The thyroid gland is a butterfly-shaped gland located in the front of your lower neck. It produces hormones that help your body regulate your metabolism.

Your thyroid needs iodine to function properly. That iodine comes from the food you eat. No other organs use or absorb much iodine from your blood. Excess iodine in your body is excreted in the urine.

Radioiodine is used for treatment of several thyroid conditions. It is given by nuclear medicine specialists. Depending on the dose of the radioiodine, you may be able to go home the day of the procedure. For higher doses, you need to stay in a special room in the hospital and have your urine monitored for the radioactive iodine being excreted.

• You will swallow radioiodine in the form of capsules (pills) or a liquid.
• Your thyroid will absorb most of the radioactive iodine.
• The nuclear medicine team may do scans during your treatment to check where the iodine has been absorbed.
• The radiation will kill cells in the thyroid gland and, if the treatment is for thyroid cancer, any thyroid cancer cells that might have traveled and settled in other organs.

Most other cells do not take up iodine, so the treatment is very safe. Very high doses can sometimes decrease the production of saliva (spit) or injure the colon or bone marrow.

Radioiodine therapy is used to treat hyperthyroidism and thyroid cancer.

Hyperthyroidism occurs when your thyroid gland makes excess thyroid hormones. Radioiodine treats this condition by killing overactive thyroid cells and by shrinking an enlarged thyroid gland. This stops the thyroid gland from producing too much thyroid hormone.

The nuclear medicine team will try to calculate a dose that leaves you with normal thyroid function. But, this calculation is not always completely accurate. As a result, the treatment can lead to hypothyroidism (underactive thyroid gland), which needs to be treated with thyroid hormone supplementation.

Radioactive iodine treatment is also used in the treatment of some thyroid cancers after surgery has already removed the cancer and most of the thyroid gland. The radioactive iodine kills any remaining thyroid cancer cells that might remain after surgery. You may receive this treatment 3 to 6 weeks after the surgery to remove your thyroid. It also can kill cancer cells that may have spread to other parts of your body.

Many thyroid experts now believe this treatment has been overused in some people with thyroid cancer because we now know that some people have a very low risk for cancer recurrence. Talk to your health care provider about the risks and benefits of this treatment for you.

Risks of radioiodine therapy include:

• Low sperm count and infertility in men for up to 2 years after treatment (rare)
• Irregular periods in women for up to one year (rare)
• Very low or absent thyroid hormone levels that require medicine for thyroid hormone replacement (common)

Short-lasting side effects include:

• Neck tenderness and swelling
• Swelling of the salivary glands (glands at the bottom and back of the mouth where saliva is produced)
• Dry mouth
• Gastritis (inflammation of the lining of the stomach)
• Taste changes
• Dry eyes

Women should not be pregnant or breastfeeding at the time of treatment, and they should not become pregnant for 6 to 12 months following treatment. Men should avoid conception for at least 6 months following treatment.

People with Graves disease also have a risk of worsening hyperthyroidism after radioiodine therapy. Symptoms usually peak about 10 to 14 days after treatment. Most symptoms can be controlled with medicines called beta blockers. Very rarely radioactive iodine treatment can cause a severe form of hyperthyroidism called thyroid storm.

You may have tests to check your thyroid hormone levels before the therapy.

You may be asked to stop taking any thyroid hormone medicine before the procedure.

You will be asked to stop any thyroid-suppressing medicines (propylthiouracil, methimazole) at least one week before the procedure (very important or the treatment will not work).

You may be placed on a low-iodine diet for 2 to 3 weeks before the procedure. You will need to avoid:

• Foods that contain iodized salt
• Dairy products, eggs
• Seafood and seaweed
• Soybeans or soy-containing products
• Foods colored with red dye

You may receive injections of thyroid-stimulating hormone to increase the uptake of iodine by thyroid cells.

Just before the procedure if it is being given for thyroid cancer:

• You may have a body scan to check for any remaining cancer cells that need to be destroyed. Your provider will give you a small dose of radioiodine to swallow.
• You may receive medicine to prevent nausea and vomiting during the procedure.

Chewing gum or sucking on hard candy may help with dry mouth. Your provider may suggest not wearing contact lenses for days or weeks afterward.

You may have a body scan to check for any remaining thyroid cancer cells after the radioiodine dose is given.

Your body will pass the radioactive iodine in your urine and saliva.

To prevent exposure to others after therapy, your provider will ask you to avoid certain activities. Ask your provider how long you need to avoid these activities -- in some cases, it will depend on the dose given.

For about 3 days after treatment, you should:

• Limit your time in public places
• Not travel by airplane or use public transportation (you may set off the radiation detection machines in airports or at border crossings for several days after treatment)
• Drink plenty of fluids
• Not prepare food for others
• Not share utensils with others
• Sit down when urinating and flush the toilet 2 to 3 times after use

For about 5 or more days after treatment, you should:

• Stay at least 6 feet away from small children and pregnant women
• Not return to work
• Sleep in a separate bed from your partner (for up to 11 days)

You should also sleep in a separate bed from a pregnant partner and from children or infants for 6 to 23 days, depending upon the dose of radioiodine given.

You'll likely need to have a blood test every 6 to 12 months to check thyroid hormone levels. You also may need other follow-up tests.

If your thyroid becomes underactive after treatment most people will need to take thyroid hormone supplement pills for the rest of their life. This replaces the hormone the thyroid would normally make.

The side-effects are short term and go away as time passes. High doses have a low risk for long-term complications including damage to salivary glands and risk for malignancy.

Radioactive iodine therapy; Hyperthyroidism - radioiodine; Thyroid cancer - radioiodine; Papillary carcinoma - radioiodine; Follicular carcinoma - radioiodine; I-131 therapy

Mettler FA, Guiberteau MJ. Thyroid, parathyroid, and salivary glands. In: Mettler FA, Guiberteau MJ, eds. Essentials of Nuclear Medicine and Molecular Imaging. 7th ed. Philadelphia, PA: Elsevier; 2019:chap 4.

National Cancer Institute website. Thyroid cancer treatment (adult) (PDQ) Health professional version. www.cancer.gov/types/thyroid/hp/thyroid-treatment-pdq#link/_920. Updated April 11, 2024. Accessed October 17, 2024.

Ross DS, Burch HB, Cooper DS, et al. 2016 American Thyroid Association guidelines for diagnosis and management of hyperthyroidism and other causes of thyrotoxicosis. Thyroid. 2016;26(10):1343-1421. PMID: 27521067 pubmed.ncbi.nlm.nih.gov/27521067/.

Weetman AP, Kahaly GJ. Graves disease. In: Robertson RP, Giudice LC, Grossman AB, et al, eds. DeGroot's Endocrinology. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 71.

Updated by: Sandeep K. Dhaliwal, MD, board-certified in Diabetes, Endocrinology, and Metabolism, Springfield, VA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.