Sodium-iodide Symporter - Iodine Uptake

Iodine Uptake

Iodine uptake mediated by thyroid follicular cells from the blood plasma is the first step for the synthesis of thyroid hormones. This ingested iodine is bound to serum proteins, especially to albumins. The rest of the iodine which remains unlinked and free in bloodstream, is removed from the body through urine (the kidney is essential in the removal of iodine from extracellular space).

Iodine uptake is a result of an active transport mechanism mediated by the NIS protein, which is found in the basolateral membrane of thyroid follicular cells. As a result of this active transport, iodide concentration inside follicular cells of thyroid tissue is 20 to 50 times higher than in the plasma. The transport of iodide across the cell membrane is driven by the electrochemical gradient of sodium (the intracellular concentration of sodium is approximately 12 mM and extracellular concentration 140 mM). Once inside the follicular cells, the iodide diffuses to the apical membrane, where it is metabolically oxidized through the action of thyroid peroxidase to iodinium (I+) which in turn iodinates tyrosine residues of the thyroglobulin proteins in the follicle colloid. Thus, NIS is essential for the synthesis of thyroid hormones (T₃ and T₄).

Apart from thyroid cells NIS can also be found, although less expressed, in other tissues such as the salivary glands, the gastric mucosa, the kidney, the placenta, the ovaries and the mammary glands during pregnancy and lactation. NIS expression in the mammary glands is quite a relevant fact since the regulation of iodide absorption and its presence in the breast milk is the main source of iodine for a newborn. Note that the regulation of NIS expression in thyroid is done by the thyroid-stimulating hormone (TSH), whereas in breast is done by a combination of three molecules: prolactin, oxytocin and β-estradiol.