Iodine in biology ring iodine atom, and also convert T3 to 3,3'-Diiodothyronine (T2) by removing an inner ring atom. Both of the latter products are inactivated hormones which have essentially no biological effects and are quickly prepared for disposal. A family of non-selenium-dependent enzymes then further deiodinates the products of these reactions. Selenium also plays a very important role in the production of glutathione, the body's most powerful antioxidant. During the production of the thyroid hormones, hydrogen peroxide is produced in large quantities, and therefore high iodine in the absence of selenium can destroy the thyroid gland (often described as a sore throat
Iodine be primordial. As such, iodine is monoisotopic and its atomic weight is known to great precision, as it is a constant of nature. The longest-lived of the radioactive isotopes of iodine is iodine-129, which has a half-life of 15.7 million years, decaying via beta decay to stable xenon-129. Some iodine-129 was formed along with iodine-127 before the formation of the Solar System, but it has by now completely decayed away, making it an extinct radionuclide that is nevertheless still useful in dating the history of the early Solar System or very old groundwaters, due to its mobility in the environment.