What is radioactive iodine?

Radioactive iodine (RAI) is a treatment that uses radiation to treat thyroid cancer. The radiation comes from a radioactive substance called iodine-131. You usually take radioactive iodine as a capsule that you swallow. Radioactive iodine is a treatment for papillary and follicular thyroid cancers.

RAI may be given to:

  • destroy any normal thyroid tissue still in the neck after surgery – this is called radioiodine remnant ablation (RRA)
  • treat any thyroid cancer that could not be surgery
  • reduce the risk of thyroid cancer coming back (recurrence)
  • treat thyroid cancer that has come back after treatment.

Usually, the thyroid gland takes iodine from our diet and uses it to make thyroid hormones. When you have RAI, the thyroid cells take in the iodine and get a very high dose of radiation. This destroys the thyroid cells and any cancer cells that may be left after surgery.

RAI does not affect other parts of the body in the same way. This is because other cells do not absorb iodine as much as thyroid cells.

You may be offered RAI treatment again if your tests show:

  • there are still some thyroid cells after treatment
  • the cancer has come back.

Preparing for radioactive iodine treatment

Before you have radioactive iodine (RAI) treatment, you will usually be asked to start eating a low-iodine diet. You may be asked to stop taking your thyroid hormones.

Low-iodine diet

You will usually be asked to have a low-iodine diet for 1 to 2 weeks before your treatment starts. This is because having too much iodine in your body may make the treatment less effective. Your doctor or specialist nurse will give you more information and tell you when to start the low-iodine diet.

We have information about which foods to eat, avoid or limit on a low-iodine diet below.

  • Foods you can eat freely
    • fresh or frozen fruit and vegetables, including potatoes
    • cooked green vegetables like spinach and broccoli
    • fresh or frozen meat
    • ordinary table salt and sea salt
    • rice and dried pasta
    • fresh bread (including fresh sliced bread)
    • non-dairy spread such as Vitalite or Pure or Flora non-dairy
    • olive oil, vegetable oils and nut oils
    • water, soft or fizzy drinks, fruit juices and alcoholic drinks
    • tea and coffee without milk
    • milk substitutes such as rice, coconut, almond or soya milk (please check labels for ingredients and avoid brands that contain carrageenan which is a seaweed derivative)
    • dark or plain chocolate with a minimum of 70% cocoa
    • crisps.
  • Foods you should limit
    • milk – limit to a maximum of 25ml per day, which could be used in tea and coffee (about 5 to 7 teaspoons of milk per day)
    • butter – limit to a very thin layer (1 teaspoon or 5g) per day
    • cheese – 1 ounce or 25g once per week
    • other dairy products like yoghurt, dairy ice cream – limit to 1 portion per week
    • egg – limit to 1 per week
    • products containing egg like mayonnaise, custard, fresh egg pasta, egg fried rice, pancakes, Yorkshire puddings etc.
  • Foods you should avoid
    • fish, seafood, seaweed, kelp, laverbread
    • raw green vegetables like spinach or broccoli
    • cakes and biscuits containing butter and eggs
    • milk or white chocolate
    • takeaway meals and fast foods, or restaurant foods (as ingredients are unknown)
    • iodised salt and Pink Himalayan salt, if imported or bought outside the UK
    • vitamins and mineral supplements, nutritional supplements and cough mixtures (unless prescribed by your medical team, for example vitamin D)

What if I eat something high in iodine by mistake?

Eating a high iodine food by mistake should not affect your treatment. You should keep following the low-iodine dietary advice provided here.

Thyroid hormones

Radioactive iodine needs a high level of thyroid-stimulating hormone (TSH) to be effective. There are 2 ways to increase the TSH level in your blood. The most common way is to be given a drug called Recombinant human thyroid-stimulating hormone.

Recombinant human thyroid-stimulating hormone (rhTSH, Thyrogen®)

You may be given recombinant human thyroid-stimulating hormone (rhTSH). This is also called thyrotropin alfa (Thyrogen®).

The drug is like the TSH produced in your body. It allows you to keep taking your hormone-replacement tablets. This means you avoid the symptoms of thyroid hormone withdrawal. You have 2 injections, 24 hours apart. You have them on the 2 days before your RAI treatment. The injections are given into the buttock (intramuscularly). You usually have the injections as an outpatient.

rhTSH is not suitable for everyone. Your doctor or specialist nurse can tell you if you can have this treatment.

rhTSH has very few side effects. Possible side effects include:

Only a small number of people will get these side effects. They are usually mild and only last for 1 to 2 days. Paracetamol may help with any aches or pains.

Less common way of increasing your TSH

If you are not able to have the rhTSH injection, your doctor may advise you to stop taking your thyroid hormones (T3 or T4). This is because thyroid hormone replacement stops your body from making TSH. You usually stop taking them for a few weeks before and during your treatment. This allows your body to make its own TSH. Your doctor or nurse will tell you when to stop taking them. It is important to stop taking thyroid hormones when you are told to, or the RAI treatment may not work as well as it should.

While you are not taking your thyroid hormones, you will have the symptoms of thyroid hormone withdrawal. These are likely to include: 

  • tiredness (fatigue), weakness and a lack of energy
  • hair thinning
  • poor concentration
  • constipation
  • feeling cold
  • a low mood.

These symptoms should stop when you start taking the thyroid hormone tablets again, but it can take some time.

