Iodine-131 is a radioactive form of iodine used primarily to treat thyroid conditions, including hyperthyroidism and thyroid cancer. It also serves as a diagnostic tool for detecting cancer that has spread beyond the thyroid. With a half-life of eight days, it delivers a concentrated dose of radiation to thyroid cells before largely clearing from the body on its own.
How Iodine-131 Works
The thyroid gland has a unique appetite for iodine. Specialized proteins on thyroid cells actively pull iodine from the bloodstream, concentrating it at 20 to 40 times the level found in plasma. When you swallow iodine-131 as a capsule or liquid, these same proteins can’t distinguish it from regular iodine. They absorb the radioactive version just as eagerly.
Once inside thyroid cells, iodine-131 emits two types of radiation: beta particles and gamma rays. The beta particles do the therapeutic work. They travel only a few millimeters into surrounding tissue, which means they damage thyroid cells while largely sparing nearby structures. That damage comes in the form of DNA breaks that kill the cells or prevent them from dividing. The gamma rays pass through the body and can be picked up by a camera, which is what makes iodine-131 useful for imaging as well.
Treating an Overactive Thyroid
The most common use of iodine-131 is treating hyperthyroidism, particularly Graves’ disease. The goal is to destroy enough thyroid tissue to bring hormone levels back to normal. You typically swallow a single capsule in an outpatient visit, and the radioactive iodine does its work over the following weeks and months.
Success rates depend on the dose. In clinical comparisons, a higher fixed dose cured about 85% of Graves’ disease patients, while a lower dose cured roughly 70%. One large study using a standardized higher dose found that at the one-year mark, 64% of patients had become hypothyroid (meaning the thyroid was now underactive and needed hormone replacement) and another 30% had reached normal thyroid function. Becoming hypothyroid after treatment is extremely common, and most patients end up taking a daily thyroid hormone pill for life. That trade-off is generally considered preferable to the risks of uncontrolled hyperthyroidism.
Thyroid Cancer Treatment After Surgery
For differentiated thyroid cancer, the most common type, iodine-131 plays a different role. It’s given after the thyroid has been surgically removed, and it serves three distinct purposes depending on how aggressive the cancer is.
The first is remnant ablation. Even after a total thyroidectomy, small bits of thyroid tissue remain. Destroying them with a relatively low dose of iodine-131 (30 to 100 millicuries) makes follow-up monitoring more reliable, because doctors can then use a protein called thyroglobulin as a clean marker for cancer recurrence.
The second is adjuvant therapy. When a patient has intermediate or higher risk features but no confirmed residual cancer, a moderate dose (30 to 150 millicuries) targets any microscopic disease that may be lurking. This improves disease-free survival.
The third is treating known persistent disease, including cancer that has spread to lymph nodes, lungs, or bone. These patients receive higher doses, typically 100 to 200 millicuries, and may need multiple rounds of treatment. Even in metastatic cases, iodine-131 can be effective because many thyroid cancer cells retain the ability to absorb iodine. The 2025 American Thyroid Association guidelines continue to recommend radioactive iodine as a core part of the treatment pathway for these patients, alongside surgery and, in some cases, targeted therapies.
Diagnostic Scanning
Beyond treatment, iodine-131 is used for whole-body scans to detect thyroid cancer that has spread. While a different isotope, iodine-123, is sometimes preferred for routine imaging because it delivers less radiation, iodine-131 appears to be more sensitive for finding metastases. In a head-to-head comparison, iodine-131 detected cancer spread in regions including the neck, chest, lungs, and bone that iodine-123 missed. For detecting leftover thyroid tissue after surgery, both perform equally well, but when the stakes involve finding distant metastases, iodine-131 has a meaningful advantage.
Preparing for Treatment
Before receiving iodine-131, you’ll need to follow a low-iodine diet to “starve” your thyroid cells and make them absorb the radioactive iodine more aggressively. The target is less than 50 micrograms of iodine per day, which means avoiding seafood, dairy, eggs, iodized salt, and many processed foods. Guidelines from different thyroid associations recommend anywhere from one to three weeks on this diet, though recent research suggests that as few as four days may be sufficient in regions with adequate iodine intake. One common mistake is confusing a low-iodine diet with a low-salt diet. You can still use non-iodized salt freely.
If the treatment is for thyroid cancer after surgery, your doctors will also need your thyroid-stimulating hormone (TSH) levels to be very high. This is achieved either by stopping thyroid hormone pills for several weeks or by receiving an injection that temporarily raises TSH without the unpleasant symptoms of hormone withdrawal.
Side Effects and Recovery
Short-term side effects are generally mild. Some people experience neck tenderness, nausea, or a metallic taste in the mouth. The more notable concern is salivary gland damage, since salivary glands also concentrate iodine to a lesser degree. In one study of thyroid cancer patients receiving treatment doses, 26% developed some form of salivary gland toxicity. About 15% had symptoms like pain and swelling within the first 48 hours. More concerning, 21% still had chronic symptoms, typically dry mouth, at the 12-month mark. The risk increases with repeated treatments. Some patients developed chronic salivary gland swelling years or even decades later. Sucking on sour candy or lemon drops during and after treatment is commonly recommended to stimulate saliva flow and help flush radioactive iodine out of the glands more quickly.
After treatment, you’ll need to limit close contact with others for a period, particularly children and pregnant women, because your body will emit low levels of radiation as the iodine-131 decays and leaves your system. Depending on the dose, this isolation period is typically one to several days. The Nuclear Regulatory Commission provides guidance on precautions during this window, including sleeping in a separate bed, using a separate bathroom if possible, and maintaining distance from others.
Pregnancy and Fertility Considerations
Iodine-131 is absolutely not given during pregnancy or while breastfeeding, as it would be taken up by the fetal or infant thyroid and cause permanent damage. Women are advised to delay pregnancy for 6 to 12 months after treatment. Breastfeeding must be stopped before receiving radioactive iodine and cannot be resumed afterward for that child, though future breastfeeding with a subsequent pregnancy is not affected. Men may experience a temporary drop in sperm count after higher doses, which typically recovers within several months.