A trial to see if radioactive iodine treatment is necessary for low risk thyroid cancer (IoN)

Please note - this trial is no longer recruiting patients. We hope to add results when they are available.

Cancer type:

Thyroid cancer




Phase 2/3

This is a trial to find out if people need radioactive iodine treatment after surgery for thyroid cancer that is at low risk of coming back. The trial is supported by Cancer Research UK.

More about this trial

The people in this trial have papillary thyroid cancer or follicular thyroid cancer. These 2 types are also called differentiated thyroid cancer.

After surgery to remove thyroid cancer, you need to take thyroid hormone tablets to replace the hormones your thyroid gland would normally make. You may also have radioactive iodine treatment. Your doctor may call this radioiodine ablation. It destroys any thyroid tissue and cancer cells that might be left behind after surgery.

But if you have had your thyroid gland completely removed (a total thyroidectomy) and your cancer has not spread outside your thyroid gland, it is very unlikely that any cancer cells are left after surgery. Doctors don’t know if radioactive iodine is really necessary for people in this situation.

The aim of the trial is to see if radioactive iodine treatment is necessary for people who have differentiated thyroid cancer that has not spread and is at low risk of coming back after surgery.

Who can enter

You may be able to enter this trial if you

  • Have papillary or follicular thyroid cancer
  • Have had surgery to remove your thyroid gland (a total thyroidectomy) in the last 6 months and your doctors think there is a low risk of your cancer coming back
  • Are well enough to be up and about for at least half the day (performance status 0, 1 or 2)
  • Are aged 16 or over
  • Are willing to use reliable contraception from when you join the trial until at least 6 months (4 months for men) after your last dose of radioactive iodine, if there is any chance you or your partner could become pregnant – the trial team will tell you more about this

You cannot enter this trial if

  • You have anaplastic or medullary thyroid cancer
  • Your surgeon was not able to completely remove your thyroid cancer
  • You have had a CT scan with an injection of contrast medium in the last 2 months
  • You have had any other treatment for thyroid cancer (apart from surgery)
  • You have had any other type of cancer that could affect the results of this trial
  • You have certain problems with your digestive system or would not be able to have radioactive iodine for some reason (the trial team can advise you about this)
  • You have any other medical condition that the trial team think could affect you taking part
  • You are pregnant or breastfeeding

Trial design

The trial will recruit up to 560 people. It is a randomised trial. The people taking part are put into treatment groups by a computer. Neither you nor your doctor will be able to decide which group you are in.

Half the people taking part have radioactive iodine treatment, the other half do not.

The trial team will ask you to fill in some questionnaires

  • When you join the trial
  • After 2 months
  • Between 6 and 9 months later
  • Then every 6 months for the next 5 years

The questionnaires will ask about side effects and how you’ve been feeling.  This is called a quality of life study.

The trial team will ask your permission to keep a sample of cancer that was removed when you had surgery, and to take an extra blood sample. Researchers will use these samples to learn more about thyroid cancer and how best to treat it. If you don’t want to give these samples for research you don’t have to. You can still take part in the main trial.

Hospital visits

When you join the trial, you have a blood test and an ultrasound scan of your neck to see how much (if any) thyroid tissue is left after your operation.

You will then have another blood test to measure thyroglobulin levels. Before you have this test, you have to stop your thyroid hormone tablets for a while, or have a drug called recombinant human thyroid stimulating hormone (rhTSH). Whether you stop your tablets or have rhTSH will depend on the usual practice at the hospital where you are having your treatment.

If you are in the group having radioactive iodine, you will have this after having the scan of your neck, possibly on the same day. You may stay in hospital for between 1 and 3 days while the radioactive iodine takes effect. If you stopped taking your hormone tablets, you can start them again about 2 days after having the radioactive iodine.

If you are in the group not having radioactive iodine treatment and you stopped taking your hormone tablets, you can start them again straight after having the blood test.

You have a blood test 2 months later. The doctors may change the dose of thyroid hormones you take after looking at the result of this test. You will not have to stop your hormone tablets or have rhTSH at this visit.

Between 6 and 9 months after having the first neck ultrasound scan (and radioactive iodine treatment if you had it), you have another ultrasound scan and a blood test. You will also have a 2nd blood test to measure thyroglobulin levels. As before, you have to stop your hormone tablets for a while, or have rhTSH for 2 days before this blood test.

You then go to hospital every 6 months for the next 5 years. You have a blood test at each visit. You have an ultrasound scan twice in the 1st year and then once a year for the next 4 years.

Side effects

The risk of your cancer coming back is very small. Doctors don’t know if not having radioactive iodine increases the risk at all. The trial team will monitor you for 5 years and if there is any sign that your cancer has come back, they will talk to you about the best treatment to have. This may be more surgery or radioactive iodine. We know from research that even if your cancer comes back, it is still highly likely to be cured.

There are some possible side effects of radioactive iodine treatment. In the short term these include

  • Temporary taste changes
  • Tender or swollen salivary glands
  • Neck discomfort and swelling a few days after treatment (this is uncommon)
  • Feeling sick (this is very uncommon)
  • Irritation of your bladder or stomach (this is rare)

Possible long term side effects include

  • Permanent damage to your salivary glands, causing a dry mouth
  • An overflow of tears due to blocked drainage from glands in your eyes (this is rare)

If you have radioactive iodine treatment, there is a very small risk of getting another cancer or leukaemia later in life. And for women, there is a slight increase in the risk of miscarriage in the first year after treatment.

Recruitment start:

Recruitment end:

How to join a clinical trial

Please note: In order to join a trial you will need to discuss it with your doctor, unless otherwise specified.

Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Chief Investigator

Dr Ujjal Mallick

Supported by

Cancer Research UK
Experimental Cancer Medicine Centre (ECMC)
NIHR Clinical Research Network: Cancer
University College London (UCL)

Other information

This is Cancer Research UK trial number CRUK/11/010.

Freephone 0808 800 4040

Last review date

CRUK internal database number:


Please note - unless we state otherwise in the summary, you need to talk to your doctor about joining a trial.

Around 1 in 5 people take part in clinical trials

3 phases of trials

Around 1 in 5 people diagnosed with cancer in the UK take part in a clinical trial.

Last reviewed:

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