Iodine-123 is a radioactive
Radioactive decay
Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...

 of iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....

 used in nuclear medicine
Nuclear medicine
In nuclear medicine procedures, elemental radionuclides are combined with other elements to form chemical compounds, or else combined with existing pharmaceutical compounds, to form radiopharmaceuticals. These radiopharmaceuticals, once administered to the patient, can localize to specific organs...

 imaging, including single photon emission computed tomography
Single photon emission computed tomography
Single-photon emission computed tomography is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information...

 (SPECT). The isotope's half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...

 is 13.22 hours; the decay by electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...

 to tellurium-123 emits gamma radiation with predominant energies of 159 keV
Kev can refer to:*Kev Hawkins, a fictional character.*Kevin, a given name occasionally shortened to "Kev".*Kiloelectronvolt, a unit of energy who symbol is "KeV".* Krefelder Eislauf-VereinKEV can refer to:...

 (this is the gamma primarily used for imaging) and 127 keV. In medical applications, the radiation is detected by a gamma camera
Gamma camera
A gamma camera, also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy...

. The isotope is typically applied as iodide
An iodide ion is the ion I−. Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt,...

-123, the anionic form.


Iodine-123 is produced in a cyclotron
In technology, a cyclotron is a type of particle accelerator. In physics, the cyclotron frequency or gyrofrequency is the frequency of a charged particle moving perpendicularly to the direction of a uniform magnetic field, i.e. a magnetic field of constant magnitude and direction...

 by proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....

 irradiation of enriched xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. The element name is pronounced or . A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...

 in a capsule. Xenon-124 absorbs a proton and immediately loses a neutron and proton to form xenon-123, or else loses two neutrons to form caesium-123, which decays to xenon-123. The xenon-123 formed by either route then decays to iodine-123, and is collected on the side of the capsule under refrigeration, then eluted with dilute sodium hydroxide in a halogen disproportionation
Disproportionation, also known as dismutation is used to describe a specific type of redox reaction in which a species is simultaneously reduced and oxidized so as to form two different products....

 reaction, similar to collection of iodine-125
Iodine-125 is a radioisotope of iodine which has uses in biological assays, nuclear medicine imaging and in radiation therapy as brachytherapy to treat prostate cancer and brain tumors. It is the second longest-lived radioisotope of iodine, after iodine-129.Its half-life is around 59 days and it...

 after it is formed from xenon by neutron irradiation (see that article for more). Iodine-123 is usually supplied as the iodide and hypoiodate in dilute sodium hydoxide solution, at high isotopic purity.

I-123 for medical applications has also been produced at Oak Ridge National Laboratories by proton cyclotron bombardment of 80% isotopically entriched tellurium-123.


The detailed decay mechanism is electron capture
Electron capture
Electron capture is a process in which a proton-rich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...

 to form an excited state of the nearly-stable nuclide tellurium-123 (half live so long that it is considered stable for all practical purposes). This exited state of Te-123 produced is not the metastable nuclear isomer
Nuclear isomer
A nuclear isomer is a metastable state of an atomic nucleus caused by the excitation of one or more of its nucleons . "Metastable" refers to the fact that these excited states have half-lives more than 100 to 1000 times the half-lives of the other possible excited nuclear states...

 Te-123m (the decay of I-123 does not involve enough energy to produce Te-123m), but rather is a lower-energy nuclear isomer of Te-123 that immediately gamma decays to ground state Te-123 at the energies noted, or else (13% of the time) decays by internal conversion
Internal conversion
Internal conversion is a radioactive decay process where an excited nucleus interacts with an electron in one of the lower atomic orbitals, causing the electron to be emitted from the atom. Thus, in an internal conversion process, a high-energy electron is emitted from the radioactive atom, but...

 electron emission (127 keV), followed by an average of 11 Auger electron
Auger electron
The Auger effect is a physical phenomenon in which the transition of an electron in an atom filling in an inner-shell vacancy causes the emission of another electron. When a core electron is removed, leaving a vacancy, an electron from a higher energy level may fall into the vacancy, resulting in...

s emitted at very low energies (50-500 eV). The latter decay channel also produces ground-state Te-123. Especially because of the internal conversion decay channel, I-123 is not an absolutely pure gamma-emitter, although it is sometimes clinically assumed to be one.

