Iodine-131
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.
Overview
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.
- β (mean 191.6 keV, 89.9%; smaller components at 69.4 and 96.6 keV)
- γ 364.5 keV (81.2%)
- γ 637.0 keV (7.2%)
- γ 284.3 keV (6.1%)
Theranostic partners
- Iodine-123 · A radioactive form of iodine that decays quickly and gives off gamma rays, used to take a picture of the thyroid or, attached to a nerve-tracing molecule, of tumours of the adrenal gland and nervous system, before deciding whether a therapeutic dose of iodine-131 makes sense.
- Iodine-124 · A long-lived, PET-visible form of radioactive iodine used to map how a thyroid cancer takes up iodine over several days, so the dose of the treatment isotope, iodine-131, can be planned before it is given.
High-dose radioactive MIBG delivers radiation from inside neuroblastoma cells that take up noradrenaline; used for relapsed disease and tested in upfront therapy.
An injectable tracer that mimics a stress hormone and gets taken up by certain nerve-related tumours, such as pheochromocytoma and childhood neuroblastoma, so a scan afterwards can show where they are.
Azedra was a radioactive drug that homed in on adrenal-type tumours (phaeochromocytoma and paraganglioma) that could not be removed and delivered radiation from inside. It was approved in 2018 and discontinued by its maker in 2024.
A cancer-targeting antibody carrying a radioactive form of iodine was used, as a single course, to treat a type of blood cancer called non-Hodgkin lymphoma. The manufacturer stopped selling it in 2014 because too few patients were using it.
Iomab-B is Actinium's radioactive antibody that clears the bone marrow before a transplant for older patients with active relapsed acute myeloid leukaemia; the phase 3 SIERRA trial met its primary endpoint but the FDA asked for another study.
The original targeted radiotherapy: thyroid cells soak up iodine, so radioactive iodine destroys leftover thyroid tissue and metastases while sparing everything else.
This paper is a map of the radioactive-cancer-drug field at a pivotal moment, explaining to non-specialists and drug developers alike why these treatments work differently from chemotherapy and what needed to be fixed - supply, dosing science, regulation - before more of them could reach patients.
Radionuclide therapy is a standard option for MIBG-avid metastatic pheochromocytoma and paraganglioma; somatostatin receptor targeted lutetium therapy is the alternative for SSTR-avid disease.
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