Small doses of radioactive iodine can be used to help diagnose cancer of the thyroid (thyroid radioisotope scan), and to see if any papillary or follicular thyroid cancer cells have been left behind after a thyroidectomy (whole-body radioisotope scanning). When given in larger doses, radioactive iodine can be used to treat any remaining cancer cells.

Radioactive iodine is only useful for the detection and treatment of any cancer cells that may remain, if all of the healthy thyroid tissue has been removed. It is common for an operation for papillary or follicular thyroid cancer to be followed by a treatment with radioactive iodine, known as thyroid ablation, to destroy the tiny amounts of normal tissue which are often left behind.

The radioactive substance used for treatment is the same as that used for radioisotope scanning, but given in much larger doses. It is mainly taken as capsules, but can be taken as a drink or injected into a vein in your arm (intravenously). It is a way of giving radiotherapy internally, rather than externally as high-energy rays.

The thyroid cancer cells absorb the iodine and receive a very high dose of radiation, which will help to destroy them. Radioactive iodine has very little effect on other parts of the body since other cells do not absorb iodine as much as the thyroid cells. Radioactive iodine treatment can be repeated if further tests show that cancer cells are still present.

If you are taking thyroid hormones (T3 or T4) these are usually stopped for 2–4 weeks before this treatment. Most people find that stopping the hormone replacement therapy makes them feel very tired, but it is important that you do this or the radioactive iodine treatment will not work.

To help overcome the potential problems of stopping your hormone replacement therapy, it may be possible to be treated with recombinant human thyroid stimulating hormone (rhTSH). This drug, also known as thyrotropin alfa (Thyrogen®), is given as two injections into a muscle, usually in the buttock.  It allows you to carry on taking your hormone replacement tablets and avoid the symptoms of thyroid hormone withdrawal.

Before treatment you may be asked to start eating a low iodine diet, as too much iodine in your body will make the treatment less effective. You will be given advice about which foods to avoid, but you should not have:

• fish and seafood
• table salt that has had iodine added
• cough medicines
• foods that contain the pink food colouring E127, such as salami, glace cherries and tinned strawberries
• vitamin supplements that contain iodine.

You should also try to cut down on the amount of dairy products that you eat, as these also contain some iodine. This includes foods such as:

• milk and milk products
• eggs
• cheese.

Unfortunately, unlike external radiotherapy, this treatment makes you slightly radioactive for about 4–5 days. During this time the radioactivity will be gradually lost from your body in your urine, blood, saliva and sweat. This means that for a few days you will need to be looked after in hospital until the radioactivity has reduced to a safe level.

Very rarely, you may have pain, tiredness and/or breathlessness after having radioactive iodine treatment. If these side effects occur, let your nurse or doctor know, as medicines can be prescribed to help.

Because of the possibility of unnecessary radiation exposure to the hospital staff and your friends and relatives, certain safety measures are taken while the radioactive iodine is still in your body.

The staff looking after you will explain these restrictions to you in detail before you start your treatment. Each hospital has different routines, and it is worth visiting beforehand to discuss the procedure with the nursing and medical staff.

You may be admitted to the ward the day before your treatment so that the staff can go over the procedure with you. This is a good time to ask questions and it may help to make a list of them beforehand.

• You will probably be in a side room, away from the main ward.
• You may have the room to yourself or be with someone else having similar treatment.
• Lead screens may be put on either side of your bed or in the doorway to block any radiation that is given out.
• Visitors will be restricted, and only allowed to stay in the room or sit at the end of the bed for a short time, if at all. Visitors will be able to talk to you from outside the room through an intercom.
• Children under 18 and women who are pregnant will not be allowed to visit.
• Staff and visitors will be asked to stand away from your bed to reduce their exposure to the rays.
• An instrument called a Geiger counter will be used to monitor the level of radiation in the room. The nurses may wear a small counter.
• You will probably be in the single room for a short time, sometimes only one or two days. You can bring books and magazines into your room, watch TV or listen to the radio.

These safety measures and visiting restrictions can make you feel very isolated, frightened and depressed at a time when you want people around you. If you have these feelings it is important to let the staff looking after you know. People are different in the way they handle their fears; some find it easier to know everything about their treatment, while others prefer to know as little as possible. If you want any explanations, the staff on the ward will be happy to help you. It often helps to bring your fears or worries into the open by talking to the staff or to family and friends.

It is advisable not to become pregnant, or father a child, while being treated for thyroid cancer, and for a year afterwards. If you are pregnant you cannot be given radioactive iodine treatment. For this reason, you should tell your doctor if you are, or think you might be, pregnant.

Your fertility should not be affected by radioactive iodine treatment, although there is a small risk if you need to have repeated treatments. Your doctor or nurse can give you more information and support about this.

If you are breastfeeding, you should stop a couple of days before you are treated with radioactive iodine. Although it is not safe to start breastfeeding again after your treatment, it will be safe for you to do so after future pregnancies.