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Naturally occurring iodine (₅₃I) consists of one stable isotope,¹²⁷I, and is amononuclidic element for atomic weight. Radioisotopes ofiodine are known from¹⁰⁸I to¹⁴⁷I.

The longest-lived of those,¹²⁹I, has a half-life of 16.14 million years, which is too short for it to exist as aprimordial nuclide. It is, however, found in nature as a trace isotope and universally distributed, produced naturally bycosmogenic sources in the atmosphere and bynatural fission of the actinides. Today, however, most is artificial asfission product; likekrypton-85 the contribution of past nuclear testing and of operating reactors are dwarfed by release fromnuclear reprocessing.^[3]

All other iodine radioisotopes have half-lives less than 60 days, and four of these are used as tracers and therapeutic agents in medicine –¹²³I,¹²⁴I,¹²⁵I, and¹³¹I. All industrial use of radioactive iodine isotopes involves these four. In addition, one other isotope has a half-life in the same range –¹²⁶I (12.93 days; decays almost equally to tellurium or to xenon).

The isotope¹³⁵I has a half-life less than seven hours, which is inconveniently short for those purposes. However, the unavoidablein situ production of this isotope is important in nuclear reactor control, as it decays to¹³⁵Xe, the most powerful knownneutron absorber, and thenuclide responsible for the so-callediodine pit phenomenon.

In addition to commercial production,¹³¹I (half-life 8 days) is one of the common radioactivefission products ofnuclear fission, and thus occurs in large amounts insidenuclear reactors. Due to its volatility, short half-life, and high abundance in fission products,¹³¹I (along with the short-lived iodine isotope¹³²I, which is produced from the decay of¹³²Te with a half-life of 3 days) is responsible for the most dangerous part of the short-termradioactive contamination after environmental release of theradioactive waste from a nuclear power plant. For that reason,iodine supplements (usuallypotassium iodide) are given to the populace after nuclear accidents or explosions (and in some cases prior to any such incident as acivil defense mechanism) to reduce the uptake of radioactive iodine compounds by thethyroid.

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Radioisotopes of iodine are calledradioactive iodine orradioiodine. Dozens exist, but about a half dozen are the most notable inapplied sciences such as the life sciences and nuclear power, as detailed below. Mentions of radioiodine inhealth care contexts refer more often to iodine-131 than to other isotopes.

Of the many isotopes of iodine, only two are typically used in a medical setting: iodine-123 and iodine-131. Since¹³¹I has both a beta and gamma decay mode, it can be used for radiotherapy or for imaging.¹²³I, which has no beta activity, is more suited for routine nuclear medicine imaging of the thyroid and other medical processes and less damaging internally to the patient. There are some situations in which iodine-124 and iodine-125 are also used in medicine.^[8]

Due to preferential uptake of iodine by the thyroid, radioiodine is extensively used in imaging of and, in the case of¹³¹I, destroying dysfunctional thyroid tissues. Other types of tissue selectively take up certain iodine-131-containing tissue-targeting and killing radiopharmaceutical agents (such asMIBG). Iodine-125 is the only other iodine radioisotope used in radiation therapy, but only as an implanted capsule inbrachytherapy, where the isotope never has a chance to be released for chemical interaction with the body's tissues.

Thegamma-emitting isotopes iodine-123 (half-life 13.223 hours), and (less commonly) the longer-lived and less energetic iodine-125 (half-life 59.392 days) are used asnuclear imaging tracers to evaluate the anatomic and physiologic function of the thyroid. Abnormal results may be caused by disorders such asGraves' disease orHashimoto's thyroiditis. Both isotopes decay byelectron capture (EC) to the correspondingtellurium nuclides, but in neither case are these themetastable nuclides^123mTe and^125mTe (which are of higher energy, and are not produced from radioiodine). Instead, the excited tellurium nuclides decay immediately (half-life too short to detect). Following EC, the excited¹²³Te from¹²³I emits a high-speed 127 keVinternal conversion electron (not abeta ray) about 13% of the time, but this does little cellular damage due to the nuclide's short half-life and the relatively small fraction of such events. In the remainder of cases, a 159 keV gamma ray is emitted, which is well-suited for gamma imaging.