The Auger electrons from the radioisotope have been found in one study to do little cellular damage, unless the radionuclide is incorporated chemically directly into cellular DNA, which is not the case for present radiopharmaceuticals which use I-123 as the radioactive label nuclide. The damage from the more penetrating gamma radiation and 127 keV internal conversion electron radiation from the initial decay of Te-123 is moderated by the relatively short half-life of the isotope.

Medical applications

123I is the most suitable isotope of iodine for the diagnostic study of thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...

 diseases. The half-life of approximately 13.3 h (hours) is ideal for the 24-h (hour) iodine uptake test and 123I has other advantages for diagnostic imaging thyroid tissue and thyroid cancer metastasis. The energy of the photon, 159 keV, is ideal for the NaI (sodium iodide) crystal detector of current gamma cameras and also for the pinhole collimators. It has much greater photon flux than I-131. It gives approximately 20 times the counting rate of I-131 for the same administered dose. The radiation burden to the thyroid is far less (1%) than that of 131I. Moreover, scanning a thyroid remnant or metastasis with 123I does not cause "stunning" of the tissue (with loss of uptake), because of the low radiation burden of this isotope. (For the same reasons, I-123 is never used for thyroid cancer or Graves disease treatment, and this role is reserved for I-131.)

Iodine-123 is supplied as sodium iodide
Sodium iodide
Sodium iodide is a white, crystalline salt with chemical formula NaI used in radiation detection, treatment of iodine deficiency, and as a reactant in the Finkelstein reaction.-Uses:Sodium iodide is commonly used to treat and prevent iodine deficiency....

 (NaI), sometimes in basic solution in which it has been dissolved as the free element. This is administered to a patient in capsule form, by intravenous injection, or (less commonly due to problems involved in a spill) in a drink. (Iodine-131 is usually administered in a drink, due to the heavy radiation dose to local tissues which results before a capsule could dissolve). The iodine is taken up by the thyroid gland and a gamma camera
Gamma camera
A gamma camera, also called a scintillation camera or Anger camera, is a device used to image gamma radiation emitting radioisotopes, a technique known as scintigraphy...

 is used to functional images of the thyroid
The thyroid gland or simply, the thyroid , in vertebrate anatomy, is one of the largest endocrine glands. The thyroid gland is found in the neck, below the thyroid cartilage...

 for diagnosis. Quantitative measurements of the thyroid can be performed to calculate the iodine uptake (absorption) for the diagnosis of hyperthyroidism
Hyperthyroidism is the term for overactive tissue within the thyroid gland causing an overproduction of thyroid hormones . Hyperthyroidism is thus a cause of thyrotoxicosis, the clinical condition of increased thyroid hormones in the blood. Hyperthyroidism and thyrotoxicosis are not synonymous...

 and hypothyroidism
Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.Iodine deficiency is the most common cause of hypothyroidism worldwide but it can be caused by other causes such as several conditions of the thyroid gland or, less commonly, the pituitary gland or...

. Dosing can vary; a small dose can start at 11.1 MBq [300 µCi], while it is commonly an amount such as 2-4 mCi. There is a study that indicates a given dose can effectively result in effects of an otherwise higher dose, due to impurities in the preparation. The dose of radioiodine I-123 is typically tolerated by individuals who may be otherwise allergic to iodine, such as those who cannot tolerate contrast mediums containing larger doses of iodine such as used in CT scan, intravenous pyelogram
Intravenous pyelogram
An intravenous pyelogram is a radiological procedure used to visualize abnormalities of the urinary system, including the kidneys, ureters, and bladder.-Procedure:...

 (IVP) and similar imaging diagnostic procedures.

Iodine-123 is also used as a label in other imaging radiopharmaceuticals e.g.metaiodobenzylguanidine (MIBG).


Removal of radioiodine contamination can be difficult and use of a decontaminant specially made for radioactive iodine removal is advised. Two common products designed for institutional use are Bind-It (from Laboratory Technologies, Inc.) and I-Bind. General purpose radioactive decontamination products are often unusable for iodine, as these may only spread or volatilize it.
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