Revised 2015

What is radioactive iodine?

Radioactive iodine (I-131) has been used to treat over-active thyroid disorders (hyperthyroidism) since the early 1940s. It is an effective method of treatment. It is now being used more often as a first-line treatment rather than surgery. Radioactive iodine is taken up by the thyroid, and destroys the cells in the thyroid gland. This has the effect of reducing the amount of thyroxine made by the thyroid gland and may also reduce the size of the gland.

When is radioactive iodine treatment used?

  • As treatment for an over-active thyroid gland, with or without prior antithyroid drug therapy, especially in people with toxic nodular hyperthyroidism. It may also be used as a first line treatment in Graves' disease
  • When an over-active thyroid gland due to Graves' disease fails to settle after antithyroid medication
  • To shrink the gland when there is a goitre (thyroid enlargement)

Radioactive iodine is also used in higher doses in the treatment of thyroid cancer.

Is radioactive iodine treatment safe?

Large follow-up studies have been carried out for more than 70 years in many European countries and the United States. These show that there is no increased risk of cancer from treatment with radioactive iodine. The small dose of radioactivity used means that radioactive iodine treatment is in no way like the radiation exposure following uncontrolled nuclear fallout which can cause thyroid cancer.

Is it safe to conceive after radioactive iodine treatment?

Following radioactive iodine treatment men should avoid fathering a child for four months, and women should avoid conceiving for six months. Your treatment centre will give you detailed information on this. After that time there is no problem with having a baby or with the development of the baby and many people have gone on to conceive and have healthy children following radioactive iodine.

Who should NOT have radioactive iodine treatment?

Radioactive iodine treatment is not given to:

  • Pregnant women - radioiodine crosses the placenta and can affect the thyroid gland developing in the unborn baby
  • Breast-feeding women - the radioactive iodine passes via the milk to the baby’s thyroid gland
  • Anyone who is vomiting or incontinent
  • People with active thyroid eye disease as it may worsen the eye disease unless steroids are given at the same time

If the thyroid gland is very over-active, radioactive iodine can cause dangerously high levels of thyroid hormone, a condition known as thyroid crisis or storm. Your doctor will therefore prescribe antithyroid drugs to control the hyperactivity first before giving you radioactive iodine.

Are there any side effects of treatment with radioactive iodine?

Sometimes the thyroid gland is a bit tender after treatment. This will usually clear up after a few days. A common longer term side-effect of radioactive iodine treatment is an under-active thyroid gland (hypothyroidism), so it is very important to have regular thyroid blood tests starting from one month after the treatment to identify and treat this early, with levothyroxine. If you have existing thyroid eye disease, which can occur in Graves’ disease, this may sometimes be aggravated by radioactive iodine treatment (especially in smokers). Make your doctor is aware of any eye symptoms you have, to check if these might be thyroid related. Your doctor will advise ways to minimise this, such as a course of treatment with steroid tablets, early treatment with levothyroxine and stopping smoking. This will normally be done after assessment by an eye specialist.

How is radioactive iodine taken?

You will be prescribed radioactive iodine by a doctor who is licensed to do so. Your treatment will take place in a hospital but you do not need to be admitted to hospital as an inpatient. The treatment and its effects will be discussed at the hospital before the radioactive iodine is given and you will be asked to give your consent to the treatment. The dose is taken either as a simple capsule swallowed with mouthfuls of water, or as a drink. The gland has to be working when this treatment is taken, so antithyroid tablets should be stopped usually at least one week before the treatment is given and not taken for at least one week afterwards. You will be given instructions on when to stop the tablets. Your doctor will discuss with you before treatment whether you should restart the medication afterwards and when.

What safety measures should I take?

The amount of radioactive iodine used to control an over-active thyroid gland can be given without complicated arrangements, as long as you are not working with radioactive substances or on procedures that would be upset by the radioactivity, such as in a photographic laboratory.

After treatment you should avoid close contact with babies, children under five years, pregnant women and pets. As a general rule you should keep at arm’s length for two to three weeks, but the length of time depends on the dose used and the people you will be in contact with. You will receive more detailed guidance from the clinical team where you are treated. Recent reports have highlighted the fact that people who have recently had radioactive iodine treatment can trigger radiation detectors used for security purposes, for example at airports and seaports. This can occur for up to four months after treatment. If you are travelling within this time period you should carry a letter from your hospital explaining the treatment you have had.

What happens after radioactive iodine treatment?

Follow-up after radioactive iodine treatment is essential. There is no single correct dose and about 10% of patients need a further dose. There are no additional problems associated with having more than one dose. Some of those for whom the first dose is effective will need treatment with antithyroid medication until the over-activity is fully settled.

Over two-thirds of those who have radioactive iodine treatment will develop hypothyroidism (an under-active thyroid). This can occur anytime from one month after the treatment and is most common within the first 12 months after treatment but can occur later. You should have a blood test about four weeks after treatment, and should then be checked every one to three months in the first year - usually by your hospital clinic. It is very important not to miss these checks even if you feel well, as they can pick up an under-active thyroid before it has time to have effects on you. After that you should see your GP for an annual blood test, or at any time beforehand, if symptoms of hypothyroidism develop. It is straightforward to treat an under-active thyroid gland. Levothyroxine, which is thyroxine in tablet form, is used to replace the thyroxine that your thyroid gland is unable to produce.

What about radioactive iodine treatment for children and teenagers?

Radioactive iodine is an effective and safe treatment option for children and teenagers with an over-active thyroid gland, but will usually be given as a second-line treatment after a reasonably long course of antithyroid medication. This is particularly the case for younger children.

Some important points….

  • Radioactive iodine has been used for several decades to treat an over-active thyroid gland safely
  • It should not be used in people who are pregnant or planning pregnancy shortly after treatment
  • After radioactive iodine you should keep babies, children under five, pregnant women and pets at arm’s length for two to three weeks (or longer if advised by your clinical team)
  • You will need a blood test four weeks after the treatment and every one to three months until stable and once a year after that to check your thyroid hormone levels. Contact your doctor if you notice symptoms suggestive of hypothyroidism
  • Radioactive iodine may worsen Graves’ thyroid eye disease - if you have eye problems tell your doctor as special precautions or a delay in treatment may be required
  • Check with your doctor whether your prescription is exempt from charges

It is well recognised that thyroid problems often run in families and if family members are unwell they should be encouraged to discuss with their own GP whether thyroid testing is warranted.

If you have questions or concerns about your thyroid disorder, you should talk to your doctor or specialist as they will be best placed to advise you. You may also contact the British Thyroid Foundation for further information and support, or if you have any comments about the information contained in this leaflet.

The British Thyroid Foundation
The British Thyroid Foundation is a registered charity: England and Wales No 1006391, Scotland SC046037

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Endorsed by:

The British Thyroid Association - medical professionals encouraging the highest standards in patient care and research

The British Association of Endocrine and Thyroid Surgeons - the representative body of British surgeons who have a specialist interest in surgery of the endocrine glands (thyroid, parathyroid and adrenal)

First issued: 2008
Revised: 2011, 2015
Our literature is reviewed every two years and revised if necessary.

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