# Radioactive iodine (I-131)

Source: https://onco.cc/drugs/radioactive-iodine/  
OnCo record `radioactive-iodine` (Treatment). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.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: Treatment
- 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

## Notes

- Correction 23 Sept 2026: the record linked the somatostatin receptor 2 target, which belongs to the SSTR2 radioligands (Lutathera, RYZ101) and not to radioiodine. I-131 enters thyroid cells through the sodium-iodide symporter (SLC5A5, NIS), which has no target page in OnCo yet, so the record now names no target; the mechanism text says which transporter it uses. Found by the open drug engine, where the wrong link showed I-131 as an approved SSTR2 radioligand.

## Sources

- Wikipedia: https://en.wikipedia.org/wiki/Iodine-131
- FDA label (DailyMed): https://dailymed.nlm.nih.gov/dailymed/search.cfm?query=Radioactive%20iodine
- UniProt Q92911: sodium/iodide cotransporter (SLC5A5, NIS), the uptake route of iodide into thyroid cells: https://www.uniprot.org/uniprotkb/Q92911/entry

## Connected records

- cancers: [Thyroid cancer](https://onco.cc/cancers/thyroid/)
- technologies: [Radioiodine therapy and whole-body iodine scanning](https://onco.cc/technologies/radioiodine-therapy/), [Radioligand therapy (beta emitters)](https://onco.cc/technologies/radioligand-therapy/), [Thyroglobulin, calcitonin and CEA in thyroid cancer follow-up](https://onco.cc/technologies/thyroid-cancer-markers/)
- terms: [Low-risk differentiated thyroid cancer (ATA risk)](https://onco.cc/terms/low-risk-dtc/), [Radioiodine therapy (I-131)](https://onco.cc/terms/radioiodine-term/), [Radioiodine-refractory (RAI-R) thyroid cancer](https://onco.cc/terms/rai-refractory/), [Theranostics](https://onco.cc/terms/theranostics/)
- trials: [ESTIMABL2](https://onco.cc/trials/estimabl2/), [HiLo](https://onco.cc/trials/hilo/), [IoN](https://onco.cc/trials/ion-trial/)
- pairings: [MAPK inhibitor redifferentiation → radioiodine](https://onco.cc/pairings/redifferentiation-rai/)
- roadmaps: [Radiotherapy roadmap: X-rays → shaped beams → fewer fractions, particles and FLASH](https://onco.cc/roadmaps/radiation-roadmap/)
- pathways: [Thyroid cancer (KEGG map)](https://onco.cc/pathways/thyroid-cancer-signalling/)

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JSON: https://onco.cc/api/v1/entities/radioactive-iodine.json