**2.1 Iodine absorption and metabolism**

Ingested iodides are rapidly and nearly completely absorbed (>90%) from the duodenum into the blood and most of the iodides are excreted by kidneys. Sodium iodide symporter (NIS) on the apical membrane of enterocytes mediates active iodide uptake. Normally about one fifth of absorbed iodides are taken up by thyroid follicular cells and this is used for thyroid hormone synthesis, yet the clearance of circulating iodide varies with iodine intake. In the condition of an adequate iodine supply, ≤10% of absorbed iodides are taken up by the thyroid and in chronic iodine deficiency, this fraction can exceed 80%.(Zimmermann and Crill) The basal membrane of the thyroid follicular cell is able to actively transport iodide to the interior of the cell against a concentration gradient by the action of the NIS, which co-transports one iodide ion along with two sodium ions. The process of concentrating iodide in the thyroid follicular cells is called iodide trapping and presence of the NIS is essential for the process.(Hall 2011) Thyroid hormones are produced by oxidation, organification and coupling processes in the thyroid gland and they are finally released into the blood stream for their action.
