| General | |
|---|---|
| Symbol | 123I |
| Names | iodine-123, radioiodine |
| Protons(Z) | 53 |
| Neutrons(N) | 70 |
| Nuclide data | |
| Natural abundance | 0 |
| Half-life(t1/2) | 13.223 hours[1] |
| Isotope mass | 122.905590[2]Da |
| Parent isotopes | 123Xe |
| Decay products | 123Te |
| Decay modes | |
| Decay mode | Decay energy (MeV) |
| electron capture | 1.228MeV[3] |
| Isotopes of iodine Complete table of nuclides | |
Iodine-123 (123I) is aradioactiveisotope ofiodine used innuclear medicine imaging, includingsingle-photon emission computed tomography (SPECT) or SPECT/CT exams. The isotope'shalf-life is 13.223 hours;[1] the decay byelectron capture to tellurium-123 emitsgamma radiation with a predominant energy of 159keV (this is the gamma primarily used for imaging). In medical applications, the radiation is detected by agamma camera. The isotope is typically applied asiodide-123, theanionic form.
Iodine-123 is produced in acyclotron byproton irradiation ofxenon-124 in a capsule, which on absorbing a proton of the used energy will lose a neutron and proton to formxenon-123, or else two neutrons to formcaesium-123, which decays to it. The xenon-123 formed by either route then decays to iodine-123, and is trapped on the inner wall of the irradiation capsule under refrigeration, then eluted with sodium hydroxide in a halogendisproportionation reaction, similar to collection ofiodine-125 after it is formed from xenon byneutron irradiation (seearticle on125I for more details).
Iodine-123 is usually supplied as [123
I]-sodium iodide in 0.1 Msodium hydroxide solution, at 99.8% isotopic purity.[4]
123I for medical applications has also been produced atOak Ridge National Laboratory by proton cyclotron bombardment of 80% isotopically enriched tellurium-123.[5]
The detailed decay mechanism iselectron capture (EC) to form anexcited state of theobservationally stable nuclide tellurium-123 . The excited state of123Te produced is not themetastablenuclear isomer123mTe (the decay of123I does not involve enough energy to produce123mTe), but rather is a much shorter-lived excited state of123Te that immediately decays toground state123Te by emitting agamma ray at the energy noted, or else (13% of the time) byinternal conversion electron emission,[6] followed by an average of 11Auger electrons emitted at very low energies (50-500 eV). The latter decay channel also produces ground-state123Te. Especially because of the internal conversion decay channel,123I is not an absolutely pure gamma-emitter, although it may be considered so for clinically purposes.[citation needed]
The Auger electrons from the radioisotope have been found in one study to do little cellular damage, unless the radionuclide is directly incorporated chemically into cellularDNA, which is not the case for presentradiopharmaceuticals which use123I as the radioactive label nuclide. Thedamage from the more penetrating gamma radiation and internal conversion electrons from the initial decay of123Te is moderated by the relatively shorthalf-life of theisotope.[7]
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| Chemical and physical data | |
| Formula | 123I− |
| Molar mass | 122.91 g/mol |
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123I is the most suitable isotope of iodine for the diagnostic study ofthyroid diseases. The half-life (13.223 hours) is ideal for the 24-houriodine uptake test and123I has other advantages for diagnostic imaging thyroid tissue and thyroid cancermetastasis. The energy of the photon, 159 keV, is ideal for the NaI (sodium iodide)crystal detector of currentgamma cameras and also for the pinholecollimators. It has much greater photon flux than131I. It gives approximately 20 times the counting rate of131I for the same administered dose, while the radiation burden to the thyroid is far less (1%) than that of131I. Moreover, scanning a thyroid remnant or metastasis with123I does not cause "stunning" of the tissue (with loss of uptake), because of the low radiation burden of this isotope.[8] For the same reasons,123I is never used for thyroid cancer orGraves diseasetreatment, and this role is reserved for131I.
123I is supplied assodium iodide (NaI), sometimes in basic solution in which it has been dissolved as the free element. This is administered to a patient by ingestion under capsule form, byintravenous injection, or (less commonly due to problems involved in a spill) in a drink. The iodine is taken up by thethyroid gland and agamma camera is used to obtain functional images of thethyroid for diagnosis. Quantitative measurements of the thyroid can be performed to calculate the iodine uptake (absorption) for the diagnosis ofhyperthyroidism andhypothyroidism.
Dosing can vary; 7.5–25megabecquerels (200–680 μCi) is recommended for thyroid imaging[9][10] and for total body while an uptake test may use 3.7–11.1 MBq (100–300 μCi).[11][12] 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.[13] The dose of radioiodine123I is typically tolerated by individuals who cannot toleratecontrast mediums containing larger concentration of stable iodine such as used inCT scan,intravenous pyelogram (IVP) and similar imaging diagnostic procedures. Iodine is not anallergen.[14]
123I is also used as a label in other imagingradiopharmaceuticals, such asmetaiodobenzylguanidine (MIBG) andioflupane.
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[15] and I-Bind.[citation needed] General purpose radioactive decontamination products are often unusable for iodine, as these may only spread or volatilize it.[citation needed]
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