Iodine possesses 37 known isotopes of mass number varying between 108 and 144 and 16 nuclear isomers. Of these isotopes, only iodine-127 (I-127) is stable and represents all of the natural iodine.

Of the 30 iodine radioisotopes, the most stable is iodine-129 (I-129) with a half-life of 15.7 million years. All others have a half-life of less than 60 days. Four of them are used as radiogenic tracers and therapeutic agents: I-123, I-124, I-125 and I-131.

Because iodine is absorbed by the thyroid, iodine radioisotopes are widely used in medical imaging (iodine-131 in particular) or to destroy thyroid dysfunctional tissues or other types of absorbent tissue Selectively, iodine-131-containing radiopharmaceuticals. Iodine 125 is the only other radioisotope of iodine used in radiotherapy.

Iodine 125 (I-125) is the isotope of iodine whose nucleus consists of 53 protons and 72 neutrons. It is an electron-disrupting radioisotope in Tellure 125 (Te-125) with a half-life of 59.4 days. As iodine 123 is a gamma ray emitter.

Because of its relatively long half-life and low-energy photon emission that can be detected by gamma-ray counters, Iodine 125 (I-125) is the isotope used for labeling antibodies in Radioactive immunoassays and other gamma-ray counting procedures involving in vitro proteins. The same characteristics make this isotope a useful tool for brachytherapy.