Iodine131

Iodine-131, often denoted I-131, is a radioactive isotope of iodine (atomic number 53) with a mass number of 131. It decays by beta minus emission to xenon-131, with a half-life of about 8.02 days, and emits gamma photons of 364 keV that enable imaging and measurement.

I-131 is produced primarily as a fission product in nuclear reactors and is also formed by neutron activation in some processes. It is volatile and can be released in nuclear accidents and medical waste, leading to environmental contamination. Its uptake and distribution in the body are governed by the same sodium-iodide transport system that concentrates stable iodine in the thyroid.

In medicine, I-131 is used for diagnosis and treatment of thyroid disorders because the thyroid concentrates iodine. Therapeutically, it ablates overactive thyroid tissue or thyroid cancer metastases. Diagnostically, it has been used for thyroid uptake studies and whole-body scans, though many facilities prefer I-123 or technetium-99m for routine imaging due to lower radiation doses.

Safety and regulation: Because I-131 concentrates in the thyroid, exposure poses radiation risks, especially to children and pregnant individuals. Medical use involves controlled dosing, shielding, and monitoring. After exposure, potassium iodide may be administered to saturate the thyroid and reduce uptake in case of accidental release.

In radiation protection and medical history, I-131 has been a focus due to thyroid cancer risk after nuclear accidents such as Chernobyl and Fukushima. It remains widely used in nuclear medicine under strict regulatory oversight.