In the radioactive iodine uptake (RAIU) test, you swallow a small amount of radioactive iodine before you receive a scan. The test reveals how well your thyroid is functioning.
The thyroid is a gland in the neck that controls the body’s metabolism. It does so by making a hormone called thyroxine (T4) in response to a pituitary hormone called thyroid-stimulating hormone (TSH). The thyroid absorbs iodine from the body in order to produce T4.
A radioactive iodine uptake (RAIU) test is one of two kinds of scans used to diagnose thyroid diseases. The other is called a thyroid scan. An RAIU shows how well your thyroid functions, while a thyroid scan shows the gland’s size, shape, and position.
As part of the RAIU, you’ll take a pill or liquid containing radioactive iodine. The scan will show how much of this radioactive iodine is absorbed by the thyroid.
Did you know?
The radioactive iodine uptake test is also known as a thyroid uptake.
A doctor may recommend an RAIU scan if you have:
- symptoms of an overactive thyroid
- blood work indicating an overactive thyroid
- a goiter, or enlarged thyroid
The RAIU can provide valuable information for diagnosis and treatment. In some cases, a doctor may recommend that you also have a thyroid scan along with the RAIU.
The dose of radiation used in this test is small and isn’t associated with any dangerous side effects. However, as an added precautionary measure, the test isn’t recommended for pregnant or nursing people.
There are potential risks in exposing a fetus or baby to radioactive material. Inform your doctor if you’re nursing or if there’s any chance you may be pregnant. They can use other means, such as blood work and physical exams, to monitor your condition.
Also, tell your doctor if you have adverse reactions to iodine. They could interfere with your ability to take this test.
Did you know?
Although some people may have negative reactions to iodine, it’s not considered a true allergen.
Your healthcare team will provide specific instructions on how to prepare. To start, you may have to avoid foods, supplements, or medications that could interfere with the test.
Sources of iodine
You may need to avoid foods and products containing iodine for 1 to 4 weeks before your RAIU scan.
Foods that naturally contain iodine include:
Some prepared foods and takeout meals may be high in iodized salt.
Iodine may also appear in:
- some vitamins and nutritional supplements, such as ones that offer thyroid support
- foods containing red dye
- over-the-counter (OTC) medications containing red dye
Certain medications and solutions may increase the amount of iodine the thyroid absorbs. Your doctor may speak with you about temporarily stopping them before your scan.
These medications and solutions include:
- medications to treat an overactive thyroid
- medications to treat an underactive thyroid
- Lugol’s solution, an iodine-based antiseptic
- a saturated solution of potassium iodide
You’ll have to fast for a few hours before the test, and you may need blood work to see how your thyroid works around the time of the scan.
Tell your doctor if you’ve had other imaging tests with iodine-based contrast in the last 2 months. These tests can influence the results of your RAIU scan.
In addition, let your doctor know if you’ve had diarrhea recently since it may affect your ability to absorb iodine.
The procedure is simple and painless.
You’ll swallow a pill or liquid containing radioactive iodine. It will take time for the iodine to make its way into your system so your thyroid can absorb it.
Avoid eating for at least 1 hour after swallowing the radioactive iodine. Until the test is over, you’ll have to follow the same dietary restrictions you did in preparation for the test.
Return to the test center at certain intervals, usually 4 to 6 hours and 24 hours after ingesting the radioactive iodine. At these times, you’ll sit down, and a technician will place a device called a gamma probe over the outside of your neck. The gamma probe measures how much radioactive iodine the thyroid has absorbed at the time of the scan.
Each scan takes about 5 minutes, although you may have to sit for additional images if the first ones are unclear.
You’ll excrete the radioactive iodine in your urine for a few hours or days after the test.
The doctor will analyze your results in context with your blood work and the other tests you may have had, such as a thyroid scan.
Your thyroid has retained less iodine than would be expected if the level of radioactivity in your thyroid is unusually low. This generally means your thyroid has become inflamed and isn’t retaining or producing T4 properly. Another name for an inflamed thyroid is thyroiditis.
Your thyroid is absorbing more iodine than would be expected if the level of radioactivity in your thyroid is high. This means your thyroid is overactive, and you’re producing too much T4. In other words, you have hyperthyroidism.
The most likely causes of hyperthyroidism are Graves’ disease (an autoimmune condition) or hyperactive thyroid nodules. These nodules, or lumps, may grow and increase the total output of thyroid hormone into the blood.
You and your doctor will discuss how best to proceed. If you haven’t already had a thyroid scan, your doctor may recommend one to learn more about your condition.
Medications can help address many cases of thyroiditis or hyperthyroidism. More invasive measures, such as fluid drainage or surgery, may be used to shrink or remove bothersome nodules.
Some people won’t require treatment, at least not immediately. Instead, your doctor may monitor your condition to see if it improves.