Iodine appears in everything from thyroid physiology to environmental monitoring, and its isotopes span a wide range of behaviors — some used in hospitals, others tracked as contaminants. Knowing the differences makes it easier to interpret test results or choose the right isotope for a study.
There are 6 Iodine Isotopes, ranging from I-123 to I-131. For each, you’ll find below Half-life (s/d/yr),Decay mode(s),Natural abundance (%) so you can quickly compare stability, decay pathways and how common each isotope is in samples you’ll find below.
Which iodine isotope is typically used for diagnostic imaging and why?
I-123 is commonly used for diagnostic imaging because it emits gamma rays suitable for SPECT and has a relatively short half-life that minimizes patient dose; I-131 also emits gammas but its beta emission and longer half-life make it better suited for therapy rather than routine imaging.
Do any iodine isotopes occur naturally in measurable amounts?
Natural iodine is almost entirely the stable isotope I-127; radioactive iodine isotopes are either produced artificially or appear in trace amounts after nuclear events, so their natural abundance is effectively negligible for most practical purposes.
Iodine Isotopes
| Isotope | Half-life (s/d/yr) | Decay mode(s) | Natural abundance (%) |
|---|---|---|---|
| I-123 | 0.55 d | Electron capture (gamma emission) | 0.00 |
| I-124 | 4.18 d | Positron emission and electron capture | 0.00 |
| I-125 | 59.49 d | Electron capture (low-energy photons) | 0.00 |
| I-127 | Stable | Stable (non-radioactive) | 100.00 |
| I-129 | 15,700,000 yr | Beta-minus decay | 0.00 |
| I-131 | 8.02 d | Beta-minus (gamma emissions) | 0.00 |
Images and Descriptions
I-123
Short-lived radioisotope used in medical SPECT imaging for thyroid and brain studies. Produced in cyclotrons, it decays by electron capture with gamma emission; prized for clean imaging and minimal long-term contamination.
I-124
Positron-emitting iodine used for PET imaging and research tracers (immuno-PET). Made in cyclotrons, it decays over days by positron emission and EC; useful for longer-duration PET studies but requires on-site production.
I-125
Medium-lived isotope widely used for laboratory radiolabeling and some brachytherapy applications. Produced by neutron irradiation or cyclotron routes, it decays by electron capture emitting low-energy photons and requires careful handling.
I-127
The only stable, naturally abundant iodine isotope and the chemical standard for iodine. Non-radioactive and essential for thyroid hormones, it forms the basis of natural iodine chemistry and is not a radiological hazard.
I-129
Very long-lived radioisotope produced by cosmic rays and nuclear fission; present as trace contamination from reactors and fallout. Its 15.7-million-year half-life makes it useful for environmental tracing and nuclear forensics.
I-131
Common fission product known for thyroid uptake and medical use in therapy and diagnostics. Produced in reactors and generators, it decays in about eight days with beta and gamma emissions and poses significant radiological hazard in accidents.
