Define about Iodine Metabolism?
Now, we will very briefly study how iodine is absorbed, distributed in the body and ' excreted out. Like other nutrients, dietary iodide is either found free or bound to amino acids. It is primarily found as iodide or iodate. The latter form is reduced to iodide by glutathione in the gut. Iodide is rapidly 'and completely absorbed throughout the gastrointestinal tract and very little iodine appears in faeces. Iodine bound to amino acids is also absorbed but less efficiently. The thyroid hormones: thyroxine (T4) and triiodothyronine (T3) are also absorbed unaltered. Therefore, T4 medication can be administered orally.
After absorption, free iodide appears in the blood and circulates to all tissues. Thyroid gland traps most of the ingested iodide (80%). This is achieved against an iodide gradient (often 40 to 50 times plasma concentration) by sodium-dependent active transport system. This mechanism is regulated by thyroid stimulating hormone (TSN) secreted by pituitary. Thyroid takes up almost 120 mcg of iodide per day. Other tissues such as salivary glands, gastric mucosa, choroid plexus and mammary glands also concentrate the element by a similar active mechanism. Several sulphur-containing compounds such as thiocyanate, isothiocyanate and goitrill inhibit active transport mechanism by competing for uptake with iodide. Thus, iodide uptake by thyroid gland may be reduced. These are called goitrogens and their goitrogenic activity can be overcome by iodine supplementation.