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This page gives information for patients considering Iodine-131 iodide (radioiodine) therapy for the treatment of benign thyroid disease. Radioiodine therapy is an effective treatment for overactive and / or enlarged thyroid glands. It has been used all over the world for more than 70 years. Approximately 10,000 people of all ages are treated with radioiodine each year in the UK.

The thyroid is a gland in the neck that makes hormones that control your blood pressure and body temperature. To perform its normal function, the thyroid needs iodine, which is absorbed from the food we eat. Radioiodine is a special type of iodine that is radioactive. Radioiodine is taken up by the thyroid in the same way as normal iodine. The radiation that it gives out destroys cells in the thyroid, reducing its ability to make thyroid hormones.

Radioiodine is recommended as a first-line treatment for Graves’ disease and toxic goitre by the National for Health and Care Excellence (NICE). Evidence shows that radioiodine is more effective in the long term than anti-thyroid drugs and equally as effective as surgery. In many cases, only one treatment is required, and anti-thyroid medication can then be stopped, making for a more simplified medication regime.

All treatments and procedures have risks, and we will talk to you about the risks of radioiodine therapy.

Problems that may happen straight away

Most patients experience no symptoms at all following treatment. In some cases, the thyroid gland may feel a bit tender, but this will generally clear up on its own after a few days.
Although radioiodine is beneficial for you, it can cause other people to be unnecessarily exposed to radiation. Other people can receive a radiation dose if you are in close contact (less than 1 metre) with them for a significant amount of time, or if there is a transfer of any radioiodine excreted from your body to others through their direct contact with your body fluids. Because of this you should make all possible effort to minimise the radiation dose to others from your treatment, and we will advise you how you can do this. Information on the restrictions we ask you to follow can be found in the section below 'What should I expect after radioiodine therapy?'

Problems that may happen later

The usual dose of radioiodine aims to make your thyroid produce reduced amounts of thyroid hormones within 3-4 weeks of treatment. The thyroid gland becomes underactive (hypothyroid) in 70-80% of patients two years after receiving radioiodine treatment. This number does depend on the reason the radioiodine is given – it is less for patients who have a thyroid nodule. If you become hypothyroid this will require lifelong treatment with replacement hormones.

In some cases, the dose of radioiodine given may not be enough, and over-activity of the gland will persist. If so, a further dose of radioiodine can be given after two months.

Problems that are rare, but serious

Radioiodine may worsen thyroid eye disease in patients with Graves’ disease (especially in smokers). Therefore, radioiodine treatment may be postponed until the eye problems are stable or treatment is given with either steroid or thyroid medication to prevent any problems. Please let the thyroid doctors know if you have any eye trouble.

It is important to note that while any exposure to radiation can result in a small increase in risk of cancer, there is no evidence of a clinically important increase in the incidence of thyroid cancer, leukaemia or other tumours has been shown to occur in patients who receive radioiodine treatment.

Radioiodine should not be given to someone who is pregnant or breastfeeding as it can be passed on to the baby. We also ask all patients avoid conceiving a child for six months following treatment.

If you decide not to undergo radioiodine therapy, options include continuation of anti-thyroid medication, or consideration of surgery. If you change your mind at a later date and wish to reconsider radioiodine treatment, this can always be discussed with your treating Endocrinologist.

The alternative treatment options for an overactive thyroid include medication to control the symptoms (for example, racing heart – this will not treat the underlying hyperactive thyroid) and to treat the thyroid itself (medications that treat the hyperactive thyroid include propylthiouracil and carbimazole). These can have side-effects which your doctor will be able to discuss with you. Other options include surgery for the thyroid gland; this can treat the hyperactive thyroid but carries the risk of surgery and general anaesthetic.

Anti-thyroid drugs (Carbimazole, Propylthiouracil) should be stopped at least five days prior to treatment as they prevent radioiodine being taken up by the thyroid. Your doctor will advise you about this. Only restart this medication after treatment if advised by your doctor.

It is important that you have a low iodine diet for the week prior to your therapy to maximise the likelihood of therapy success. This means you should avoid shellfish, fish oil supplements, and food containing artificial red food colourings (including cough mixtures) for the week before your therapy, as they may contain iodine which could stop the radioiodine therapy from working efficiently.
You may also need to make some arrangements with work or childcare if required due to restrictions following treatment.

We want to involve you in all the decisions about your care and treatment. If you decide to go ahead with treatment, by law we must ask for your consent and will ask you to sign a consent form. This confirms that you agree to have the procedure and understand what it involves. Staff will explain all the risks, benefits and alternatives before they ask you to sign a consent form. If you are unsure about any aspect of your proposed treatment, please don’t hesitate to speak with a senior member of staff again.

Normally the radioiodine is given as a small capsule which you will be asked to swallow with a drink of water.


When you come for your appointment, the doctor will review your thyroid uptake scan to make sure that you are suitable for therapy. Often the thyroid uptake scan will be scheduled for the morning and the therapy on the same afternoon.

