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Your thyroid gland absorbs nearly all of the iodine in your body. Because of this, radioactive iodine (RAI, also called I-131) can be used to treat thyroid cancer. The RAI collects mainly in thyroid cells, where the radiation can destroy the thyroid gland and any other thyroid cells (including cancer cells) that take up iodine, with little effect on the rest of your body. The radiation dose used here is much stronger than the one used in radioiodine scans, which are described in Tests for Thyroid Cancer.
This treatment can be used to ablate (destroy) any thyroid tissue not removed by surgery or to treat some types of thyroid cancer that have spread to lymph nodes and other parts of the body.
Radioactive iodine therapy helps people live longer if they have papillary or follicular thyroid cancer (differentiated thyroid cancer) that has spread to the neck or other body parts, and it is now standard practice in such cases. But the benefits of RAI therapy 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 surgery. Discuss your risks and benefits of RAI therapy with your doctor. Radioactive iodine therapy cannot be used to treat anaplastic (undifferentiated) and medullary thyroid carcinomas because these types of cancer do not take up iodine.
For RAI therapy to be most effective, you must have a high level of thyroid-stimulating hormone (TSH or thyrotropin) in the blood. This hormone is what makes thyroid tissue (and cancer cells) take up radioactive iodine. If your thyroid has been removed, there are a couple of ways to raise TSH levels before being treated with RAI:
Most doctors also recommend that you follow a low iodine diet for 1 or 2 weeks before treatment. This means avoiding foods that contain iodized salt and red dye #3, as well as dairy products, eggs, seafood, and soy.
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 injected into a vein. You may also be given medicine to prevent nausea. Usually, only one treatment is needed. A small number of people may need a second treatment.
Your body will give off radiation for some time after you get RAI therapy. Depending on the dose and where you are being treated, 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.
If you are allowed to go home after treatment, 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.
If you are going home the same day as your treatment, you should not use public transportation (taxis, rideshares, buses, or trains) . If possible, drive yourself. If this is not 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.
Some questions you may want to ask your care team before going home:
To learn more, see Radiation Therapy Safety.
Short-term side effects of RAI treatment may include:
Chewing gum or sucking on hard candy may help with salivary gland problems.
Radioiodine treatment also reduces tear formation in some people, leading to dry eyes. If you wear contact lenses, ask your doctor how long you should keep them out.
Men who receive large total doses of radiation because of many treatments with RAI may have lower sperm counts or, rarely, become infertile. Radioactive iodine may also affect a woman’s ovaries, and some women may have irregular periods for up to a year after treatment. Many doctors recommend that women avoid becoming pregnant for 6 months to a year after treatment. No ill effects have been noted in the children born to parents who received radioactive iodine in the past.
People who have had RAI therapy may have a slightly increased risk of developing leukemia, stomach cancer, and salivary gland cancer in the future. Doctors disagree on exactly how much this risk is increased, but most of the largest studies have found that this is an extremely rare complication.
Talk to your health care team if you have any questions about the possible risks and benefits of your treatment.
The American Cancer Society medical and editorial content team
Our team is made up of doctors and oncology certified nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.
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American Thyroid Association (ATA) Guidelines Taskforce on Thyroid Nodules and Differentiated Thyroid Cancer. Revised American Thyroid Association management guidelines for patients with thyroid nodules and differentiated thyroid cancer. Thyroid. 2016; 26:1-133.
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Schneider DF, Mazeh H, Lubner SJ, Jaume JC, and Chen H. Chapter 71: Cancer of the Endocrine System. In: Niederhuber JE, Armitage JO, Dorshow JH, Kastan MB, Tepper JE, eds. Abeloff’s Clinical Oncology. 5th ed. Philadelphia, Pa. Elsevier: 2014.
Tuttle RM. Differentiated thyroid cancer: Radioiodine treatment. UpToDate website. https://www.uptodate.com/contents/differentiated-thyroid-cancer-radioiodine-treatment. Updated June 8, 2018. Accessed February 20, 2019.
Last Revised: February 28, 2023
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