# Radioactive iodine (I-131)

Source: https://nuclide.cc/drugs/radioactive-iodine/  
Nuclide record `radioactive-iodine` (Agent). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

The original targeted radiotherapy: thyroid cells soak up iodine, so radioactive iodine destroys leftover thyroid tissue and metastases while sparing everything else.

## Summary

In use since 1946 for differentiated thyroid cancer; the archetype of theranostics (I-123 or I-131 scans image the same uptake). Roles: remnant ablation after thyroidectomy (now omitted in low-risk disease after ESTIMABL2, IoN, HiLo), adjuvant treatment of intermediate/high-risk disease, and treatment of iodine-avid metastases. Given after TSH stimulation (withdrawal or recombinant TSH). Salivary damage, secondary malignancy at high cumulative doses, and refractoriness in dedifferentiated tumours are the limits.

## Fields

- Kind: Agent
- Status: approved
- Last checked: 2026-09-07
- Brand: Sodium iodide I-131
- Modality: Radiopharmaceutical (beta/gamma emitter, natural uptake)
- Mechanism: Sodium-iodide symporter (NIS) concentrates iodide in thyroid follicular cells; I-131 beta particles (mean path ~0.8 mm) irradiate the cell and neighbours; gamma emission enables imaging.
- Approvals: US 1951: Hyperthyroidism and thyroid carcinoma (first radiopharmaceutical approval)
- Dosing: Oral capsule or solution; 1.1 GBq (30 mCi) for remnant ablation (HiLo); 3.7-7.4 GBq for adjuvant or metastatic treatment; low-iodine diet and TSH stimulation beforehand

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Iodine-131
- FDA label (DailyMed): https://dailymed.nlm.nih.gov/dailymed/search.cfm?query=Radioactive%20iodine

## Connected records

- isotopes: [Iodine-123](https://nuclide.cc/isotopes/i-123/), [Iodine-124](https://nuclide.cc/isotopes/i-124/), [Iodine-131](https://nuclide.cc/isotopes/i-131/)
- indications: [Functional assessment of thyroid nodules](https://nuclide.cc/indications/thyroid-nodule-assessment/), [Graves' disease and hyperthyroidism](https://nuclide.cc/indications/graves-hyperthyroidism/), [Thyroid cancer](https://nuclide.cc/indications/thyroid/)
- technologies: [Radioiodine therapy and whole-body iodine scanning](https://nuclide.cc/technologies/radioiodine-therapy/), [Radioligand therapy (beta emitters)](https://nuclide.cc/technologies/radioligand-therapy/)
- targets: [NIS (sodium-iodide symporter)](https://nuclide.cc/targets/nis-symporter/), [Somatostatin receptor 2](https://nuclide.cc/targets/sstr2/)
- terms: [Radioiodine therapy (I-131)](https://nuclide.cc/terms/radioiodine-term/), [Theranostics](https://nuclide.cc/terms/theranostics/)
- trials: [ESTIMABL2](https://nuclide.cc/trials/estimabl2/), [HiLo](https://nuclide.cc/trials/hilo/), [IoN](https://nuclide.cc/trials/ion-trial/)
- institutions: [Board of Radiation and Isotope Technology](https://nuclide.cc/institutions/brit/)
- companies: [PDRadiopharma](https://nuclide.cc/companies/pdradiopharma/)

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