Radioactive iodine therapy

This page was reviewed under our medical and editorial policy by

Maurie Markman, MD, President, Medicine & Science

This page was updated on June 7, 2022.

With a diagnosis of thyroid cancer, doctors may recommend removal of your thyroid, followed by radioactive iodine treatment to kill off any cancer cells that remain in the body.

While the words “radioactive iodine” may sound harmful, it’s a standard, safe treatment for thyroid cancer. This nuclear medicine targets only the cells in the thyroid.

Radioactive iodine and the thyroid

Radioactive iodine, also called radioiodine or RAI, refers to two forms of iodine.

  • I-123 is used to make images of the thyroid and monitor its activity. It’s harmless and doesn’t kill thyroid cells.
  • I-131 does kill thyroid cells. In addition to cancer treatment, doctors deploy I-131 to treat an overactive thyroid (hyperthyroidism) and overgrown thyroid (goiters).

In this article, radioactive iodine will refer to treatment with I-131.

The thyroid gland, which straddles the windpipe in the neck, needs iodine to make several hormones. It obtains iodine from food, as your body can’t make it. Thyroid hormones control metabolism and affect how the digestive system, heart and muscles work, as well as maintain healthy bones and brain development. A low supply of iodine may cause goiters and hypothyroidism, characterized by dry skin, slow heart rate, weight gain and more frequent and intense menstrual periods. A shortage may also cause cognitive disabilities in infants and babies in utero.

Unlike other parts of the body, the thyroid is the only part of the body that absorbs and retains iodine, making radioactive iodine therapy a targeted way of treating thyroid cancer directly without harming most other tissues. It gets to work quickly, killing cancer cells in the thyroid.

  • It’s regarded as standard practice for treatment of papillary or follicular thyroid cancer that has grown beyond the thyroid (higher stages).
  • For cancers contained within the gland, surgery without radioactive iodine treatment may be preferred (lower stages).

RAI isn’t used to treat anaplastic and medullary thyroid cancers because these types don’t absorb iodine. It also isn’t recommended if you’re breastfeeding, pregnant or considering getting pregnant in the next six months, or have thyroiditis (inflammation of the thyroid gland).

Preparing for radioactive iodine therapy

Because treatment requires that the thyroid absorb as much of the radioactive iodine as possible, your doctor may order medications and a special diet to boost absorption.

The tiny pituitary gland at the base of your brain makes thyroid-stimulating hormone, or TSH. Your body needs elevated blood levels of TSH for the thyroid to increase its absorption of iodine. To do this, you may be injected with thyrotropin (a hormone that regulates thyroid hormone production) on the two days prior to treatment. If you take thyroid hormone pills, you may be told to stop. Stopping causes hypothyroidism, which, in turn, makes the pituitary gland produce more TSH.

Low iodine diet

Your care team may prescribe a low-iodine diet for a week or two before treatment and for at least two days after treatment. Iodine exists in many foods—it occurs naturally in seafood, is added to table salt, and plays a role in food processing and animal feeds. Avoid these foods that contain high levels of iodine:

  • Iodized salt
  • Seafood, shellfish and seaweed (nori, kelp, dulse)
  • Milk and dairy products
  • Whole eggs and egg yolks (not egg whites)
  • Breads and other commercially baked goods containing iodate dough conditioners
  • Certain food additives: red dye #3, agar, alginate and carrageenan
  • Blackstrap molasses (not unsulfured molasses)
  • Herbal supplements and vitamins containing iodine

Also prior to treatment:

  • Limit daily consumption of grains and cereals to a piece of bread or a half cup of pasta.
  • Don’t eat soy milk, tofu or other soy products, because animal studies have shown these interfere with absorption of radioactive iodine.

Treatment and safety precautions

Taking RAI typically means ingesting I-131 radioactive iodine pills or a small amount of liquid that makes you radioactive temporarily. You may be given medicine beforehand to counter any nausea the RAI may cause.

After swallowing your dose, you need to keep away from people for a period of time, and ensure you don’t accidentally contaminate others, especially children and pregnant women.

Protect members of your household by staying at least 3 feet away. Don’t take public transport. If someone drives you home, sit as far as possible away from them—in the rear seat on the opposite side of the driver, for example. If allowed to drive home, do so alone.

Your body will excrete RAI through urine, stool, saliva and sweat. You will need to avoid touching, kissing and having sex or sleeping with anyone for a time. Sleep in a separate room and use a separate bathroom, if possible, following directions on how to clean up the bathroom after each use and safely dispose of tissues or other refuse soiled with blood or secretions. You cannot share dishes or eating utensils, clothing, washcloths or towels until the period of high radioactivity has passed. How many days you must stay away from others depends upon the dose size.

Radioactive iodine side effects and risks

RAI treatment may cause a dry mouth, so suck on sour hard candies or chew gum to stimulate your salivary glands. It may also dry up tears, so your doctor may suggest not wearing contact lenses for a while. Drink lots of water to flush out the radioactivity.

Other side effects may include:

  • Changes in taste or loss of taste
  • Sore neck and swelling
  • Menstrual irregularity
  • Drop in sperm counts, or infertility in men (in rare cases)
  • Increased risk of developing salivary gland, kidney or stomach cancer, or leukemia

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