Absorption and Regulation of Iodine

Absorption of Iodine

Iodine from the diet is absorbed throughout the gastrointestinal tract. Dietary iodine is converted into the iodide ion and rapidly absorbed in the intestine. The iodide ion is bio-available and absorbed totally from food and water. Iodide entering the circulation is actively trapped by thyroid glands.

About 20% of iodine of the total iodine, is taken up by thyroid glands, and the rest of 80% is taken up by kidneys for excretion.?

Out of stored iodine in thyroid glands, 95% is present in the form of thyroglobulin. Out of this 95%, 2/3 is present in the form of iodothyronine, which is the inactive form, and 1/3 is present in the active form, which is T3 and T4.

The fate of Iodine in Thyroid Glands:

The thyroid glands contain thyroid follicles the lumen of these cells is full of colloid. Follicular cell synthesizes thyroglobulin (a glycoprotein) from carbohydrates and amino acids including tyrosine, obtained during the circulation of blood. Thyroglobulin moves into the lumen of follicular cells where it becomes available for the production of hormones.

NIS System:

The sodium/iodide cotransporter, also known as the sodium/iodide symporter (NIS) is a protein. It is a transmembrane glycoprotein that transports two sodium cations (Na+) for each iodide anion (I?) into the cell. NIS-mediated uptake of iodide into follicular cells of the thyroid gland is the first step in the synthesis of thyroid hormone The active transport of iodine is based upon the counter-transport of sodium and iodine in capillaries. The active transport can be inhibited by Anion thiocyanate.

Production of Thyroid Hormones:

Thyroid peroxidase catalyzes the oxidation of iodide into active form I2. The conversion process requires hydrogen peroxidase and thyroid peroxidases.

Iodide H2o2+Thyroid peroxidases→ Iodine

From blood to cell iodine is taken up by the NIS system, but from cell to lumen iodide is transferred to the lumen by the tendon. This active I2 binds with tyrosine in thyroglobulin to form mono or diiodotyrosine. These join to form thyroid hormones triiodothyronine (T3) and thyroxine (T4).

The summary of the production of Thyroid hormones is as follows:

The fate of Iodine in Kidneys:

The excretion of iodine in the urine is a good measure of iodine intake. In a normal population with no evidence of clinical iodine deficiency, either in the form of endemic goiter or endemic cretinism, urinary iodine excretion reflects the average daily iodine requirement. Therefore, for determining the iodine requirements, the important indexes are serum T4 and TSH levels (indicating normal thyroid status) and urinary iodine excretion.?

Regulation of Iodine

Regulation of Iodine by the Body:

The body needs iodine to make thyroid hormones. These hormones control the body's metabolism; and many other important functions. The body also needs thyroid hormones for proper bone and brain development during pregnancy and infancy. Iodine is mostly concentrated in the thyroid gland. The thyroid gland actively absorbs?iodine?from the blood to produce and release these hormones into the blood, actions regulated by a second hormone, called thyroid-stimulating hormone (TSH), produced by the pituitary gland.

Regulation of Iodine by Diet:

??A healthy adult body contains 15-20 mg of iodine, 70-80% of which is stored in the thyroid gland. Thyroid glands can store iodine for up to 3 months.?Iodine is found naturally in some foods and is also added to salt that is labeled as "iodized." You can get recommended amounts of iodine by eating a variety of foods, including Fish (cod and tuna), seaweed, shrimp, and other seafood, which are generally rich in iodine.?