Radioactive iodine (RAI / I-131) ablation

What Is Radioactive Iodine (RAI) Therapy for Thyroid Cancer?

Radioactive iodine therapy — often called RAI, I-131 ablation, or simply iodine therapy — is a treatment usually given after surgery for thyroid cancer. You swallow a measured dose of radioactive iodine as a capsule or a drink. There is no injection, and it is not external beam radiation.

It works because of a simple fact about the thyroid: its cells naturally soak up iodine to make thyroid hormone. The two most common thyroid cancers — papillary and follicular — keep this ability, so they absorb iodine too. The radioactive iodine collects in those cells, and the radiation it gives off destroys them from the inside. Because most other tissues do not take up iodine, the treatment targets thyroid tissue while largely sparing the rest of the body.

RAI is used for two related jobs: to remove any normal thyroid tissue left after surgery (this is called remnant ablation), and to treat any cancer cells that remain or have spread, as long as they still take up iodine. In short, it is a targeted, swallowed treatment that reaches thyroid cells throughout the body.

Who Actually Needs Radioactive Iodine — and Who Doesn't

A common worry is whether RAI is automatic after thyroid surgery. It is not. Whether you need I-131 ablation depends on your individual risk — factors such as the size of the tumour, whether it had spread to lymph nodes or beyond, and your overall risk of the cancer coming back.

People with very small, low-risk cancers often do not need it at all, in keeping with our promise of no unnecessary treatment. Others, with higher-risk disease, benefit clearly. And because medullary and anaplastic thyroid cancers do not absorb iodine, RAI is simply not used for them. The right answer is different for each person.

At CION, this is never one doctor's call. Your case is reviewed by a multidisciplinary tumour board — surgical, medical, radiation and nuclear medicine specialists together — so the recommendation is matched to your exact stage and type.

If you already have a surgery record, pathology report or scan, you are welcome to have it reviewed for free as a written second opinion — it often makes the next step clearer than it first appears.

How Radioactive Iodine Therapy Happens, Step by Step

RAI treatment unfolds over a couple of weeks rather than in one short visit. Here is the usual sequence, so you know exactly what to expect at each stage.

How to Prepare for I-131 Ablation — and Why Each Step Matters

RAI works best when the preparation behind it is right. Each step below makes your thyroid cells hungrier for the iodine, so the treatment reaches the tissue it needs to. Your care team gives you exact, written instructions — this is the plain-language version of why they matter.

• Low-iodine diet for 1–2 weeks — avoiding iodised salt, seafood and certain foods lowers competing iodine, so the radioactive iodine is taken up strongly
• Raise your TSH — by pausing thyroid hormone tablets for a few weeks, or with an injection that lifts TSH, your thyroid cells become eager to absorb iodine
• Avoid recent iodine contrast scans — CT dye contains iodine that can flood the body and blunt the treatment, so spacing it out matters
• Tell your team about medications — some drugs and supplements contain iodine; sharing your full list lets your team adjust timing safely
• Confirm pregnancy and breastfeeding status — radioactive iodine is not given during pregnancy or breastfeeding, so this is always checked beforehand
• Plan the after-treatment precautions — arranging brief distancing from young children and pregnant women, and good hydration, as your team advises

Not sure which of these apply to you? Speak to a CION specialist — we walk you through the prep before anything is scheduled.

Side Effects, Precautions and What Comes Next

Most people tolerate radioactive iodine well, but some short-term effects are common. These can include mild nausea, a dry or altered sense of taste, a sore or swollen neck, and tenderness of the salivary glands, because they also take up a little iodine. Drinking plenty of fluids and sucking on sour sweets, as your team advises, can help protect them.

For a few days after a treatment dose you give off a small amount of radiation, so you follow simple precautions — keeping some distance from young children and pregnant women, good toilet hygiene, and staying well hydrated to clear iodine from the body. These are temporary; most of the iodine leaves the body within days, and the radioactivity itself fades steadily over the following week or two.

At CION, your treatment is planned by a multidisciplinary tumour board rather than one doctor's opinion, and every precaution is explained in writing before you begin. You can read about the wider treatment pathway, including surgery, on our thyroid cancer treatment in Hyderabad page, see how the imaging that guides RAI works on our radioiodine whole-body scan overview, and learn how follow-up blood tests track your recovery on our thyroglobulin test guide. If your thyroid is being removed, our life without a thyroid page explains daily life afterwards. For symptoms, types and care, visit the main thyroid cancer hub.

Understanding exactly what RAI involves is what lets your treatment be matched precisely to your situation — so nothing is over-treated, and nothing is missed.

People Who Walked the Thyroid Journey — and What Came Next

Real stories from patients who brought a report to our team, got a clear answer on treatment, and walked the journey with us.

Disclaimer: This content is for informational purposes only and does not constitute medical advice, diagnosis, or treatment. Always consult a qualified oncologist for guidance specific to your situation. This page is periodically reviewed and updated by CION's medical team in line with current clinical guidelines.