Before the treatment, the doctor will check that you have discontinued your thyroid medication and explain the precautions you will need to follow after treatment. This will include precautions specific to you, based on the questions you answered when you were referred for treatment. If you are of childbearing potential, we will also perform a pregnancy test. The doctor will then answer any of your questions and make sure that you are happy to proceed with the therapy by signing the consent form.

The doctor will then prepare the capsule. We ask you to tip the capsule back into your mouth without touching it with your hands, either from a cup or a straw. The doctor will then ask you to sit in the waiting room for 30 minutes to make sure that you are feeling well. After that you are able to go home.

If you have trouble swallowing capsules, please inform your doctor before the treatment. You can be given a drink of liquid radioiodine, but this needs to be pre-arranged at least two weeks before your appointment.

Following your treatment with radioiodine, you will need to take steps to prevent radiation exposure to other people by increasing your distance from them. This may mean taking time off work or arranging alternative childcare for any children you look after.

The amount of radiation in your body decreases with time after treatment until it disappears, and so the risk presented to others by your contact with them will decrease with time. Young children are particularly sensitive to the effects of radiation, so the restrictions are stricter for contact with children.

At your appointment, you will give you a yellow card with your restrictions written on it and the date they expire. We ask you to carry this card with you at all times while the restrictions are in place. Normally your appointment letter is enough evidence to allow you to take time off work, but if you require an additional letter, please contact the Nuclear Medicine admin team on 020 3447 0565.
If you have any questions or concerns about these restrictions or about radiation, please contact the Nuclear Medicine physics team on 07984 288 596.

Close contact restrictions

To reduce the radiation dose to those around you, you should minimise the time you spend with them, and maximise the distance between you. We define “close contact” as a distance of 1 metre or less and “prolonged contact” as a time of 15 minutes or more. The restrictions we give you depend on the dose that has been planned for you. An example of a typical patient who has received 600 MBq of I-131 is given.

Following the treatment we ask that you:

  • Have no close contact pregnant or breastfeeding women for 25 days.
  • Have no close contact with any children or adults in your household for 14 days. After this time we ask you to limit the time spent in close contact with children under 5 to a maximum of 15 minutes per day for a further
    • 6 days for children aged 3-5 years (a total of 20 days after treatment)
    • 11 days for children aged under 3 years (a total of 25 days after treatment). Due to the difficulty in avoiding close contact with very young children we advise that they are cared for elsewhere for 25 days following treatment where possible.
  • Sleep separately (more than 1 metre from others) for 24 days. Ensure that your bed is not against a wall that has a bed on the other side during this time.
  • Do not use public transport for journeys lasting longer than 2 hours for 14 days.
  •  Avoid repeated extended contact with the same members of the public for 10 days.
  • Take time off work, or work from home, for
    • 25 days if your work involves close contact with children or pregnant women
    • 21 days if you spend more than 4 hours per day at 1 metre from other adults
    • 10 days if you spend 4 hours or less per day at more than 1 metre from other adults
    • No time off is required if you can maintain a distance of 2 metres from others at work provided that you can follow the hygienic precautions below

Hygienic precautions

Most of the radioiodine not collected in the thyroid gland will be eliminated in your urine during the first two days after treatment. Very small amounts are also excreted through your saliva, sweat and faeces. It is possible for the radioiodine excreted from your body to be transferred to others if there is contact with your body fluids. This could either happen through direct contact with the fluid or from fluids deposited in places where other people may come into contact with them, such as urine on a toilet seat.

We ask you to follow these hygienic precautions for three days after treatment to minimise the chance of contamination:

  • Always sit down when using the toilet and flush the toilet twice after use. Wash your hands thoroughly as soon as possible afterwards.
  • Use a separate toilet or bathroom if available and ensure that the toilet and bathroom are kept clean.
  • Do not share a toothbrush or towel with anyone else, and wash your clothes separately.
  • Wash crockery and cutlery separately and keep them for your own use where possible.
  • Wear gloves when preparing and cooking food and when handling any children’s items or other items used regularly by others – e.g. phones, tablets, personal computers.
  • Refrain from kissing and sexual activity.


Your follow up will be with the Endocrinology team, who have the overall management of your thyroid condition. You should be in contact with them in the week following your therapy (via email: and should have your blood tested every for four weeks for a period of four months, to assess how your thyroid is functioning following therapy. If you have any questions with regard to radiation risk or other procedures to follow, you will be given contact details for the Nuclear Medicine department at the time of your therapy.

You must contact the Nuclear Medicine Clinic if you vomit within four hours of the treatment, as your therapy may need to be repeated.

You should also contact the Nuclear Medicine Clinic to inform them if you vomit or are accidentally incontinent within 24 hours of the treatment.

More information and support can be found from the British Thyroid Foundation ( or from NHS direct (

Guidance can also be found on the website of the National Insititute for Health and Care Excellence (NICE) (

UCLH cannot accept responsibility for information provided by other organisations.

  • Thyroid Disease: assessment and management, National Institute for Health and Care Excellence (NICE) guidelines [NH145], published 20 November 2019