The term "radioactive iodine" is used to describe specific forms of the element iodine that are generated when tellurium atoms are bombarded with neutrons released from a nuclear reactor or with protons accelerated in a cyclotron. This bombardment shifts the balance of the protons and neutrons in tellurium atoms to create unstable particles called radioactive isotopes. In the normal state, the nucleus of a stable iodine atom contains 53 protons and 74 neutrons, contributing to a combined atomic weight of 127. 123-Iodine is a radioactive isotope that contains 53 protons and 70 neutrons, contributing to a combined atomic weight of 123. 131-Iodine is a radioactive isotope that contains 53 protons and 78 neutrons, contributing to a combined atomic weight of 131. Radioactive isotopes are considered to be unstable particles that do not exist in a defined state. As soon as they are generated, they begin to undergo a process of decay that involves a shift from a higher energy state to a lower energy state. As radioactive isotopes decay, they release small bursts of energy in the form of radiation. As 123-Iodine decays, it releases small bursts of energy in the form of gamma radiation. As 131-Iodine decays, it releases small bursts of energy in the form of gamma radiation and larger bursts of energy in the form of beta radiation.
How does it work?
Radioactive iodine in the form of 123-Iodine or 131-Iodine may be used in the evaluation and treatment of a range of different thyroid disorders. The tissue that makes up the thyroid gland is unique in that it is the only tissue in the body that takes up iodine and stores it for later use. This property allows for targeted delivery of doses of 123-Iodine or 131-Iodine, limiting any exposure to other parts of the body. 123-Iodine or 131-Iodine are used for different purposes based on the different properties of the different forms of radiation they release as they decay.
The gamma radiation released from 123-Iodine as it decays travels a long distance without damaging any of the tissue it passes through as it leaves the thyroid gland. Special instruments called scanners can detect this gamma radiation to measure the uptake of radioactive isotope or to generate images that reflect the function of different regions of the thyroid gland. 123-Iodine is generally used as a tracer in radiographic studies performed to evaluate thyroid disorders. It is not used to treat thyroid disorders.
The gamma radiation released from 131-Iodine as it decays can also be used to generate images that reflect the distribution of thyroid tissue in the body. 131-Iodine may be used for this purpose when high doses are administered during the treatment of thyroid cancer. The beta radiation released from 131-Iodine as it decays travels a much shorter distance than gamma radiation, but in doing so it effectively destroys much of the tissue that it passes through. 131-Iodine can be used to destroy residual thyroid tissue in the treatment of thyroid cancer. It can also be used to destroy overactive thyroid tissue in the treatment of Graves' disease and other forms of hyperthyroidism. When a therapeutic dose of 131-Iodine is administered in the setting of Graves' disease, most of the dose is taken up and stored by follicular cells that produce and secrete excess amounts of thyroid hormone. Over time, as the 131-Iodine decays, the beta radiation that is released gradually destroys these overactive follicular cells.