# Iodine-131

Source: https://nuclide.cc/isotopes/i-131/  
Nuclide record `i-131` (Isotope). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

A radioactive form of iodine. Because the thyroid gland naturally absorbs iodine, swallowing it lets the radiation destroy thyroid cells or thyroid cancer cells almost anywhere they have spread, with very little effect on the rest of the body.

## Summary

Iodine-131 decays by beta emission with an associated gamma emission and a physical half-life of 8.02-8.04 days. Its principal beta emissions average around 190 keV with higher-energy components up to roughly 606 keV, and the dominant imaging gamma is 364.5 keV (about 81% abundance); roughly 90% of local irradiation from sodium iodide I-131 comes from the beta component and 10% from gamma, per the FDA label.

Because thyroid follicular cells and well-differentiated thyroid cancer retain the sodium/iodide symporter, oral 131I concentrates selectively in thyroid tissue, remnant tissue after thyroidectomy, and iodine-avid metastases, delivering an internal beta dose. It has been used for benign thyroid disease and differentiated thyroid cancer ablation for more than 70 years and remains a WHO essential medicine.

Commercial 131I is produced in a nuclear reactor overwhelmingly by neutron activation of tellurium-130 (130Te(n,γ)131Te/131mTe, which beta-decays to 131I), recovered by dry or wet distillation; it can also be recovered as a uranium-235 fission product, though current suppliers do not routinely co-recover it that way. As a widely produced, long-established reactor product with many qualified suppliers, supply is generally adequate, though individual reactor outages can cause local shortages.

## Fields

- Kind: Isotope
- Status: approved
- Last checked: 2026-09-22
- Also known as: 131I; I-131; radioiodine
- Tags: therapy; beta emitter; gamma emitter

## Sources

- FDA label: Sodium Iodide I-131 Solution (therapeutic): https://www.accessdata.fda.gov/drugsatfda_docs/label/2018/016517s016lbl.pdf
- FDA label: Sodium Iodide I 131 Capsules (diagnostic): https://www.accessdata.fda.gov/drugsatfda_docs/label/2016/021305s041lbl.pdf
- Medical Isotope Production and Utilization (NAS/NCBI, 2016): https://www.ncbi.nlm.nih.gov/books/NBK396163/

## Connected records

- drugs: [131I-MIBG (iobenguane I-131) therapy](https://nuclide.cc/drugs/i131-mibg/), [Iobenguane I-123](https://nuclide.cc/drugs/i-123-mibg/), [Iobenguane I-131 (therapeutic)](https://nuclide.cc/drugs/iobenguane-i-131/), [Iodine-131 tositumomab](https://nuclide.cc/drugs/i-131-tositumomab/), [Iomab-B](https://nuclide.cc/drugs/iomab-b/), [Radioactive iodine (I-131)](https://nuclide.cc/drugs/radioactive-iodine/)
- technologies: [Hot cells and radiation shielding](https://nuclide.cc/technologies/hot-cells-and-shielding/), [MIBG imaging and 131I-MIBG therapy](https://nuclide.cc/technologies/mibg-theranostics/), [Monte Carlo dosimetry](https://nuclide.cc/technologies/monte-carlo-dosimetry/), [Radio-antibody & radio-ADC](https://nuclide.cc/technologies/radioimmunotherapy/), [Radioiodine therapy and whole-body iodine scanning](https://nuclide.cc/technologies/radioiodine-therapy/), [Research reactors for medical isotopes (Mo-99, Lu-177, I-131)](https://nuclide.cc/technologies/research-reactor-isotope-production/), [SPECT dosimetry workflow for radionuclide therapy](https://nuclide.cc/technologies/spect-dosimetry-workflow/)
- indications: [Functional assessment of thyroid nodules](https://nuclide.cc/indications/thyroid-nodule-assessment/), [Graves' disease and hyperthyroidism](https://nuclide.cc/indications/graves-hyperthyroidism/), [High-risk neuroblastoma](https://nuclide.cc/indications/neuroblastoma-high-risk/), [Metastatic pheochromocytoma and paraganglioma](https://nuclide.cc/indications/metastatic-ppgl/), [Neuroblastoma (paediatric)](https://nuclide.cc/indications/neuroblastoma/), [Pheochromocytoma and paraganglioma (PPGL)](https://nuclide.cc/indications/pheochromocytoma-paraganglioma/), [Thyroid cancer](https://nuclide.cc/indications/thyroid/)
- trials: [COG ANBL1531](https://nuclide.cc/trials/anbl1531/), [ESTIMABL2](https://nuclide.cc/trials/estimabl2/), [Study of Iopofosine I-131 (CLR 131) in Select B-Cell Malignancies (CLOVER-1) With Expansion in Waldenstrom](https://nuclide.cc/trials/nct02952508/)
- terms: [Authorized User (AU)](https://nuclide.cc/terms/authorized-user/), [MIBG Curie and SIOPEN scores](https://nuclide.cc/terms/curie-siopen-score/), [Patient release criteria after radioactive treatment](https://nuclide.cc/terms/patient-release-criteria/), [Radioiodine therapy (I-131)](https://nuclide.cc/terms/radioiodine-term/), [SDH deficiency (SDHB immunohistochemistry loss)](https://nuclide.cc/terms/sdh-deficiency/), [Theranostics](https://nuclide.cc/terms/theranostics/)
- companies: [Cellectar Biosciences](https://nuclide.cc/companies/cellectar/), [HTA (Atom Hi-Tech)](https://nuclide.cc/companies/hta-atom-hi-tech/), [IRE (Institut national des Radioéléments)](https://nuclide.cc/companies/ire/), [Jubilant Radiopharma](https://nuclide.cc/companies/jubilant-radiopharma/), [Lantheus](https://nuclide.cc/companies/lantheus/), [NTP Radioisotopes](https://nuclide.cc/companies/ntp-radioisotopes/), [PDRadiopharma](https://nuclide.cc/companies/pdradiopharma/), [Y-mAbs Therapeutics](https://nuclide.cc/companies/y-mabs/)
- ideas: [A coordinated reserve and shared schedule for the world's medical isotope reactors](https://nuclide.cc/ideas/idea-reg-medical-isotope-reactor-reserve/), [A global medical isotope supply observatory with forecasts and shortage alerts](https://nuclide.cc/ideas/idea-reg-isotope-supply-observatory/), [Pretargeted alpha radioimmunotherapy](https://nuclide.cc/ideas/idea-pretargeted-alpha-therapy/)
- people: [Dale L. Bailey](https://nuclide.cc/people/dale-bailey/), [Katherine K. Matthay](https://nuclide.cc/people/katherine-matthay/), [Neeta Pandit-Taskar](https://nuclide.cc/people/neeta-pandit-taskar/)
- key papers: [Efficacy and safety of high-specific-activity 131I-MIBG therapy in advanced pheochromocytoma or paraganglioma](https://nuclide.cc/key-papers/paper-pryma-high-specific-activity-i131-mibg-ppgl-jnm-2019/), [Radiopharmaceutical therapy in cancer: clinical advances and challenges](https://nuclide.cc/key-papers/paper-sgouros-tat-review-nat-rev-2020/)
- institutions: [Board of Radiation and Isotope Technology](https://nuclide.cc/institutions/brit/)
- collections: [US NRC 10 CFR Part 35 (Medical Use of Byproduct Material)](https://nuclide.cc/collections/nrc-regulations-part35/)
- targets: [NIS (sodium-iodide symporter)](https://nuclide.cc/targets/nis-symporter/), [Norepinephrine transporter (NET)](https://nuclide.cc/targets/norepinephrine-transporter/)

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