Radioactive Iodine: What to Expect, and the Low-Iodine Diet

Radioactive iodine (RAI) is a capsule of iodine-131 that some people have after a total thyroidectomy for thyroid cancer, to clear leftover thyroid tissue or lower the chance of the cancer returning. To prepare, your TSH is raised, usually with two injections, and you follow a low-iodine diet for about one to two weeks. In Australia it usually means a short hospital stay in a shielded room, then a few days of precautions at home. Your team will give you your own instructions, and they should always come first.

This one is personal for us: Jilda, who co-founded ThyroBase, had radioactive iodine after her total thyroidectomy. If RAI is on your calendar, here is what to expect, in plain language.

What RAI is for

After surgery for papillary, follicular or oncocytic thyroid cancer, RAI can do one of three jobs:

• clear leftover healthy thyroid tissue, so follow-up tests are easier to read
• lower the chance of the cancer coming back
• target cancer that is still there, in the neck or elsewhere

It does not work for medullary or anaplastic thyroid cancer. RAI is also used for an overactive thyroid, which is a different process with different precautions, so this guide focuses on thyroid cancer.

Not everyone needs it any more

RAI used to be given almost routinely after thyroid cancer surgery. The 2025 American Thyroid Association guideline now says people with low-risk cancer do not routinely need it, and two large trials found that low-risk people who skipped it did no worse at five years. For people at higher risk or with known spread, it remains an important treatment.

Whether you have it depends on your cancer's risk features. That is a conversation with your team, and if you had RAI years ago, it was the standard of care at the time.

Preparing: raising your TSH

RAI works best when TSH is high, because TSH drives any remaining thyroid cells to take up iodine. There are two ways to do this:

• Two injections of recombinant TSH (Thyrogen) on the days before treatment. You keep taking your levothyroxine. This is now the usual approach.
• Stopping levothyroxine for three to four weeks, so your own TSH rises. This is less common now, and usually much harder going for a few weeks, because you become underactive while you wait.

Your team will decide which suits you. Do not stop or change your tablet unless they tell you to.

Preparing: the low-iodine diet

For about one to two weeks before RAI, you will usually be asked to keep your iodine intake very low, so the remaining thyroid cells are hungry for the treatment. A few things are worth knowing:

• It cuts iodised salt, not salt. Plain non-iodised salt is generally fine in small amounts. Cutting all salt can cause a dangerous drop in sodium, especially if you have stopped your levothyroxine, so do not go salt-free.
• Bread is different in Australia. By law, shop-bought bread here is made with iodised salt, so it is usually off the list during the diet.
• The usual foods to pause include seafood, seaweed and sushi, dairy, egg yolks and some soy products, but use your hospital's list rather than an overseas one, because Australian foods differ.
• Longer is not better. Guidelines say there is little reason to go beyond two weeks, and stricter diets have not been shown to work better.

The honest picture is that the diet is standard practice, but the evidence for it is thinner than most people assume, and people often find it stressful. Your hospital's dietitian is the right person to ask.

Supplements, scans and medicines

• Pause supplements and powders, ours included, during your low-iodine diet unless your treating team says otherwise. Kelp tablets, many multivitamins, some pregnancy vitamins and many protein powders contain iodine. Ask your team a few weeks before RAI which supplements to stop and when.
• Tell your team about any CT scan with contrast dye in the past couple of months. The dye contains a lot of iodine, and they may want to check it has cleared.
• Tell your team if you take amiodarone, a heart-rhythm medicine that contains a lot of iodine. Never stop it yourself; your cardiologist and RAI team will plan around it.

The treatment and the days after

In Australia, RAI usually means a short stay of one to three days in a shielded single room, because your body gives off radiation for a while after the capsule. When you go home, you will be given written precautions for a few days, based on time, distance and hygiene: keeping some distance from others, especially children and anyone pregnant, sleeping apart, and extra care in the bathroom. How many days depends on you and your dose, so follow your own instructions rather than anyone else's.

A few days after treatment you will usually have a whole-body scan, using the radiation already in your body, to see where the iodine went. It is normal for it to show some thyroid tissue in the neck.

Side effects

Dry mouth, swelling or tenderness of the salivary glands and changes in taste are the most common side effects, and most settle. Some people feel nauseous or have a tender neck for a few days. Drink plenty of water and look after your teeth. Advice on sour lollies differs, including on when to start them, so ask your team what they recommend.

If you could become pregnant, you will be asked to wait at least six months after RAI before trying. A large study found no increase in pregnancy problems when conception was six months or more after treatment.

After RAI

If you stopped your tablet to prepare, your team will tell you when to restart. If you had the injections, you will usually have kept taking it throughout. Either way, do not change your dose on your own.

Follow-up is mostly a thyroglobulin test, a marker made only by thyroid tissue, every six to twelve months at first, plus neck ultrasounds. For people with low-risk cancer and consistently clear results, routine checks eventually step down.

The part nobody prepares you for

Worry and tiredness after thyroid cancer are common and often under-acknowledged. Australian research with thyroid cancer survivors found many were told they were lucky, and felt their ongoing concerns were dismissed. If that is you, you deserve support, not a shrug. Cancer Council's 13 11 20 is a good first call, and your GP can help too. Our guide to fatigue after thyroidectomy may also help.

Questions to ask your team

1. Do I need RAI, and what is it for in my case?
2. Will I have the injections, or stop my tablet?
3. When should my low-iodine diet start and end, and can I have your food list?
4. Which supplements and medicines should I pause, and when?
5. What precautions should I follow at home, and for how many days?

