Iodine is a very important mineral/chemical element for the body and is the heaviest of its group called halogens. Iodine is needed for optimal thyroid function and aids in proper construction of cellular membranes, especially in your mitochondria. Iodine is also used in many cases to protect against or combat radioactive isotopes as they compete for the same receptors in the body.
Other halogens such as bromine, fluorine and chlorine can also compete with iodine receptors and cause thyroid issues as well as reduce waters dielectric properties. In English they stop iodine from doing its job and reduce waters ability to hold a DC electric charge (bad). They can be found in water, bread, diary as well as other food sources, they are normally artificially added and do not show up in unprocessed fresh sources.
Iodine can be found in greatest quantities naturally in sea vegetables and seafood. A lack of Iodine can cause many issues with temperature regulation, energy levels, body fat levels and pain around the sternum.