Iodine-131

Iodine-131 (131I), also called radioiodine, is a radioisotope of iodine.

131I decays with a half-life of 8.0197 days with beta and gamma emissions. This nuclide of iodine atom has 78 neutrons in nucleus, the stable nuclide 127I has 74 neutrons. On decaying, 131I transforms into 131Xe:

$$ {^{131}_{53}I} \rightarrow \beta + {^{131}_{54}Xe} $$

131I is a fission product with a yield of 2.8336%, and was released in nuclear weapons tests and the Chernobyl accident. However, the short half-life means it is not present in cooled spent nuclear fuel, unlike iodine-129.

It is used in nuclear medicine both diagnostically and therapeutically. Examples of its use in radiation therapy include the treatment of thyrotoxicosis and thyroid cancer. Diagnostic tests exploit the mechanism of absorption of iodine by the normal cells of the thyroid gland. As an example iodine-131 is one of the radioactive isotopes of iodine that can be used to test how well the thyroid gland is functioning.

131I is also used as a radioactive label for radiopharmaceuticals that can be used for imaging and therapy e.g. 131I-metaiodobenzylguanidine (131I-MIBG) for imaging and treating phaeochromocytoma and neuroblastoma.

If 131I is present in high levels in the environment from radioactive fallout, it is absorbed by the body and may cause damage to the thyroid. This can be mitigated by taking iodine supplements, raising the total amount of iodine in the body and therefore reducing uptake and retention in tissues and lowering the relative proportion of radioactive iodine. Such supplements were distributed to the population living nearest to the Chernobyl nuclear power plant after the disaster.

Patients receiving radioiodine treatment are warned not to have sexual intercourse for one month (or shorter, depending on dose given), and women are told not to become pregnant for six months afterwards. These guidelines vary from hospital to hospital and will depend also on the dose of radiation given. One also advises not to hug or hold children when the radiation is still high, and a one or two metre distance to others may be recommended.

Many airports now have radiation detectors in order to detect the smuggling of radioactive materials that may be used in nuclear weapons manufacture. Patients should be warned that if they choose to travel by air, they may set off radiation detectors at airports up to 12 weeks after their treatment with 131I. A physician's letter does not exempt one from interrogation by airport security personnel, because these letters are easily forged. For security reasons, there is no information available in the public domain on which airports use radiation detectors.