Preparing for radioactive iodine treatment for thyroid cancer

Radioactive iodine treatment is a type of internal radiotherapy. It uses a radioactive form of iodine called iodine 131 (I-131). It is a useful treatment in thyroid cancer because the thyroid gland absorbs and stores most of the iodine in your body. The thyroid gland gets iodine from certain foods and uses this to make essential thyroid hormones.

Radioactive iodine is a targeted treatment. The radioactive iodine circulates throughout your body in your bloodstream. But it is mainly taken up by thyroid cells, having little effect on other cells. Thyroid cancer cells in your body pick up the iodine. The radiation in the iodine then kills the cancer cells.

It is only suitable for some types of thyroid cancer. It is a treatment for:

  • follicular thyroid cancer
  • papillary thyroid cancer

It can treat the cancer even if it has spread. But even if you have one of these types of thyroid cancer, this treatment may not be necessary or suitable for you. Not all of the cancer cells take up the iodine so you may have a test dose to see if they do.

When do you have radioactive iodine?

You might have radioactive iodine treatment:

  • after surgery, to kill any cancer cells that may have been left behind
  • to treat thyroid cancer that has spread
  • to treat thyroid cancer that has come back after it was first treated

You may only need to have this treatment once. But it can be repeated every 3 months if needed, until there is no sign of any thyroid cancer on your scans.

Before your radioactive iodine treatment

For your treatment to work, one of the hormone levels in your body needs to be high. This hormone is called thyroid stimulating hormone or TSH. A high level of TSH helps any thyroid cancer cells in the body to take up radioactive iodine.

To increase the levels of TSH, your doctor will ask you to do one of the following:

  • have an injection of a man-made TSH called recombinant human TSH (rhTSH) once a day for 2 days
  • stop taking your thyroid hormone tablets a few weeks before treatment (thyroid withdrawal)

Your healthcare team will explain what you need to do and what is going to happen. They will also let you know when to re-start your hormone tablets if they have asked you to stop taking them.

Low iodine diet

You might need to have a low iodine diet for a few weeks before you have your treatment. This prepares the thyroid cells so that they can absorb the radioactive iodine. All food and drink contains some iodine.  A low iodine diet means that you should avoid foods that have a high level of iodine in them.

Foods you are allowed 

You can eat the following foods because they contain very low levels of iodine:

  • fruits and vegetables including potatoes
  • cooked green vegetables
  • meat
  • ordinary table salt and sea salt
  • fresh bread
  • rice and dries pasta
  • non dairy spreads such as Vitalite, Pure and non dairy Flora
  • olive oil, vegetable oils and nut oils
  • water, soft drinks, fizzy drinks, fruit juices and alcoholic drinks
  • tea and coffee without milk
  • milk substitutes such as coconut, rice, almond and soya milk avoid ones that contain an ingredient called carrageenan (as this comes from seaweed)
  • dark and plain chocolate that is 70% cocoa or more
  • crisps

Restricted food 

You can eat a small amount of these food items as they have a moderate amount of iodine:

  • milk about 5 - 7 teaspoons a day (25 ml)
  • butter a teaspoon (5 g) each day
  • cheese 25 g (1 ounce) per week
  • dairy products such as yoghurt and dairy ice cream 1 serve per week
  • 1 egg each week
  • products that contain eggs such as mayonnaise, custard, fresh egg pasta, egg fried rice, Yorkshire pudding, pancakes

Food to avoid

You shouldn't eat these foods as they have high levels of iodine:

  • fish, seafood, seaweed, kelp and laverbread
  • raw green vegetables such as spinach and broccoli
  • cakes and biscuits made with eggs or butter
  • milk chocolate and white chocolate
  • take away foods, fast foods and restaurant food as their ingredients aren't known and might contain iodine
  • iodised salt and Pink Himalaya salt that has come from outside the UK
  • vitamins and mineral supplements, nutritional supplements and cough mixtures (unless prescribed by your medical team, for example vitamin D)

You can return to your normal diet after you have had your treatment.

Pregnancy and breastfeeding

Tell your doctor if you think you may be pregnant. You should not have this treatment during pregnancy.

Talk to your doctor if you are breastfeeding. You will need to stop breastfeeding before you have treatment with radioactive iodine.

Last reviewed: 
20 May 2021
Next review due: 
20 May 2024

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    AL Mitchell and others
    Journal of Laryngology and Otology, 2016. Volume 130, Supplement 2, Pages 150 to 160

  • Low iodine diet in differentiated thyroid cancer: a review

    J. H. Li and others

    Clinical Endocrinology. 2016, 84 pages 3-12

  • Cancer and its management (8th edition)
    J Tobias and D Hochhauser
    Wiley-Blackwell, 2010

  • Low Iodine Diet for Preparation for Radioactive Iodine Therapy in Differentiated Thyroid Carcinoma in Korea

    J. H. Chung

    Endocrinology and Metabolism. 2013, Volume 28, Number 3, pages 157-163

  • Low Iodine Diet Working Group 2018 (Version 2)

    British Thyroid Foundation website, accessed May 2021

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