The term "iodine" actually describes a single atom of iodide bound to either another atom of iodide or to another compound. "Nascent iodine" was once used as simply another name for sodium iodide (an iodide atom bound to sodium). The term was then used by the American mysticist, Edgar Cayce, to describe a free form of iodine (i.e., a single atom of iodide unbound to any other atom) - apparently created by adding electromagnetic or another form of energy. However, when this free form of iodide is exposed to a positively charged ion, such as sodium or potassium, as it would be in a liquid supplement, it will bind with these to form sodium iodide or potassium iodide. Therefore, if you are buying a supplement promoted as "nascent iodine", it is most likely sodium iodide or potassium iodide. There do not appear to be any published, placebo-controlled studies on "nascent iodine" for thyroid support or any other use.
Learn more about iodine and potassium iodide, including uses, dosage, and more, in the Potassium Iodide Review and the "Iodine" section of the Multivitamin and Multimineral Supplements Review.