What Is a Radioactive Iodine Uptake Test?

A radioactive iodine uptake test, or RAIU, is typically performed with a thyroid scan to help determine thyroid health and functioning. The test helps your healthcare provider see how much radioactive iodine your thyroid has absorbed over a certain time period, usually 6 or 24 hours after taking radioactive iodine.

Purpose of Test

Iodine is essential for your thyroid in order to make thyroid hormones like thyroxine (T4) and triiodothyronine (T3). The thyroid gland actively takes up iodine from the bloodstream, concentrating it to produce hormones that are secreted throughout the body to use for energy, muscle development, and brain and heart functioning. 

The RAIU test is most commonly used to assess the underlying cause of hyperthyroidism, but can also be used to assess the nature of thyroid lumps or nodules, to assess the results of thyroid surgery or chemotherapy for thyroid cancer, and to look for metastases in somebody with thyroid cancer.

The RAIU test will most likely be done together with a thyroid scan, though a thyroid scan may be performed without a RAIU test to determine the size, shape, and position of the thyroid. However, all RAIU tests will need a thyroid scan to determine how the thyroid is functioning.

Risks and Contraindications

The risks with the RAIU test are minimal. While a radioactive substance is being used, the amount administered for the test is very small—there have been no known reported side effects associated with it. However, because there is a small amount of radiation exposure, women who are pregnant or breastfeeding should not undergo the RAIU test in order to protect both mother and child. In these cases, your healthcare provider may opt for a thyroid scan without radioactive materials or blood work to determine the next steps for treatment.

If you have an allergy to shellfish or dietary iodine, you should speak to your healthcare provider before doing the RAIU test, as they may decide this is a contraindication for you. If you’ve had an allergic reaction to iodine dye in the past, you may still be able to have the RAIU, but should also discuss it with your healthcare provider first so that they are aware and can make the best-informed decision for your health.

Before the Test

Once your healthcare provider recommends the RAIU test, they will give you a detailed list of what to do the day of your test. You’ll likely be told not to eat after midnight the night before the test. In terms of medications, make sure you let your healthcare provider know at this pre-test visit what you are taking so they can let you know if you need to stop any medication prior to the test.

Don’t stop any medication without consulting your healthcare provider first. If you’ve had digestive issues like diarrhea, a recent CT scan, or are worried about iodine in your diet, you should tell your healthcare provider during this appointment, since it may alter the results of the RAIU test.

Timing

The RAIU test takes less than 30 minutes, though you’ll need to prepare for it ahead of time. Your healthcare provider will prescribe you a pill that contains radioactive iodine. After taking the pill (usually at home), you’ll wait about six hours before going in for a scan. The scan will measure the amount of iodine that has been taken up by the thyroid. It will also produce an image of the thyroid due to the radioactive materials picked up by a gamma probe, which the healthcare provider will move over your neck where your thyroid is located.

In some cases, you will go back 24 hours later for another scan, which will show what new amounts of iodine have been taken up by the thyroid over this time period. In other instances, a single scan is taken between 6 and 24 hours after the injection. Or, an initial scan is taken after 6 hours and repeated in 24 hours. All of these scenarios are possible, so it is best to discuss with your healthcare provider which route he or she would like to take for your testing.

Once the radiologist has gotten all the required images from the scan, they will interpret the results and send them over to your healthcare provider, who can go over the results with you. This should take about a day or two. Your healthcare provider will either call you with the results or schedule you for a follow-up appointment in their office.

Location

RAIU tests can be performed in a hospital, but they are more often done in an outpatient imaging center. Depending on the center, you’ll sit in a chair or will lie on a table facing the probe that will run over your neck to record images of your thyroid. You’ll be in a room with just the test team, such as a nurse, lab technician, and other staff members.

What to Wear

Since the test is capturing your thyroid, you won’t need to wear a gown, and your street clothes should be fine to keep on. Just make sure the staff can get to your neck: you’ll want to skip any turtlenecks or high-collared shirts and jackets that day.

Food and Drink

You’ll likely need to fast for eight hours before the test and drink only clear liquids such as water prior to the scan. Sometimes you may need to go on a low-iodine diet prior to the test. You’ll also want to make sure not to have had recent imaging with iodine contrast.

Cost and Health Insurance

The RAIU test without health insurance can vary in cost, starting in the $340 range. Depending on your plan coverage, a majority of that will be covered, but the amount may vary by plan.

What to Bring

Your healthcare provider will have forwarded any prior tests, bloodwork, and other information over to the testing center for you. Just make sure to have your insurance information and cash or a credit card, in case you have to pay for the test or its co-pay before it’s done.

During the Test

Once you’ve taken the radioactive iodine pill and have made it to the imaging center for both your 6-hour and 24-hour mark, the technician will place the probe over the thyroid for imaging.

You shouldn't need anyone to drive you home after the test, but if it helps your nerves to bring a friend or family member, they can wait for you in the waiting room. Depending on your healthcare provider, they may be able to come in with you during the test.

After the Test

Because the amount of radioactive iodine you’re ingesting for the test is so small, there may not be any precautions needed after the test. At most, the imaging team may tell you to flush the toilet twice after urinating for 24 hours to prevent radioactive material from sitting in the toilet and to practice good hygiene such as handwashing.

Since no side effects have been documented with this kind of test, you should feel completely normal after and can resume all normal activity, including eating and drinking.

Interpreting Results

Once your healthcare provider has gotten the results from the imaging center, they will either follow up with a phone call or scheduled visit in their office. A normal result for the RAIU test at 6 hours is anywhere between 3 percent to 16 percent, and at 24 hours, 8 percent to 25 percent. Results that are higher than the normal ranges could signal an overactive thyroid and are most commonly a result of Graves’ disease, though they can also be because of a toxic nodular goiter (when the thyroid gland produces too much hormone).

High results could also be a consequence of toxic adenoma, which is a thyroid nodule that’s producing too much of the hormone. If results are lower than the average range, it could be due to factitious hyperthyroidism (caused by taking too much thyroid medication), subacute thyroiditis (leading to inflammation and swelling of the thyroid), silent thyroiditis (which is painless and doesn’t typically present any symptoms), or an iodine overload in the body.

Using the results from the RAIU test, your healthcare provider will discuss the findings with you, as well as how to approach or adjust your current treatment with this new knowledge.

A Word From Verywell

Learning that you may need extra thyroid testing can be nerve-wracking, but the RAIU test is one of the simplest and least-invasive ways of helping your healthcare provider learn exactly how your thyroid is functioning and to figure out if you may be dealing with hyperthyroidism. Together with a thyroid scan, the RAIU test will help pinpoint the best treatment options for you or determine how effective your current thyroid treatment is working.

While the test is a brief outpatient procedure, don’t be bashful about asking your healthcare provider any questions you may have about the radioactive iodine pill, how exactly it works, and whether or not you can get the results with the RAIU test and thyroid scan or with just a thyroid scan without the RAIU.