radioactive isotope or radioisotope, natural or artificially created isotope of a chemical element having an unstable nucleus that decays, emitting alpha, beta, or gamma rays until stability is reached. The stable end product is a nonradioactive isotope of another element, i.e., radium-226 decays finally to lead-206. Very careful measurements show that many materials contain traces of radioactive isotopes. For a time it was thought that these materials were all members of the actinide series; however, exacting radiochemical research has demonstrated that certain of the light elements also have naturally occurring isotopes that are radioactive. Since minute traces of radioactive isotopes can be sensitively detected by means of the Geiger counter and other methods, they have various uses in medical therapy, diagnosis, and research. In therapy, they are used to kill or inhibit specific malfunctioning cells. Radioactive phosphorus is used to treat abnormal cell proliferation, e.g., polycythemia (increase in red cells) and leukemia (increase in white cells). Radioactive iodine can be used in the diagnosis of thyroid function and in the treatment of hyperthyroidism. Since the iodine taken into the body concentrates in the thyroid gland, the radioaction can be confined to that organ. In research, radioactive isotopes as tracer agents make it possible to follow the action and reaction of organic and inorganic substances within the body, many of which could not be studied by any other means. They also help to ascertain the effects of radiation on the human organism (see radiation sickness). In industry, radioactive isotopes are used for a number of purposes, including measuring the thickness of metal or plastic sheets by the amount of radiation they can stop, testing for corrosion or wear, and monitoring various processes.