Excited¹²⁵Te resulting from electron capture of¹²⁵I also emits a much lower-energy internal conversion electron (35.5 keV), which does relatively little damage due to its low energy, even though its emission is more common. The relatively low-energy gamma from¹²⁵I/¹²⁵Te decay is poorly suited for imaging, but can still be seen, and this longer-lived isotope is necessary in tests that require several days of imaging, for example,fibrinogen scan imaging to detect blood clots.

Both¹²³I and¹²⁵I emit copious low energyAuger electrons after their decay, but these do not cause serious damage (double-stranded DNA breaks) in cells, unless the nuclide is incorporated into a medication that accumulates in the nucleus, or into DNA (this is never the case is clinical medicine, but it has been seen in experimental animal models).^[9]

Iodine-125 is also commonly used byradiation oncologists in low dose ratebrachytherapy in the treatment of cancer at sites other than the thyroid, especially inprostate cancer. When¹²⁵I is used therapeutically, it is encapsulated in titanium seeds and implanted in the area of the tumor, where it remains. The low energy of the gamma spectrum in this case limits radiation damage to tissues far from the implanted capsule. Iodine-125, due to its suitable longer half-life and less penetrating gamma spectrum, is also often preferred for laboratory tests that rely on iodine as a tracer that is counted by agamma counter, such as inradioimmunoassaying.

¹²⁵I is used as theradiolabel in investigating whichligands go to whichplant pattern recognition receptors (PRRs).^[10]

Iodine-124 is a proton-rich isotope of iodine with a half-life of 4.1760 days, decaying to¹²⁴Te by 77.3% electron capture and 22.7% positron emission.^[11] Iodine-124 can be made by numerous nuclear reactions in acyclotron. The most common starting material used is¹²⁴Te.

Iodine-124 as the iodide salt can be used to directly image the thyroid usingpositron emission tomography (PET).^[12] Iodine-124 can also be used as a PETradiotracer with a usefully longer half-life compared withfluorine-18.^[13] In this use, the nuclide is chemically bonded to a pharmaceutical to form a positron-emitting radiopharmaceutical, and injected into the body, where again it is imaged by PET scan.

Iodine-129 (¹²⁹I;half-life 16.1 million years) is a product ofcosmic ray spallation on various isotopes ofxenon in theatmosphere, incosmic raymuon interaction with tellurium-130, and alsouranium andplutonium fission, both in subsurface rocks and nuclear reactors. Artificial nuclear processes, in particular nuclear fuel reprocessing and atmospheric nuclear weapons tests, have now swamped the natural signal for this isotope. Nevertheless, it now serves as a groundwater tracer as indicator of nuclear waste dispersion into the natural environment. In a similar fashion,¹²⁹I was used in rainwater studies to track fission products following theChernobyl disaster.

In some ways,¹²⁹I is similar to³⁶Cl. It is a soluble halogen, exists mainly as a non-sorbinganion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies,¹²⁹I concentrations are usually reported as the ratio of¹²⁹I to total I (which is virtually all¹²⁷I). As is the case with³⁶Cl/Cl,¹²⁹I/I ratios in nature are quite small: the pre-nuclear ratio was nearly 10⁻¹²,^[3] but as mentioned artificial sources are normally now much greater.¹²⁹I differs from³⁶Cl in that its half-life is longer (16.1 vs. 0.301 million years), it is highly biophilic, and occurs in multipleionic forms (commonly, I⁻ andIO₃⁻), which have different chemical behaviors. This makes it fairly easy for¹²⁹I to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc.Excesses of stable¹²⁹Xe in meteorites have been shown to result from decay of "primordial" iodine-129 produced newly by the supernovas that created the dust and gas from which the Solar System formed. This isotope has long decayed and is thus referred to as "extinct". Historically,¹²⁹I was the firstextinct radionuclide to be identified as present in the earlySolar System. Its decay is the basis of the I-Xe iodine-xenonradiometric dating scheme, which covers the first 85 million years ofSolar System evolution.

