Thyroglobulin thyroid hormones, biosynthesis

FIGURE 31-2 Thyroid hormone biosynthesis. Iodide is taken into the follicle cell, where it is converted by thyroid peroxidase to an oxidized form of iodine (Ip). h is transported to the follicle lumen, where it is bonded to tyrosine residues of the thyroglobulin [TGB] molecule. Iodinated TGB is incorporated back into the cell, where it undergoes lysis to yield the thyroid hormones T3 and T4. See text for further discussion. 

Figure 38.2 Possible sites of inhibitory actions of soy isoflavones on iodine utilization and thyroid hormone biosynthesis and actions. Soy isoflavonoids, genistein and daidzein, inhibit oxidation of iodide by thyroid peroxidase at the apical membrane of thyroid follicular cells, followed by iodination of tyrosine residues in thyroglobulin and their coupling in colloid. In addition, they may affect deiodination of iodothyronines and interfere with thyroid hormone binding to transthyretin. Full arrows indicate the sites of inhibition. So far, only few reports concern the effect of thyroid hormone actions in target cells.

Figure 45-2. Biosynthesis of the thyroid hormones Tj and in the thyroid follicular cell and release into the bloodstream. Abbreviations are as follows TG, thyroglobulin MIT, monoiodotyrosine DIT, diiodotyrosine Tj, triiodothyronine T., thyroxine.

Figure 21.19 Biosynthesis of thyroid hormones as residues in the protein thyroglobulin. 

The biosynthesis of thyroid hormone proceeds with the coupling of an MIT and a DIT residue to form a triiodothyronine (T3) residue or of two DIT residues to form a tetraiodothyronine (T4) residue. T3 and T4 are stored in the thyroid follicle as amino acid residues in thyroglobulin. Under most circumstances, the T4/T3 ratio in thyroglobulin is approximately 13 1. 

Fig. 34.2. Summary of the major pathways for the biosynthesis and secretion of the thyroid hormones. When thyrotropin (TSH) binds to the TSH receptor at the basal membrane of the follicular cell, the biosynthesis of thyroglobulin (TG) is stimulated, as is that of thyroperoxidase (TPO) and the production of hydrogen peroxide. Noniodinated TG is synthesized by the rough endoplasmic reticulum of the follicular cell and secreted through the apical membrane of the follicular cell into the follicular lumen. Iodide enters the follicular cell by the iodide pump (NIS, sodium iodide symporter) and is then transported into the follicular lumen. In the lumen, the iodide is oxidized by TPO-O (a Ti-cation radical intermediate formed from TPO and hydrogen peroxide) at the apical...

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