Revised 2019

Radioactive iodine has been taken by mouth for the treatment of overactive thyroid disorders since the 1940s.

When is radioactive iodine used in the treatment of non- cancerous thyroid disorders?

  • To treat an overactive thyroid gland
  • To reduce thyroid activity caused by nodules
  • To shrink an enlarged thyroid
  • When an overactive thyroid gland due to Graves’ disease fails to settle after antithyroid medication
  • As a first-line therapy in Graves’ disease as an alternative to surgery or antithyroid drug therapy


  • As a drink
  • As a capsule, taken with water
  • Antithyroid tablets should be stopped at least one week before and after treatment

Are there any side effects?

  • Long-term – over two-thirds of people who have this treatment will develop hypothyroidism (underactive thyroid)

  • Short-term - rarely, some soreness around the thyroid gland

  • Existing thyroid eye disease may worsen, particularly if the eyes are currently sore or inflamed

Is radioactive iodine treatment safe?

  • Studies show there is an extremely small excess risk of thyroid or other cancers

  • It should not be given to pregnant women or women who are breast-feeding

  • Men should avoid fathering a child for at least four months, and women should avoid conceiving for at least six months, after radioactive iodine treatment

  • Fertility for both men and women is not affected

  • Avoid close contact with babies, children under six years and pregnant women for two to three weeks or as advised by your clinical team


  • Blood tests are carried out about four to six weeks after treatment

  • Further tests should be repeated every one to three months in the first year and once a year after that

  • About 10% of patients will need a second dose of radioactive iodine

  • There are no problems with having a follow-up dose

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.

This Quick Guide is one in a series about thyroid disorders. Other Quick Guides are available to read and download from the British Thyroid Foundation website.

 A leaflet containing comprehensive information about Treatment of an overactive or enlarged thyroid gland with radioactive iodine is also available.

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

Membership Information
Quick Guides

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: 2010, 2011, 2015, 2019
Our literature is reviewed every two years and revised if necessary.

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