A radioactive isotope of iodine, a nonmetallic element of the halogen group with an atomic mass of 123 and a half-life of 13.2 hours with radioisotopic activity. Selectively accumulating in the thyroid tissue, iodine I 123 emits gamma rays that can be detected with gamma scintigraphy, allowing localization of thyroid tissue. This agent may be used as a tracer in whole body scintigraphy (WBS) to localize thyroid carcinoma metastases.