Where ThyroBase fits

Nothing replaces your treating team's plan around RAI. ThyroBase's morning serve contains a little spirulina, which carries a trace of naturally occurring iodine, so pause it, like any supplement or powder, during your low-iodine diet unless your team says otherwise. ThyroBase is a daily morning and evening nutrition powder for people living with Hashimoto's, an underactive thyroid or no thyroid, designed to sit alongside your thyroid medication, never instead of it. Here is who ThyroBase is, and is not, designed for.

Radioactive iodine is short, specific and planned by your team.

Follow their list, not the internet's, and ask for support for the weeks after.

Frequently asked questions

What is the low-iodine diet before radioactive iodine?

It is a short diet, usually one to two weeks, that keeps your iodine intake very low so the remaining thyroid cells take up more of the treatment. It cuts iodised salt, seafood, seaweed, dairy, egg yolks and, in Australia, shop-bought bread. Use your hospital's food list.

Can I eat salt on a low-iodine diet?

Yes, plain non-iodised salt in small amounts is generally fine. The diet cuts iodised salt, not salt itself. Cutting out all salt can cause a dangerous drop in sodium, especially if you have stopped your levothyroxine, so follow your team's advice.

Does everyone with thyroid cancer need radioactive iodine?

No. Current guidelines say people with low-risk thyroid cancer do not routinely need it, and large trials found low-risk people who skipped it did no worse at five years. People at higher risk or with spread often still benefit. Your team will decide based on your cancer.

How long after radioactive iodine should I wait to get pregnant?

If you could become pregnant, guidelines recommend waiting at least six months after RAI. A large study found no increase in pregnancy problems when conception was six months or more after treatment. Talk to your team about your own timing.

The honest takeaway

RAI is a short, planned part of treatment for some people with thyroid cancer, not everyone. Prepare the way your team asks, with their food list and their timing, keep salt in your diet unless told otherwise, and pause supplements during the low-iodine weeks. Then look after yourself in the months after, because feeling fine on paper and feeling fine are not always the same thing.

ThyroBase is a nutritional supplement and is not intended to diagnose, treat, cure, or prevent any disease. It is designed to sit alongside your thyroid medication, not replace it. This article is general information, not medical advice. Always follow your treating team's instructions before and after radioactive iodine.

References

Ringel MD, Sosa JA, Baloch Z, et al. 2025 American Thyroid Association management guidelines for adult patients with differentiated thyroid cancer. Thyroid. 2025;35(8):841-985.
Leboulleux S, Bournaud C, Chougnet CN, et al. Thyroidectomy without radioiodine in patients with low-risk thyroid cancer. New England Journal of Medicine. 2022;386(10):923-32.
Mallick U, Newbold K, Beasley M, et al. Thyroidectomy with or without postoperative radioiodine for patients with low-risk differentiated thyroid cancer in the UK (IoN): a randomised, multicentre, non-inferiority trial. Lancet. 2025;406(10498):52-62.
Mallick U, Harmer C, Yap B, et al. Ablation with low-dose radioiodine and thyrotropin alfa in thyroid cancer. New England Journal of Medicine. 2012;366(18):1674-85.
Schroeder PR, Haugen BR, Pacini F, et al. A comparison of short-term changes in health-related quality of life in thyroid carcinoma patients undergoing diagnostic evaluation with recombinant human thyrotropin compared with thyroid hormone withdrawal. Journal of Clinical Endocrinology and Metabolism. 2006;91(3):878-84.
Borget I, Bonastre J, Catargi B, et al. Quality of life and cost-effectiveness assessment of radioiodine ablation strategies in patients with thyroid cancer: results from the randomized phase III ESTIMABL trial. Journal of Clinical Oncology. 2015;33(26):2885-92.
Herbert G, England C, Perry R, et al. Impact of low iodine diets on ablation success in differentiated thyroid cancer: a mixed-methods systematic review and meta-analysis. Clinical Endocrinology. 2022;97(6):702-29.
Al Nozha OM, Vautour L, How J. Life-threatening hyponatremia following a low-iodine diet: a case report and review of all reported cases. Endocrine Practice. 2011;17(5):e113-7.
Padovani RP, Kasamatsu TS, Nakabashi CC, et al. One month is sufficient for urinary iodine to return to its baseline value after the use of water-soluble iodinated contrast agents in post-thyroidectomy patients requiring radioiodine therapy. Thyroid. 2012;22(9):926-30.
Grewal RK, Larson SM, Pentlow CE, et al. Salivary gland side effects commonly develop several weeks after initial radioactive iodine ablation. Journal of Nuclear Medicine. 2009;50(10):1605-10.
Kim HO, Lee K, Lee SM, et al. Association between pregnancy outcomes and radioactive iodine treatment after thyroidectomy among women with thyroid cancer. JAMA Internal Medicine. 2020;180(1):54-61.
O'Neill CJ, Carlson MA, Rowe CW, et al. Hearing the voices of Australian thyroid cancer survivors: qualitative thematic analysis of semistructured interviews identifies unmet support needs. Thyroid. 2023;33(12):1455-64.
Hedman C, Strang P, Djärv T, et al. Anxiety and fear of recurrence despite a good prognosis: an interview study with differentiated thyroid cancer patients. Thyroid. 2017;27(11):1417-23.
NSW Agency for Clinical Innovation. Low iodine diet: patient factsheet.
Cancer Council NSW. Radioactive iodine treatment; Understanding thyroid cancer. 2025.
Australian Radiation Protection and Nuclear Safety Agency. Radiation Protection Series 14.2. 2008.
healthdirect Australia. Thyroid cancer; Hyperthyroidism. healthdirect.gov.au.