Thirty isotopes are recognized. Only one stable isotope, 127I is found in nature. The artificial radioisotope 131I, with a half-life of 8 days, has been used in treating the thyroid gland. Lack of iodine is the cause of goiter. Iodides, and thyroxin which contains iodine, are used internally in medicine, and as a solution of KI and iodine in alcohol is used for external wounds.
Potassium iodide finds use in photography.
The deep blue color with starch solution is characteristic of the free element.