What is radioiodine?

Radioiodine therapy has already been applied to thyroid disorders for more than 50 years, with good results. By ‘radioiodine therapy’, we mean taking capsules of radioactive iodine-131. Iodine-131 is abbreviated to I-131. It is a radioactive substance. The radioactive radiation deactivates the active thyroid cells, which will take up the iodine. The radioactive effect of iodine-131 is halved every eight days. Radioiodine therapy is therefore very active in the first week after taking the capsule. The radioactive radiation gradually disappears after that.

When is it indicated?

Iodine-131 is used for both benign and malignant thyroid disorders. A few examples:

Treatment procedure


  • It is important to avoid medications and supplements containing iodine to ensure good uptake of iodine-131 by the thyroid cells. Iodine contained in radiographic contrast agents (iodinated CT contrast media), amiodarone (Cordarone), cough syrup or mouthwashes (Betadine) can continue to circulate in the body for many weeks and may hinder successful treatment with radioactive iodine-131. Please make sure to flag any use of such products in the last few months.
  • Please also avoid iodine contained in kelp or seaweed (e.g. sushi) in the weeks preceding the treatment. The intake of large amounts of shellfish, saltwater fish, sea salt and dairy should also be avoided before the treatment.
  • Medications affecting the thyroid such as Thyroxine, Strumazol and Propylthiouracyl (PTU) must be stopped before the treatment. The Nuclear Medicine physician or the endocrinologist will explain how many weeks in advance they should be stopped.
  • In the case of thyroid cancer where the thyroid has been removed, the patient is usually given a Thyrogen injection in the buttock muscle 48 and 24 hours in advance. This injection contains a synthetic thyroid stimulating hormone (often termed ‘recombinant TSH’) which stimulates iodine-131 uptake. This can be done as an outpatient at the Endocrinology Department, with Thyroxine being taken after that.
  • Patients must report if they are pregnant, want to become pregnant or are breastfeeding, as in that case the treatment cannot be performed. It is recommended that the patient or their partner use contraception for a period of six months. Breastfeeding must be stopped several weeks before the treatment.


Radioiodine treatment consists of the administration of a small iodine-131 capsule which must be taken with water. It is important to drink a lot in the week after the capsule is taken (a minimum of 1.5 litres per day, preferably more). This ensures that any remaining radiation is cleared from the body quickly in the urine.

Depending on the dose, the radioiodine therapy can be received as an outpatient or the patient is admitted for three days. In the latter case, the patient will stay in a dedicated single room.

If you are admitted for a stay in an radioiodine therapy room:

  • You must not leave the room. The outside door of the room is closed but not locked. The room is very spacious with a very large window and own bathroom and fridge. No visitors are allowed in the room, unless they patient requires care.
  • Smoking is prohibited during your stay.
  • Urine must be passed in a sitting position. Hand washing after toilet visits is mandatory.
  • Please bring your own towels, flannels and toiletries.
  • Make sure to bring things that allow you to pass the time in a comfortable way
    (book, puzzle, mobile phone, tablet, etcetera).


The majority of the patients experience a reduction of their symptoms. Symptoms reduce gradually over a few months after taking the iodine-131 capsule. While waiting for the beneficial effect it is therefore sometimes necessary to resume taking Strumazol or medication that protects the heart against thyroid hormones for a few weeks or months (e.g. a beta blocker like Inderal). The endocrinologist will prescribe this if required.

A minority of patients will need more than one treatment in the long term.

Centres and specialist areas

Nuclear Medicine

Latest publication date: 05/02/2021
Supervising author: Dr Van Den Bossche Bieke