Iodine-131 (¹³¹
I) is abeta-emitting isotope with a half-life of 8.0249 days, and comparatively energetic (0.61 MeV) beta radiation, which penetrates 0.6 to 2.0 mm from the site of uptake. This beta radiation can be used for the destruction ofthyroid nodules or hyperfunctioning thyroid tissue and for elimination of remaining thyroid tissue after surgery for the treatment ofGraves' disease. The purpose of this therapy, which was first explored by Dr.Saul Hertz in 1941,^[14] is to destroy thyroid tissue that could not be removed surgically. In this procedure,¹³¹I is administered either intravenously or orally following a diagnostic scan. This procedure may also be used, with higher doses of radio-iodine, to treat patients withthyroid cancer.

The¹³¹I is taken up into thyroid tissue and concentrated there. The beta particles emitted by the radioisotope destroys the associated thyroid tissue with little damage to surrounding tissues (more than 2.0 mm from the tissues absorbing the iodine). Due to similar destruction,¹³¹I is the iodine radioisotope used in other water-soluble iodine-labeledradiopharmaceuticals (such asMIBG) used therapeutically to destroy tissues.

The high energy beta radiation (up to 606 keV) from¹³¹I causes it to be the most carcinogenic of the iodine isotopes. It is thought to cause the majority of excess thyroid cancers seen after nuclear fission contamination (such as bomb fallout or severe nuclear reactor accidents like theChernobyl disaster) However, these epidemiological effects are seen primarily in children, and treatment of adults and children with therapeutic¹³¹I, and epidemiology of adults exposed to low-dose¹³¹I has not demonstrated carcinogenicity.^[15]

Iodine-135 is a neutron-rich isotope of iodine with a half-life of 6.58 hours, and important tonuclear reactor physics. It is produced in relatively large amounts as afission product, and decays toxenon-135, which is anuclear poison with the largest known thermalneutron cross section, which is a cause of multiple complications in the control ofnuclear reactors. The process of buildup ofxenon-135 from accumulated iodine-135 can temporarily preclude a shut-down reactor from restarting. This is known as xenon poisoning or "falling into aniodine pit".

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Colloquially, radioactive materials can be described as "hot," and non-radioactive materials can be described as "cold." There are instances in which cold iodide is administered to people in order to prevent the uptake of hot iodide by the thyroid gland. For example, blockage of thyroid iodine uptake with potassium iodide is used innuclear medicinescintigraphy and therapy with some radioiodinated compounds that are not targeted to the thyroid, such asiobenguane (MIBG), which is used to image or treat neural tissue tumors, or iodinated fibrinogen, which is used infibrinogen scans to investigate clotting. These compounds contain iodine, but not in the iodide form. However, since they may be ultimately metabolized or break down to radioactive iodide, it is common to administer non-radioactive potassium iodide to insure that metabolites of these radiopharmaceuticals is not sequestered by the thyroid gland and result in a concentrated radiation dose to that tissue.

Potassium iodide has been distributed to populations exposed tonuclear fission accidents such as theChernobyl disaster. The iodide solutionSSKI, asaturatedsolution of potassium (K)iodide in water, has been used to block absorption of the radioiodine. Tablets containing potassium iodide are now also manufactured and stocked in central disaster sites by some governments for this purpose. In theory, many cancers might be prevented in this way, since an excess of thyroid cancer, presumably due to radioiodine uptake, is the only proven long-term effect after exposure of a population to radioactive fission products from a reactor accident or atomic bomb fallout. Taking large amounts of iodide saturates thyroid receptors and prevents uptake of most radioactiveiodine-131 that may be present from fission product exposure (although it does not protect from other radioisotopes, nor from any external radiation). The protective effect of KI lasts approximately 24 hours, so must be dosed daily until a risk of significant exposure to this isotope no longer exists; as it decays relatively rapidly with a half-life of eight days (other iodine isotopes of concern are even shorter-lived), 99.95% of the radioiodine has vanished after three months.^[16][17]

Daughter products other than iodine

• Isotopes of xenon
• Isotopes of tellurium
• Isotopes of antimony
• Isotopes of tin

• Isotopes of iodine
• Iodine
• Lists of isotopes by element
• Nuclear safety and security

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