NIS (sodium/iodide symporter) is a type of receptor protein that takes in two molecules of sodium and iodide. But there are others on cell membranes that specifically activate once an Iodide fits. Glad to hear your making the doctor think outside the norm along with you, rather than allowing your health to be determined solely by a faux analysis and standard plan of care push.
Sometimes, cells will gulp a chunk of extracellular fluid and determine what's floating around on the outside (endocytosis). It's a very interesting process. If you have time, a friend recommended Guyton's Physiology book, it's a good read. I have it, but haven't read it yet. It's very long and I'd love to read it, but I have other things I've been reading of late.