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Your thyroid gland absorbs nearly all the iodine that enters your body. Because of this, radioactive iodine (RAI, also called iodine-131 or I-131) can be used to treat some types of thyroid cancer.

What is radioactive iodine therapy?

During this treatment, radioactive iodine is put into your body by taking a pill, drinking a liquid, or getting a shot in your vein.

When RAI is put into the body, it collects mainly in thyroid cells. The radiation from this treatment can destroy the thyroid gland and any other thyroid cells that take up iodine (including cancer cells). Because most of the RAI collects in thyroid cells, the radiation has less effect on the rest of your body.

The radiation dose used in radioactive iodine therapy is much stronger than the doses used in radioiodine scans, which are described in .

When might RAI be used?

RAI can be used to treat some differentiated (papillary or follicular) thyroid cancers. It can be used to:

• Destroy any remaining thyroid tissue, if surgery can’t remove it all.
• Try to destroy any remaining thyroid tissue that can’t be seen, especially if there’s a higher risk of the cancer coming back. This is done after surgery, as an additional (adjuvant) treatment.  
• Treat some thyroid cancers that have spread to lymph nodes or other parts of the body.

RAI can often help treat papillary or follicular thyroid cancer that has spread to the neck or other parts of the body. But the benefits of RAI are less clear for people with small cancers of the thyroid gland that do not seem to have spread, which can often be removed completely with .

Radioactive iodine therapy isn’t used to treat anaplastic (undifferentiated) or medullary thyroid cancers because these types of cancer do not take up iodine.

Preparing for RAI therapy

For RAI therapy to be most effective, you need to have a high level of thyroid-stimulating hormone (TSH or thyrotropin) in your blood. This hormone makes thyroid cells (including cancer cells) take up radioactive iodine.

Raising your TSH levels before treatment

If your thyroid has been removed, there are 2 ways to raise your TSH levels before being treated with RAI.

• Thyroid hormone withdrawal: People taking thyroid hormone pills can stop them for several weeks. This causes hypothyroidism (very low thyroid hormone levels), which makes the pituitary gland release more TSH. This intentional hypothyroidism is temporary, but it can still cause symptoms like tiredness, depression, weight gain, constipation, muscle aches, and reduced concentration.
• TSH injections: TSH levels can also be raised by getting injections (shots) of a lab-made form of thyrotropin (Thyrogen). This drug is given daily as a shot into a muscle for 2 days, followed by RAI on the 3^rd day. An advantage of this approach is that a person doesn’t have to stop taking their thyroid hormone pills.

Following a low-iodine diet

Most doctors also recommend that you follow a low-iodine diet for 1 or 2 weeks before treatment. This typically means limiting or avoiding foods that contain iodized salt and red dye #3, as well as dairy products, eggs, seafood, and soy.

During and after the RAI procedure?

RAI is usually given in a special radiology department called nuclear medicine, either at an outpatient clinic or in the hospital. It can be taken by mouth as a pill or liquid, or it can be injected into a vein. You might also be given medicine to prevent nausea. Usually, only one treatment is needed. A small number of people may need a second treatment.

Protecting others from radiation exposure

Your body will give off low levels of radiation for some time after you get RAI therapy. You might need to be in the hospital for a few days after treatment, staying in a special isolation room to prevent others from being exposed to radiation. This will depend on the dose you’re given and where you get your treatment.

Once you are allowed to go home, you will be told how to protect others from radiation exposure and how long you need to take these precautions. Be sure you understand the instructions before you leave the hospital or clinic.

If you go home the same day as your treatment: You should not use public transportation (taxis, rideshares, buses, or trains). If possible, drive yourself. If this isn’t possible, try to sit as far away from the driver as possible. Do not have anyone who is pregnant or trying to get pregnant drive you home.

Levels of radiation in your urine: I-131 leaves the body through your urine, so you will likely be told to drink lots of fluids for the first few days after treatment, and to wipe down any surfaces the urine touches.

Questions to ask your health care team

Here are some questions you may want to ask your care team before going home.

• What symptoms should I call about, and who should I call?
• Who should I call if I have a medical emergency or have to go to the hospital?
• When is my follow-up?
• How will we know if the treatment is working?
• When can I return to work?
• How long should I use birth control?

You'll also want to find out when it is safe for you to:  

• Go out in public or use public transportation
• Share a bed with someone
• Be around pets, children, and people who are pregnant or trying to get pregnant
• Prepare food for others
• Have sex 
• Get pregnant

To learn more, see Radiation Therapy Safety.

Risks and side effects of RAI

Short-term side effects of RAI treatment may include:

• Neck tenderness and swelling
• Swelling and tenderness of the salivary glands

Chewing gum or sucking on hard candy may help with dry mouth and other salivary gland problems.

Problems with tears: Radioiodine treatment can affect the amount of tears some people make. Some people might notice dry eyes, while others might have excessive tears. If you wear contact lenses, ask your doctor how long you should keep them out.

Fertility issues for men: Men who receive large total doses of radiation because of many treatments with RAI may have lower sperm counts or, rarely, become infertile.

Fertility issues for women: Radioactive iodine may affect a woman’s ovaries, and some women may have irregular periods for up to a year after treatment. Although no ill effects have been noted in children born to parents who received RAI, many doctors recommend women avoid becoming pregnant for at least 6 months after treatment.

Second cancers: People who have had RAI therapy may have a slightly increased risk of developing some types of cancer in the future. This includes leukemia, stomach cancer, and salivary gland cancer. Doctors aren’t sure exactly how much this risk is increased, but most large studies have found that this is a very rare complication.

Talk to your health care team if you have any questions about the possible risks and benefits of your treatment.