Iodine-131 (131I), also called radioiodine, is a radioisotope of iodine which has medical and pharmaceutical uses.

Radioactive decay

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}

The primary emissions of 131I decay are 364 keV gamma rays (81% abundance) and 606 keV beta particles (89% abundance).

131I is a fission product with a yield of 2.8336% from uranium-235, 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 whose halflife is nearly a billion times that of I-131.

Effects of exposure

Iodine in food is absorbed by the body and preferentially concentrated in the thyroid where it is needed for the functioning of that gland. When 131I is present in high levels in the environment from radioactive fallout, it can be absorbed through contaminated food, and will also accumulate in the thyroid. As it decays, it may cause damage to the thyroid. The primary risk from exposure to high levels of 131I is the chance occurrence of radiogenic thyroid cancer in later life. Other risks include the possibility of non-cancerous growths and thyroiditis.

The risk of thyroid cancer in later life appears to diminish with increasing age at time of exposure. Most risk estimates are based on studies in which radiation exposures occurred in children or teenagers. When adults are exposed, it has been difficult for epidemiologists to detect a statistically significant difference in the rates of thyroid disease above that of a similar but otherwise unexposed group.

The risk 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.

Within the USA, the highest 131I fallout doses occurred during the 1950s and early 1960s to children who consumed fresh sources of milk contaminated as the result of above ground testing of nuclear weapons. The National Cancer Institute provides additional information on the health effects from exposure to 131I in fallout, as well as individualized estimates, for those born before 1971, for each of the 3070 counties in the USA from the nuclear weapons tests conducted at the Nevada Test Site.

Medical and pharmaceutical uses

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 pheochromocytoma and neuroblastoma.

Post-treatment isolation

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. "This is because a theoretical risk to a developing fetus exists, even though the amount of radioactivity retained may be small and there is no medical proof of an actual risk from radioiodine treatment. Such a precaution would essentially eliminate direct fetal exposure to radioactivity and markedly reduce the possibility of conception with sperm that might theoretically have been damaged by exposure to radioiodine. 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.


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