What is radio-iodine treatment?

Your consultant has asked us to see you about iodine (iodine 131) therapy for thyrotoxicosis. Iodine therapy uses a form of iodine that is radioactive. Iodine is taken up by the thyroid gland and the radioactivity will “slow down” the thyroid’s production of certain hormones. Radio-iodine therapy has been used for more than 40 years and is a well-established technique. We have been using radioactive iodine therapy in this Department for many years and are highly satisfied with the results.

Where else in the body does radio-iodine go?

Most of the radio-iodine goes to the thyroid. The rest will pass from your body, mostly in the urine, during the first few days.

How is the radio-iodine given?

The radio-iodine is given as a capsule to swallow with water. This is similar to any other tablet and is taken in much the same way. Occasionally the radio-iodine can be given as a drink which tastes just like water.

Is there any preparation before the treatment?

Yes, and it is important to follow the instructions exactly otherwise the treatment may not be effective.


You must tell us about all the medication you are taking, even health products or supplements bought over the counter. Kelp should be completely avoided as it is full of iodine.

    • It will be necessary to stop taking certain medication before the treatment - for example carbimazole (CBZ) and propyl-thiouracil (PTU) must be stopped at least one week days before the treatment.

    • You may also need to start Lithium carbonate (for which you will be given a prescription) for a total of 10 days.

    • You will also have been given a prescription for thyroxine. Hold this until told to start taking it.


Do not eat any fish or seafood for two days before the treatment. Also avoid iodised salt, processed meats, red food colourant (E127) and KELP.

On the day - You may have a light breakfast but no lunch. You should drink normally.

Can I have treatment if I am pregnant or breast-feeding?

No. Women who are pregnant or breast-feeding must not be given radio-iodine. If there is any possibility that you may be pregnant, you must tell us. You are advised not to become pregnant for at least four months following the treatment. You must take adequate contraceptive care.


Smoking increases the risk of thyroid eye disease, and this can be exacerbated in a small number of patients by radioiodine. We strongly recommend that all patients contempating radioiodine stop smoking.

Thyroid eye disease

Radioiodine may worsen thyroid eye disease, but only in smokers. We now minimise this risk by performing several blood tests, as another cause of worsening eye signs is undiagnosed severe hypothyroidism. With our current protocols, we can usually avoid the use of steroids.

Will there be any danger to my family or friends?

We wish to keep all radiation levels as low as reasonably practical. We aim to ensure that your family and friends will not receive radiation above the levels experienced by members of the public in their daily lives. We will discuss with you in detail how you can achieve this. The essential precautions to reduce radiation are:

    • Avoid non-essential close contact with babies, young children and pregnant women. For people with day to day contact this can be for a period of 2-4 weeks. At consultation we will discuss how this can be practically carried out.

    • You may need to take some time off work depending on your specific job.

    • You will need to avoid close contact with other people for several days

    • Please check if you can travel by public transport. Usually there will be no restriction.

  • Women should not become pregnant nor men father a child in the 4 months after treatment

On the day of treatment you will be given specific advice about these precautions. In order for you to plan your treatment we have some general advice regarding radiation protection which you can read prior to your appointment in Radiation clinic. However, the exact length of time for which these precautions apply depends on the amount of radioiodine the doctor prescribes and individual circumstances.

Investigations that will be needed during the course of treatment.

You will need several blood tests, as follows:

    1. On the day of the preliminary consultation.

    2. On the day of treatment

    3. One week after treatment

    4. 3,6,9 and 12 weeks after treatment

You will have been given request forms for all these tests by the Clinic Doctor, and they will be numbered.

Will I need to see a doctor after the radioiodine treatment?

Yes, either the doctor you saw at the clinic or your GP. Blood tests will be required to monitor the effect of the treatment on your thyroid. Full instructions will be given at the time of treatment.

How many treatments will I need?

Usually one treatment is enough, although sometimes more than one is needed. The blood tests will help decide.

Are there any side effects?

    • Very occasionally after receiving the radio-iodine you may get a sore throat. This should last for only a few days and if it does happen to you, it can be relieved by drinking plenty of fluids and sucking boiled sweets.

    • Your thyroid may become underactive. This could happen within a few months or many years after the treatment. Again the blood tests will check the state of your thyroid. If it does become underactive you will be given thyroxine tablets.

Please contact the Nuclear Medicine Department on the telephone number below if you are unable to keep an appointment. The radio-iodine has to be ordered specially for each patient and it would be appreciated if any unavoidable appointment cancellation or rearrangement could be made at least one week before the appointment date so that the order for your dose can be changed accordingly.

If you have any questions at all or need further advice please do not hesitate to contact us at Charing Cross Hospital.

Printable Version of the Patient Information Leaflet

Radioiodine radiation protection patient information

Radioiodine referral form

Radioiodine: selected research papers