin-releasing hormone and thyrotropin is negatively regulated by thyroid hormone. The general effect of thyroid hormone is to activate transcrip­tion of nuclear target genes. Thyroid hor­mone affects diverse biological processes. The processes are affected through gene regulation when thyroid hormone binds to the receptors of the nuclear or through non-genomic actions when hormone binds to cell surface and cytoplasmic pro­teins. Two major isoforms of thyroid hor­mone receptors are TRa and TRp and they are expressed in almost all tissues of the body, however, the relative expresision of these isoforms may vary among tissues. The different expression of thy­roid hormone receptors in tissues can also have different effect on them. Recep­tors can stimulate or repress the expres­sion of target genes. Thyroid hormones can modify cardiac metabolism and func­tions via multiple molecular mecha­nisms; it may affect bones via stimulation of growth hormone and insulin-like growth factor or may directly affect tar­get genes. Thyroid hormone plays impor­tant role in the development and func­tioning of brown and white adfpose tissues, has various effects on liver func­tions, including stimulation of enzymes regulating lipogenesis and lipolysis as well as oxfdative processes; also thyroid hormone highly effects the development of brain in the fetus and during the neo­natal period.