Figure 1

Mechanisms of Iodide transport in thyroid follicular cells. The first step in iodide uptake is mediated by the sodium-iodide symporter NIS, using the sodium gradient generated by the Na, K-ATPase. Active transport of potassium by the KCNE2/KCNQ1 potassium channel is also important, likely for maintaining the membrane potential of thyroid cells. At the apical membrane, pendrin and another yet unidentified transporter mediate iodide efflux. TPO, using H₂O₂ generated by the DUOX2/DUOXA system mediates the oxidation, organification and coupling reaction that result in the synthesis of the iodothyronines T4 and T3. Iodinated thyroglobulin is taken into the cell by micro- and macropinocytosis and digested in lysosomes. T4 and T3 are excreted via MCT8 and other transporters. The iodotyrosines MIT and DIT are dehalogenated by DEHAL1 and the released iodide is recycled. Purple boxes represent steps in basal iodide uptake. Orange boxes represent apical iodide uptake, oxidation, organification and coupling are mediated by TPO, represented in green boxes. The generation of H₂O₂ is represented in aqua. The recycling of iodide after digestion of iodinated thyroglobulin is represented in the red box. The secretion of thyroid hormones at the basolateral membrane is shown in the blue boxes.