RAI, or radioactive iodine therapy, is a treatment that uses a radioactive form of iodine (called I-131) to destroy thyroid cancer cells remaining after surgery. It works because thyroid cells naturally absorb iodine, so when you swallow a capsule containing radioactive iodine, the radiation concentrates in thyroid tissue and kills it from the inside. Most patients with differentiated thyroid cancer (the most common types, papillary and follicular) receive RAI after a thyroidectomy, and the combination achieves remission rates around 90%.
How RAI Destroys Cancer Cells
Thyroid cells have a specialized transporter on their surface that actively pulls iodine inside. This transporter doesn’t distinguish between normal iodine and radioactive iodine. When you take I-131, it enters thyroid cells and cancer cells the same way dietary iodine would. Once inside, the radioactive iodine emits high-energy electrons called beta particles. These particles damage the DNA of the cell, triggering cell death. Because very few other cells in the body absorb iodine this aggressively, the radiation is highly targeted, largely sparing surrounding healthy tissue.
I-131 also emits a second type of radiation, gamma rays, which pass through the body. These don’t contribute much to killing cancer cells, but they do allow doctors to take whole-body scans after treatment to see exactly where the radioactive iodine concentrated. That scan becomes a valuable map showing whether any thyroid tissue or cancer remains.
Who Gets RAI Treatment
RAI is used for patients with differentiated thyroid cancer, which includes papillary and follicular types. These cancers retain enough of the normal thyroid cell machinery to absorb iodine. After thyroidectomy, RAI destroys any microscopic cancer cells or remnant thyroid tissue the surgeon couldn’t remove. It’s also used in cases where cancer has spread to lymph nodes or distant sites like the lungs.
RAI is not used during pregnancy or breastfeeding. It’s also ineffective against thyroid cancer types that have lost the ability to absorb iodine, such as anaplastic thyroid cancer or poorly differentiated tumors that no longer behave like normal thyroid cells.
Preparing for Treatment
Preparation has two parts: a low-iodine diet and raising your TSH levels. Both steps make the treatment more effective by ensuring cancer cells are as hungry for iodine as possible when you take the radioactive dose.
The Low-Iodine Diet
For one to two weeks before treatment, you’ll follow a diet that limits iodine intake to less than 50 micrograms per day. (For context, a single teaspoon of iodized salt contains roughly 400 micrograms.) The goal is to deplete your body’s iodine stores so that when the radioactive iodine arrives, thyroid cells absorb it aggressively. You’ll avoid iodized salt, dairy products, seafood, egg yolks, and processed foods that contain iodine-based preservatives. Your doctor will also check whether you’re taking any vitamins, supplements, or medications containing iodine, and you’ll need to stop those as well. If you’ve had a CT scan with iodinated contrast dye recently, that can interfere too.
Raising TSH Levels
Thyroid-stimulating hormone (TSH) drives thyroid cells to absorb more iodine. To raise your TSH before RAI, there are two approaches. The first is stopping your thyroid hormone replacement medication for several weeks, which forces your TSH to climb naturally. The downside is that you’ll become temporarily hypothyroid, which can cause fatigue, brain fog, weight gain, depression, and muscle aches. Studies show this withdrawal period significantly impairs quality of life and can be risky for people with heart disease.
The second option is receiving two injections of a synthetic TSH (recombinant human TSH) over two days, which raises TSH without requiring you to stop your medication. This avoids the hypothyroid symptoms entirely and produces comparable side effect rates from the RAI itself. It’s especially valuable for older patients or those with other health conditions.
What Happens on Treatment Day
RAI is surprisingly simple from the patient’s perspective. You swallow a capsule or liquid containing I-131 in a hospital or clinic. There’s no surgery, no IV, no anesthesia. The entire administration takes minutes. Depending on the dose, you may go home the same day or stay in a specially shielded hospital room for a day or two.
About three to seven days later, you’ll have a post-therapy whole-body scan. This imaging session shows where the radioactive iodine collected in your body, confirming that it reached the right areas and revealing any previously undetected cancer deposits. For patients with lung metastases, the scan is most accurate at about 72 hours after the dose, when even small deposits become visible.
Side Effects: Short and Long Term
In the days following treatment, the most common side effects are nausea, swelling or tenderness in the salivary glands (the glands under your jaw and in front of your ears), and temporary changes in taste. Some people lose their sense of taste entirely for a short period. These acute effects typically resolve within a few weeks. Drinking plenty of fluids and sucking on sour candy can help stimulate saliva flow and reduce gland swelling.
Longer-term side effects are less common but worth knowing about. Chronic dry mouth from salivary gland damage is the most frequent lasting complication, and it can increase your risk of dental cavities. Some patients develop blocked tear ducts, causing watery eyes. In rare cases, particularly with very high cumulative doses, RAI has been associated with a small increased risk of developing a second, unrelated cancer years later. There’s also a rare risk of lung scarring in patients treated for widespread lung metastases.
Recurrence is possible even after successful treatment. About 15 to 20% of patients with well-differentiated thyroid cancer experience a recurrence during long-term follow-up, which is why ongoing monitoring with blood tests and imaging continues for years.
Isolation and Safety at Home
After RAI, your body emits low levels of radiation for several days. You’ll need to follow specific precautions to protect the people around you, especially children and pregnant women.
- Distance from others: Stay at least 3 feet from people for the first 8 hours, and at least 6 feet from children and pregnant women.
- Sleeping arrangements: Sleep alone and avoid prolonged intimate contact, including kissing and sexual intercourse, for three to four days.
- Bathroom use: Use a dedicated bathroom that no one else shares, and avoid public restrooms.
- Laundry and hygiene: Wash your clothes, sheets, and towels daily, separately from anyone else’s. Shower daily and wash your hands frequently.
Brief contact like handshakes or quick hugs is fine. The total isolation period for thyroid cancer patients is typically three days at minimum but can extend up to two weeks depending on your dose. Your care team will give you a specific timeline based on the amount of I-131 you received.
How Effective RAI Is
For most patients with differentiated thyroid cancer confined to the neck, the combination of surgery and RAI produces a remission rate around 90%. Five-year disease-specific survival for patients with distant metastases treated with RAI is about 86% for papillary thyroid cancer and 81% for follicular thyroid cancer. At 10 years, those numbers drop to roughly 68% and 54%, respectively. These figures reflect the more advanced cases where cancer has already spread beyond the neck, so they represent a tougher scenario than the typical RAI patient faces.
The outlook is significantly better for patients whose cancer is caught early and hasn’t spread. For them, RAI serves as a cleanup step after surgery, destroying microscopic remnants and substantially reducing the chance of recurrence.