**2.2 Sodium iodide symporter**

The rat NIS gene and the human NIS gene were cloned in 1996.(Dai, Levy et al. 1996; Smanik, Liu et al. 1996) NIS is a 13 transmembrane domain protein with an extracellular amino- and intracellular carboxyl-terminus and the expression of the NIS gene is mainly regulated by thyroid stimulating hormone (TSH). Binding of TSH to its receptor activates the NIS gene transcription and controls translocation and retention of NIS at the plasma membrane, and so this increases iodide uptake.

In addition to its expression in the thyroid follicular cells, NIS is detectable and active in some extrathyroidal tissues such as the salivary glands, gastric mucosa, lactating mammary glands, etc. Therefore, these tissues are able to take up iodide by the action of the NIS. However, contrary to thyroid follicular cells, there are no long-term retention of iodide and TSH dependency. (Baril, Martin-Duque et al. 2010) The physiologic function of the NIS in the extrathyroidal tissues is not yet clear.
