What is radioactive iodine?

Radioactive iodine is a type of radioisotope treatment that targets thyroid cells. Radioisotopes are radioactive substances given by mouth as a drink or capsules, or injected into a vein. Radioactive iodine is a treatment for papillary and follicular thyroid cancers.

Radioactive iodine is usually given:

  • to destroy any thyroid tissue that is still in the neck after surgery – this is called radioiodine remnant ablation (RRA)
  • to treat any thyroid cancer that could not be removed with surgery
  • to treat thyroid cancer that has come back after initial treatment.

The radioactive substance used for treatment is called iodine-131. You usually have it as a capsule, or occasionally as a drink.

Normally, the thyroid gland takes iodine from our diet and uses it to make thyroid hormones. When you have radioactive iodine, the thyroid cells absorb the iodine and receive a very high dose of radiation. This destroys the thyroid cells and any cancer cells that may be left after surgery.

Radioactive iodine has very little effect on other parts of the body. This is because other cells do not absorb iodine as much as thyroid cells.

If your tests show there are still some thyroid cells after treatment, or if the cancer comes back, you can have radioactive iodine treatment again.

Preparing for radioactive iodine treatment

Before you have radioactive iodine treatment, you will usually be asked to start eating a low-iodine diet. You may be asked to stop taking your thyroid hormones.

Low-iodine diet

You will usually be asked to have a low-iodine diet for 1 to 2 weeks before your treatment starts. This is because having too much iodine in your body may make the treatment less effective. Your doctor or specialist nurse will give you more information and tell you when to start the low-iodine diet.

The table below shows you which foods you can have on a low-iodine diet, and which ones you should limit or avoid.


Image: Traffic light system – eat freely, limit, avoid

Thyroid hormones

Radioactive iodine needs a high level of thyroid-stimulating hormone (TSH) to be effective.

Thyroid hormone replacement stops you making TSH. So you may need to stop taking your thyroid hormones (T3 or T4) for a few weeks before and during your treatment. Your doctor or nurse will tell you when to stop taking them.

While you are not taking your thyroid hormones, you will have the symptoms of thyroid hormone withdrawal. These are likely to include:

  • tiredness, weakness and a lack of energy
  • hair thinning
  • poor concentration
  • constipation
  • feeling cold
  • a low mood.

These symptoms should go away once you start taking the thyroid hormone tablets again, but it can take some time.

It is important to stop taking thyroid hormones when you are told to, or the radioactive iodine treatment may not be as effective.


Recombinant human thyroid-stimulating hormone (rhTSH)

Instead of having to stop your thyroid hormones, you may be given recombinant human thyroid-stimulating hormone (rhTSH). This is also known as thyrotropin alfa (Thyrogen®).

rhTSH is a drug that is similar to the TSH produced in your body. It allows you to carry on taking your hormone-replacement tablets, so you avoid the symptoms of thyroid hormone withdrawal. You have two injections 24 hours apart, on the two days before your radioactive iodine treatment. You have the injections into the buttock (intramuscularly). You usually have the injections as an outpatient.

rhTSH is not suitable for everyone. Your doctor or specialist nurse can tell you if you can have this treatment.

rhTSH has very few side effects. Possible side effects include:

These side effects are usually mild and only last for 24 to 48 hours.

Pregnancy and breastfeeding

You cannot have radioactive iodine treatment if you are pregnant or breastfeeding. It is important to tell your doctor if you are pregnant or think you might be.

If you are breastfeeding, you must stop several weeks before you have your radioactive iodine treatment. This helps to reduce the amount of radiation the breasts are exposed to. It is not safe to start breastfeeding again after your treatment. But it will be safe for you to do so if you have more children in the future.

Having your radioactive iodine treatment

You will usually be admitted to the ward on the day of your radioactive iodine treatment. You will usually have the radioactive iodine as a capsule. After this, your doctor or nurse will ask you to have a hot drink, so the iodine can be absorbed. Before and after the treatment, you can eat normally and you will be encouraged to drink plenty of fluids.

Because the iodine is radioactive, you will become radioactive for a while after the treatment. The radioactivity gradually leaves your body in your:

  • wee (urine)
  • poo (stools)
  • blood, if you are having a period
  • saliva
  • sweat.

This means you will need to stay in hospital in a single room for a few days after your treatment. During this time, the level of radioactivity will be measured regularly with a monitor (Geiger counter). You will have to follow certain safety measures and restrictions. These include:

  • no contact with people under 18 and pregnant women
  • restrictions on the length of time visitors and staff can spend with you
  • restrictions on what you can bring into hospital with you.

Each hospital has different routines for giving radioactive iodine treatment. The staff looking after you will explain the restrictions in detail before you have your treatment. You may find these safety measures and restrictions difficult, especially if you have young children. Some people feel lonely staying in a single room. Talk to your specialist nurse before treatment and let them know if you have any concerns. You may be able to visit the treatment unit to talk through the treatment.

You will need to flush the toilet twice each time you use it. You will also be encouraged to have 1 or 2 showers each day to wash any radioactivity from your skin.


Going home

When you go home, you will need to continue taking certain precautions for a few days. These may include:

  • avoiding close contact with children, babies, pregnant women and pets
  • limiting close and prolonged contact with people, for example you may not be able to share a bed or have sex for a few days
  • avoiding long journeys on public transport, or going to the cinema, the pub, or anywhere where you could be sitting or standing next to the same person for a long time.

The length of time you need to take these precautions for will depend on how quickly your body clears the radioactivity. Before you go home, the doctor who worked out how much radioactive iodine you needed (medical physicist) will give you more detailed information. They will also give you some written information about the treatment and precautions to take. You should carry this document with you for seven days after treatment.

Side effects of radioactive iodine treatment

Many people do not have any side effects after radioactive iodine treatment. You may have the following side effects:

  • Soreness or tightness around your neck or in the surgical scar.
  • Feeling a bit sick (mild nausea). This usually only lasts for a couple of days.
  • A dry mouth. This is not common during treatment, but it may happen once you go home. It can help to drink plenty of fluids.
  • Taste changes. This may not happen during treatment, but it can happen days or weeks later. It usually gets better within 4 to 8 weeks.

It is important to let your doctor or nurse know if you have any of these symptoms, as they can prescribe medicines to help.

Rarely, a dry mouth can become a long-term side effect of radioactive iodine treatment. This usually only happens if you have had more than one treatment. It happens if the salivary glands absorb the radioactive iodine. Let your doctor or specialist nurse know if you get a dry mouth or if it doesn’t improve. They will be able to help you manage it.


Pregnancy and fertility

After radioactive iodine treatment, you should not:

  • become pregnant for at least six months
  • father a child for at least four months.

It is important to use contraception during this time.

Radioactive iodine treatment does not normally affect fertility. But there is a very small risk for men who need to have more than one treatment. In this case, you may be offered sperm banking. Women have a slightly higher risk of a miscarriage in the first year after radioactive iodine treatment. Your doctor or nurse can give you more information and support about this.


Travelling after radioactive iodine treatment

For up to six months after radioactive iodine treatment, you may set off security radiation alarms at airports. It is a good idea to take the written information from the hospital, or a letter from your doctor, explaining the treatment you have had.