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Radioactive Iodine

Treatment Overview

Radioactive iodine, given in a capsule or 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 a picture 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 and saliva. How long it takes will depend on the dose you received and your age. Young people get rid of radioactive iodine faster than older adults.

Most people don't feel different after treatment. But a few people may feel a little nauseated.

Your doctor will give you written instructions. To avoid exposing other people to radioactivity, it is important to follow your doctor's instructions carefully. He or she will instruct you on how far to stay away from people, how long you need to sleep alone, and other ways to stay safe. You will be directed to avoid close contact, kissing, sex, and sharing cups, dishes, or utensils.

Some general recommendations include:footnote 1

    • Flush the toilet twice each time you use it.
    • Wash your hands well with soap and lots of water each time you use the toilet.
    • Use separate towels, face cloths, and sheets. Wash these and your personal clothing by themselves and not with other people's laundry.
    • Don't cook for other people. If cooking is necessary, use plastic gloves. Rinse gloves with water and throw them away in regular garbage.

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.

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 breastfeed your baby after you have been treated with radioactive iodine. Ask your doctor when it is safe to breastfeed.

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.

Travelling after treatment

If you have had radioactive iodine treatment and you want to travel after treatment, it is important to prepare for any problems you may have at security checks. People who have had radioactive iodine treatment can set off the radiation detection machines in airports, government buildings and border crossings for up to 4 months after treatment.

    • If you plan to travel after your treatment, check with local authorities about any steps or permission you may need to travel.
    • You may set off radiation detectors. Most police and transportation workers are aware of medical radiation, but it may be a good idea to carry some paperwork from your doctor.



  1. Sisson JC, et al. (2011). Radiation safety in the treatment of patients with thyroid diseases by radioiodine 131I: Practice recommendations of the American Thyroid Association. From the American Thyroid Association Taskforce on Radioiodine Safety. Thyroid, 21(4): 335–346.


Adaptation Date: 9/23/2021

Adapted By: HealthLink BC

Adaptation Reviewed By: HealthLink BC