Iodine Digestion and absorption
Dietary iodine is primarily bound to amino acids, but sometimes can also be found free, as either iodate (IO3-) or iodide (I++). Iodate is reduced to iodide by glutathione within the GI tract.
Dietary iodine, in the form of iodide, is rapidly absorbed by the stomach and upper small intestine. In healthy individuals, absorption is highly efficient, greater than 90%. The absorbed iodide, primarily bound to proteins, is transported by the blood to the thyroid gland and other tissues. The thyroid gland utilizes about 80% of the absorbed iodide or about 120 mcg daily. In healthy individuals, the freed up iodine resulting from the conversion of T4 to T3 can be reused to make more at a later time, as needed.
Goitrogens, mentioned earlier, are the substances existing naturally in certain foods that can interfere with iodide absorption by blocking uptake from the blood into the thyroid gland. Foods that are high in goitrogens include cabbage, kale, turnips, and other cruciferous vegetables, along with sweet potatoes, kelp, cassava, peanuts, and soybeans. Anyone who already has an iodine deficiency or underperforming thyroid gland needs to be careful not to overdo consumption of these foods.
The primary mode of iodine excretion is the urine. Small amounts of unused iodine can also be found in the feces and sweat.