Radioactive iodine (I-131) is an isotope that has been used for the treatment of hyperthyroidism in both humans and felines for over 20 years. It is considered the ideal treatment for feline hyperthyroidism. The radioactive iodine is given by an injection under the skin, and over a short period of time, destroys the abnormal thyroid tissue.
An isotope is two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei. This makes the element differ in relative atomic mass for each form, but not in chemical properties. So any isotope of an element will look, taste and smell alike, but some isotopes will emit radiation. Nuclear Radiation is emitted by certain isotopes, such as I131, as a means of releasing energy as they seek to reach a stable (non-radioactive) state.
Radioactive iodine (I131) emits two types of radiation as it decays toward a stable state. Once this happens, it is no longer radioactive. I131 has a half-life of 8 days, this means that every 8 days it is now HALF of what is was. For example if you started with 100 mCi of I131, in 8 days you would have 50mCi, after another 8 days you would have 25mCi, and so on. In most cases 11 half lives, or 88 days is enough to bring any contamination you would have to normal safe background levels.
Beta particles and gamma photons are emitted as part of this decay. The Beta particle is what makes the I131 treatment effective. The I131 given to your cat is absorbed quickly into the cat’s bloodstream and the diseased thyroid tissue sucks up as much radioactive iodine as it can hold. Once in the thyroid, the Beta particles destroy the overactive portions of the thyroid gland. Because beta particles can only travel about 2mm in tissue, the regular healthy thyroid tissue is left safe and intact.
The gamma photon emission is like an Xray. It has energy but has no mass. It is able to exit the body and can be measured with a geiger counter. Increasing the distance from the “source” drastically reduces your exposure. Limited exposure to this gamma radiation is not considered harmful to others, since the amount of damage that it does is very limited. However, prolonged exposure by others is not recommended, since the effect is cumulative.