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.
Source: https://www.cancerresearchuk.org/about-cancer/thyroid-cancer/treatment/radiotherapy/radioactive-iodine-treatment/preparing-radioactive-iodine-treatment