Pregnancy and breastfeeding

You cannot have radioactive iodine (RAI) treatment if you are pregnant or breastfeeding. It is important to tell your doctor if you are pregnant, or think you might be. If you are at an age where you can still get pregnant, you usually have a pregnancy test before you have your treatment.

If you are breastfeeding, you must stop several weeks before having RAI. This helps to lower the amount of radiation the breasts are exposed to. It is not safe to start breastfeeding again after your treatment. But it will be safe for you to do so if you have more children in the future.

Having your radioactive iodine treatment

You are usually admitted to the ward on the day of your RAI treatment. You usually have radioactive iodine (RAI) as a capsule. Before and after the treatment, you can eat normally. Your nurse will encourage you to drink plenty of fluids.

Because the iodine is radioactive, you will be radioactive for a while after the treatment. The radioactivity will slowly leave your body in your:

  • urine (pee)
  • stools (poo)
  • blood, if you are having a period
  • saliva
  • sweat.

Some people may have RAI treatment and go home on the same day. But most people need to stay in hospital, in a room on your own, for a few days after treatment. During this time, the level of radioactivity is checked regularly. You will have to follow certain safety measures and restrictions. These include:

  • having no contact with people aged under 18 and pregnant women
  • restrictions on the length of time visitors and staff can spend with you
  • restrictions on what you can bring into hospital with you.

Each hospital has different routines for giving RAI treatment. The staff looking after you will explain these before your treatment. You may find these safety measures and restrictions difficult, especially if you have young children. Some people feel lonely staying in a single room. Talk to your specialist nurse before treatment and tell them if you have concerns. You may be able to visit the treatment unit to talk through the treatment.

You will need to flush the toilet twice each time you use it. The nurses will also encourage you to shower once or twice each day. This helps to wash any radioactivity from your skin.

Side effects of radioactive iodine treatment

Many people do not have any side effects after radioactive iodine (RAI) treatment. Side effects may include some of the following:

  • Soreness or tightness around your neck or in the surgical scar.
  • Feeling a bit sick (mild nausea). This usually only lasts for a couple of days.
  • A dry mouth. This is not common during treatment, but it may happen after you go home. It can help to drink plenty of fluids.
  • Taste changes. This may not happen during treatment, but it can happen days or weeks later. It usually gets better within 4 to 8 weeks.

It is important to tell your doctor or nurse if you have any of these symptoms. They can give you medicines to help.

Rarely, a dry mouth can become a long-term side effect of RAI treatment. This usually only happens if you have had more than one treatment. It happens if the salivary glands take in the radioactive iodine. Tell your doctor or specialist nurse if you get a dry mouth or if it does not improve. They can help you manage it.

Pregnancy and fertility

After RAI treatment, it is important to use contraception to prevent pregnancy. You should not:

  • become pregnant for at least 6 months

    you may have a slightly higher risk of a miscarriage in the first year after RAI treatment. Your doctor or nurse can give you more information and support about this.

  • make someone pregnant for at least 4 months

    RAI treatment does not normally affect fertility. But there is a very small risk if you need more than one treatment. In this case, you may be offered the option of storing sperm so it can be used to make someone pregnant in the future (sperm banking).

Going home

When you go home after radioactive iodine (RAI) treatment, you will need to continue taking certain precautions for a few days. These may include:

  • avoiding close contact with children, babies, pregnant women and pets
  • limiting close and prolonged contact with people, for example you may not be able to share a bed or have sex for a few days
  • avoiding long journeys on public transport, or going to the cinema, the pub, or anywhere where you could be sitting or standing next to the same person for a long time.

How long you need to take these precautions for depends on how quickly the radioactivity leaves the body. Before you go home, the team looking after you will give you more detailed information. They will also give you some written information about the treatment and safety measures you can take. You should carry this document with you for 7 days after treatment.

Travelling after radioactive iodine treatment

For up to 3 months after RAI treatment, you may set off security radiation alarms at airports. It is a good idea to take the written information from the hospital, or a letter from your doctor, explaining the treatment you have had.

About our information

  • References

    Below is a sample of the sources used in our thyroid cancer information. If you would like more information about the sources we use, please contact us at cancerinformationteam@macmillan.org.uk

    British Medical Journal. Best Practice Guidelines, Thyroid cancer. 2020.

    European Society Medical Oncology (ESMO): Thyroid cancer, Clinical Practice Guidelines for Diagnosis, Treatment and Follow-up. 2019.

    National Institute for Health and Care Excellence (NICE). TA535: Lenvatinib and Sorafenib for treating differentiated thyroid cancer after radioactive iodine. 2018. www.nice.org.uk/guidance/ta535 [accessed May 2021].

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Senior Medical Editor, Professor Nick Reed, Consultant Clinical Oncologist.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

The language we use

We want everyone affected by cancer to feel our information is written for them.

We want our information to be as clear as possible. To do this, we try to:

  • use plain English
  • explain medical words
  • use short sentences
  • use illustrations to explain text
  • structure the information clearly
  • make sure important points are clear.

We use gender-inclusive language and talk to our readers as ‘you’ so that everyone feels included. Where clinically necessary we use the terms ‘men’ and ‘women’ or ‘male’ and ‘female’. For example, we do so when talking about parts of the body or mentioning statistics or research about who is affected.

You can read more about how we produce our information here.

Date reviewed

Reviewed: 01 June 2021
Next review: 01 June 2024
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

Our cancer information meets the PIF TICK quality mark.

This means it is easy to use, up-to-date and based on the latest evidence. Learn more about how we produce our information.