Radioactive iodine is a form of iodine that emits radiation, used primarily to treat thyroid conditions like hyperthyroidism and thyroid cancer. It works because the thyroid gland naturally absorbs iodine from the bloodstream to make hormones, and it can’t distinguish between regular iodine and a radioactive version. Once the thyroid takes up the radioactive form, the radiation destroys overactive or cancerous thyroid cells from the inside.
How the Thyroid Absorbs Iodine
Your thyroid gland sits at the front of your neck and is one of the few organs in the body that actively concentrates iodine. It does this through a specialized pump on its cell surfaces that pulls iodine out of the blood using sodium as an energy source. For every iodine molecule, the pump hauls in two sodium ions, pushing iodine into the cell against its natural gradient. This is the same mechanism your thyroid uses every day to collect iodine from food.
This biological quirk is what makes radioactive iodine therapy possible. When you swallow a capsule or liquid containing radioactive iodine, the thyroid pulls it in just like dietary iodine. Once inside thyroid cells, the radiation damages their DNA, causing the cells to stop functioning and eventually die. Because very few other tissues in the body concentrate iodine this aggressively, the radiation is largely confined to the thyroid, sparing the rest of the body.
The Two Main Isotopes
Not all radioactive iodine is the same. The two isotopes used in medicine, I-131 and I-123, serve very different purposes.
I-131 emits both beta particles (which destroy tissue) and gamma rays (which can be detected by a camera). This dual nature makes it useful for both treatment and post-treatment scanning. Its physical half-life is about eight days, meaning half the radiation decays every eight days. I-131 is the form used to treat hyperthyroidism and thyroid cancer.
I-123 is a pure gamma emitter, meaning it produces images without delivering a destructive dose to tissue. It’s used strictly for diagnostic scans. Studies comparing the two have found that I-123 actually produces sharper images. In one comparison involving thyroid cancer patients, I-123 detected all 35 areas of residual thyroid tissue, while I-131 missed three of them. I-123 also avoids a phenomenon called “stunning,” where a diagnostic dose of I-131 temporarily reduces the thyroid’s ability to absorb a later therapeutic dose.
Treating Hyperthyroidism
For conditions like Graves’ disease and toxic thyroid nodules, where the thyroid produces too much hormone, radioactive iodine is one of the most common treatments worldwide. The goal is to destroy enough thyroid tissue to bring hormone levels back to normal, or more often, to intentionally make the thyroid underactive so hormone levels can be controlled with a daily pill.
Treatment is straightforward. You swallow a single capsule or drink a liquid, typically in an outpatient setting. Cure rates after a single dose range from 80% to 100%, with one large study reporting an 87.7% success rate from one treatment. Some patients need a second dose. Higher doses are sometimes recommended for people with severe heart failure or other serious health conditions, to reduce the chance of needing retreatment.
Treating Thyroid Cancer
After surgery to remove a cancerous thyroid, small amounts of thyroid tissue almost always remain. Radioactive iodine is used to destroy these remnants, a process called ablation. The doses for cancer treatment are significantly higher than those used for hyperthyroidism.
Research has consistently shown that higher doses are more effective at eliminating remnant tissue, particularly when the initial surgery didn’t remove the entire thyroid. Current practice typically involves a total thyroidectomy followed by a high dose of I-131. After treatment, a follow-up whole-body scan can detect whether any thyroid tissue or cancer cells remain, since those cells will still light up on imaging if they’ve absorbed the radioactive iodine.
This approach works for differentiated thyroid cancers, the most common types. It is not used for rarer forms like medullary or anaplastic thyroid cancer, because those cancer cells don’t absorb iodine the way normal thyroid cells do.
Preparing for Treatment
Before receiving radioactive iodine, you’ll typically follow a low-iodine diet. The logic is simple: if your body is low on iodine, your thyroid cells will be “hungrier” and absorb more of the radioactive dose, making treatment more effective.
The diet means avoiding iodized salt, sea salt, seafood, dairy products, and egg yolks. Soy products, commercially baked breads (which often contain iodine-based dough conditioners), and supplements containing iodine or kelp are also off the list. Even certain food dyes, specifically FD&C Red Dye #3 found in maraschino cherries and some candies, contain iodine. Grain products like pasta and bread are limited to small amounts. The diet is temporary, typically lasting one to two weeks before treatment.
What Happens After Treatment
Most of the radioactive iodine that your thyroid doesn’t absorb leaves your body through urine within the first two days. But during that window, your body is emitting enough radiation to potentially affect the people around you, so you’ll need to follow precautions.
The Nuclear Regulatory Commission recommends a set of practical steps tailored to each patient’s situation. These generally include:
- Sleeping alone in a separate bedroom
- Using a dedicated bathroom if possible, sitting on the toilet, and flushing twice
- Using separate or disposable kitchen utensils and not sharing food
- Washing laundry separately
- Staying away from children and pregnant women
- Avoiding public transportation
- Drinking plenty of fluids to help flush the iodine out faster
As a general guideline, these precautions are recommended for three to five effective half-lives of the isotope, which for I-131 translates to roughly one to five weeks depending on the dose. Your treatment team will give you a specific timeline based on how much radiation you received.
Side Effects
The most significant long-term side effect is damage to the salivary glands. These glands also concentrate iodine, though not as aggressively as the thyroid, and they can absorb enough of the radioactive dose to become inflamed or damaged. Studies report chronic salivary problems in 16% to 54% of patients, with abnormal salivary gland function detected on imaging in 37% to 72% of cases. Symptoms include dry mouth, swelling, and changes in taste.
The common advice to suck on sour candies or lemon drops during treatment, which is meant to flush iodine through the glands faster, has not been clearly shown to help. One counterintuitive finding is that stimulating saliva production also increases blood flow to the glands, which can actually deliver more radioactive iodine to them. A newer approach called sialendoscopy, where a tiny scope is threaded into the salivary ducts to relieve blockages, has shown promise for treating early-stage salivary inflammation but is less effective once damage is established.
Short-term side effects can include neck tenderness, nausea, and a metallic taste in the mouth. Most patients who receive treatment for hyperthyroidism will eventually become hypothyroid, meaning their thyroid no longer produces enough hormone. This is expected and managed with daily thyroid hormone replacement for life.
Potassium Iodide in Emergencies
Radioactive iodine also has a darker context: nuclear accidents and fallout. When a nuclear event releases I-131 into the environment, it can settle on crops, contaminate milk, and be inhaled. The thyroid absorbs it just as readily from environmental exposure as from a medical dose.
Potassium iodide (KI) tablets are the standard countermeasure. They work by flooding the thyroid with non-radioactive iodine, filling it to capacity so it can’t take in the dangerous form. According to the CDC, a single dose of KI keeps the thyroid saturated for about 24 hours. The key is timing: it needs to be taken shortly before or soon after exposure to be effective. KI only protects the thyroid; it does not shield any other part of the body from radiation.