Iodine-125 is an iodine radioisotope that is used in biological analyzes, imaging of nuclear medicine and in radiation therapy as brachytherapy for the treatment of a number of conditions, including prostate cancer, uveal melanomas, and brain tumors. Because of its relatively long half-life and the emission of low-energy photons, which can be detected with gamma counter detectors, Iodine-125 is the preferred isotope for labeling antibodies in radioimmunoassay and other procedures for counting gamma radiation with the participation of proteins outside the body. The same properties of the isotope make it useful for brachytherapy and for some nuclear medicine scanning procedures in which it is attached to proteins, and where a longer half-life than I-123 is required for a test of several days. Iodine-125 can be used when imaging the thyroid gland, but iodine-123 is preferred for this purpose because of the better penetration of radiation and a shorter half-life. For radiation ablation of tissues that absorb iodine (for example, the thyroid gland) or that absorb iodine-containing radiopharmaceutical, the preferred isotope is iodine-131. Iodine-125 is useful for determining glomerular filtration rate in the diagnosis or monitoring of patients with kidney disease.