Microminerals (trace elements)Iodine Dr. Sherin Bakhashab
Iodine • Sources: • Iodized table salt will provide daily body needs. • Fish, and sea food. • Recommended daily allowance (RDA):for adult100-150 µg/ day. • Absorption: Iodine is usually ingested as an iodide (I-) or iodate compound (IO3) and is rapidly absorbed in the intestine. • The total iodine content of the body 15-20 mg of iodine, 70-80% of which is stored in the thyroid gland.
Iodine • Required to produce thyroid hormones that control cell metabolism, neuromuscular tissue growth and development, especially the fetal perinatal brain. • These hormones require iodine for bioactivity. • The iodine present in the thyroid gland is distributed as follows:
Synthesis and secretion • Thyroxine (T4) and triiodothyronine (T3) are the physiologically active substances that are manufactured by the thyroid and are iodine containing compounds. • Only ¼ or 1/5 of the total iodine in the gland is present in the form of T3 , T4 . • 93 % of metabolically active hormones is secreted by the thyroid gland = Thyroxine (T4) • 7 % = triiodothyronine (T3) • Triiodothyronine is about 4 times as potent as T4
Synthesis and secretion • Physiologic anatomy: • Composed of large number of closed follicles. • Filled with a secretory substance called colloid. • The colloid contains the protein thyroglobulin which is the storage form of thyroxine.
Synthesis and secretion • Thyroglobulin: • Is a glycoprotein having a total of 140 tyrosine residues. • It is synthesized by thyroid follicular cells and then secreted into the cavity which is filled with the colloid.
Thyroid hormone synthesis • Iodine trapping: • The absorbed iodide present in the diet is trapped into the follicular cells by an active-transport mechanism that utilizes energy derived from Na+ , K+-ATPase. • Trapping iodide may be inhibited by certain pharmacological agents: thiocynates (-SCN), perchlorates (-ClO4), nitrates (-NO3). These act by competing with iodide for transport mechanism
Thyroid hormone synthesis • Coupling: • MIT + DIT → T3 • DIT + DIT → T4 • Proteolysis: • The release of T3 and T4 involves proteolytic degradation of thyroglobulin. • MIT and DIT are also released in this process, and they are deiodinated and hence do not leave the cell. • T3 and T4 are released to the blood stream. • Deiodination of MIT and DIT requires deiodinase and NADPH.
Transport of T3 and T4 and Excretion • Within the plasma T3 and T4 are transported almost in association with 2 proteins, and so called thyroxine-binding protein which acts as carrier agent for the hormones • 99% of thyroid hormones are bound to: • Thyroxine-binding globulin • Thyroxine-binding prealbumin and albumin • T3 and T4 are metabolized in the peripheral tissue by deamination and decarboxylation to tetra- and tri-iodoacetic acid. • Deiodination may occur in the peripheral tissues, and the liberated iodide is excreted in the urine.
Iodine Deficiency • Iodine deficiency disorders refer to spectrum of health consequences resulting from inadequate intake of iodine.