Radioactive iodine

Treatment Overview

Radioactive iodine, given in a liquid form, is absorbed and concentrated by the thyroid gland. The treatment destroys thyroid tissue but does not harm other tissue in the body.

See an illustration of the thyroid gland.

While radiation can cause thyroid cancer, treatment of hyperthyroidism with radioactive iodine does not increase your chances of getting thyroid cancer.

What To Expect After Treatment

Within days, the radioactive iodine passes out of your body in your urine.

To avoid exposing other people to radioactivity, it is important to do the following for the first 5 days after your treatment:

  • Drink plenty of fluids.
  • Avoid spending a lot of time around others, especially children and pregnant women.
  • Do not sit next to someone in a motor vehicle for more than 1 hour.
  • Avoid kissing or sexual intercourse.
  • Sleep in your own room.
  • Use separate towels, washcloths, and sheets. Wash these and your personal clothing separately for 1 week.

To further reduce the chance of exposing other people to radioactivity:

  • Wash your hands with soap and lots of water each time you use the toilet.
  • Keep the toilet very clean. Men should urinate sitting down to avoid splashing. Also, flush the toilet 2 or 3 times after each use.
  • Rinse the bathroom sink and tub thoroughly after using them.
  • Use separate (or disposable) eating utensils for the first few days and wash them separately.

Why It Is Done

Radioactive iodine may be used to treat hyperthyroidism in people who have noncancerous (benign) thyroid nodules that make too much thyroid hormone.

Radioactive iodine is also used if you have your thyroid removed (thyroidectomy) because of thyroid cancer. Radioactive iodine therapy destroys any remaining thyroid tissue or cancer cells that were not removed during surgery.

How Well It Works

In almost all cases, your thyroid hormone levels will return to normal or below normal after radioactive iodine treatment. This may take 8 to 12 weeks or longer. If your thyroid hormone level does not go down after 6 months, you may need another dose of radioactive iodine.

If you have thyroid cancer and you are treated with radioactive iodine, it may take from several weeks to many months for your body to get rid of any remaining cancer cells.

Your thyroid nodule is unlikely to grow after being treated with radioactive iodine.


The risks from radioactive iodine treatment include:

  • Metallic taste in your mouth.
  • Dry mouth.
  • Sore throat.
  • Neck pain. Radioactive iodine treatment can make your neck swell up or hurt.
  • Nausea or vomiting, which is usually mild.
  • Constipation or diarrhea.
  • Fatigue.
  • Unusually low (hypothyroidism) or unusually high (hyperthyroidism) thyroid levels.
    • Hypothyroidism occurs in about 10% of people within 5 years after they are treated with radioactive iodine for thyroid nodules.1
    • You can have temporary hyperthyroidism caused by an inflammation of your thyroid gland after radiation treatment.

What To Think About

If you are pregnant, you should not receive radioactive iodine treatment. This kind of treatment can damage your fetus's thyroid gland or expose your fetus to radioactivity.

You should not breast-feed your baby after you have been treated with radioactive iodine. Ask your doctor when it is safe to breast-feed.

If you have thyroid cancer and your primary tumor is smaller than 1 cm (0.4 in.), experts do not recommend radioactive iodine unless your cancer has spread (metastasized) to other areas of your body.2

Different people with thyroid cancer will receive different doses of radioactive iodine. If you are young and you do not have a great risk of your cancer coming back, you will probably need less radioactive iodine than an older person. Sometimes this means that a younger person who receives radioactive iodine treatment will not have to stay overnight in a hospital.

If you have had radioactive iodine treatment and you want to travel 3 to 4 days after treatment, it is important to prepare for any problems you may have at airport security. People who have had radioactive iodine treatment can set off the radiation detection machines in airports.

If you plan on traveling within 3 to 4 days of your radioactive treatment:

  • Check with local authorities about any special procedures or considerations.
  • Ask your doctor to write a letter that describes the radiation isotope used, the date and time of treatment, the dose, and its biological half-life (how long it takes for half of the radioactive iodine to be eliminated from the body). The letter should include your doctor's 24-hour telephone numbers so that authorities can call your doctor if they need to verify the information in the letter.
  • Keep in mind that you will have to wait for permission to travel.

Antithyroid medications are sometimes used before radioactive iodine to treat a noncancerous nodule that is making too much thyroid hormone and causing hyperthyroidism.

Complete the special treatment information form (PDF)(What is a PDF document?) to help you understand this treatment.



  1. Hegedüs L (2004). The thyroid nodule. New England Journal of Medicine, 351(17): 1764–1771.
  2. Sherman SI (2003). Thyroid carcinoma. Lancet, 361(9356): 501–511.

Last Updated: April 7, 2009

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