A radioactive iodine uptake (RAIU) test uses a radioactive tracer and a special probe to measure how much tracer the thyroid gland absorbs from the blood. The test can show how much tracer is absorbed by the thyroid gland. The RAIU test often is done along with a thyroid scan, which shows if the tracer is evenly spread in the gland. This helps your doctor know if the thyroid gland is working properly. The radioactive tracer commonly used in this test is iodine.
A radioactive iodine uptake test is done to find problems with how the thyroid gland works, such as hyperthyroidism.
Why It Is Done
A radioactive iodine uptake (RAIU) test is done to:
- Find the cause of an overactive thyroid gland (hyperthyroidism).
- Plan treatment for hyperthyroidism.
- Plan treatment for patients who have had thyroid cancer surgery.
How To Prepare
Tell your doctor if you:
- Take any medicines regularly. Be sure your doctor knows the names and doses of all your medicines. Your doctor will instruct you if and when you need to stop taking any of the following medicines that can change the RAIU test results:
- Thyroid hormones
- Antithyroid medicines
- Medicines or supplements that contain iodine, such as iodized salt, kelp, cough syrups, multivitamins, and the heart medicine amiodarone
- Are allergic to any medicines, such as iodine. But even if you are allergic to iodine, you will likely be able to have this test because the amount used in the tracer is so small that your chance of an allergic reaction is very low.
- Have ever had a serious allergic reaction (anaphylaxis) from any substance, such as the venom from a bee sting or from eating shellfish.
- Have had any test using radioactive materials or iodine dye, such as a CT scan, 4 weeks before the RAIU test. These other tests may change the results of the RAIU test.
- Are or might be pregnant.
- Are breastfeeding.
Before an RAIU test, blood tests may be done to measure the amount of thyroid hormones (TSH, T3, and T4) in your blood.
To prepare for an RAIU test:
- Follow your doctor's instructions about not eating before the test. Most often, you will need to stop eating 8 hours before the test.
- Tell your doctor about all of the medicines, vitamins, and supplements you are taking. You may need to stop taking some medicines or supplements for a while before the test.
Your doctor may ask you to eat a low-iodine diet.
For an RAIU, you will swallow a dose of radioactive iodine. Iodine can be taken as a capsule or a fluid 4 to 24 hours before the test. Iodine has little or no taste.
Just before the test, you will remove your dentures (if you wear them) and all jewelry or metal objects from around your neck and upper body.
You will be asked to sign a consent form that says you understand the risks of the test and agree to have it done.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form ( What is a PDF document? ).
How It Is Done
A radioactive iodine uptake (RAIU) test is done in the nuclear medicine section of a hospital's radiology department by a person trained in nuclear medicine (nuclear medicine technologist).
For this test, you will lie on your back with your head tipped backward and your neck extended. It is important to lie still during this test. A special machine is placed over your thyroid gland to measure the amount of tracer absorbed by the thyroid gland. This is not an X-ray machine—it is a scanner that detects the radiation given off by the tracer. This test takes about 10 minutes and is done 3 to 6 hours after you are given the tracer. Another scan may be done in 24 hours.
After an RAIU test, you can do your regular activities. But you will be asked to take special precautions when you urinate. This is because your body gets rid of the radioactive tracer through your urine. This takes about 24 hours. During this time, it is important to flush the toilet twice each time you use it and wash your hands thoroughly after each time you urinate.
How It Feels
You may find it uncomfortable to lie still with your head tipped backward.
Anytime you're exposed to radiation, there's a small chance of damage to cells or tissue. That's the case even with the low-level radioactive tracer used for this test. But the chance of damage is very low compared with the benefits of the test.
Most of the tracer will leave your body through your urine or stool within a day. So be sure to flush the toilet right after you use it, and wash your hands well with soap and water. The amount of radiation in the tracer is very small. This means it isn't a risk for people to be around you after the test.
This test is not done for pregnant women because of the chance of exposing the baby (fetus) to radiation. This test is also not recommended for breastfeeding women or young children.
A radioactive iodine uptake (RAIU) test uses a radioactive tracer and a special probe to measure how much tracer the thyroid gland absorbs from the blood. The radioactive tracer used in this test is iodine. An RAIU test is done to check for thyroid gland problems, such as hyperthyroidism.
The amount of radioactive tracer in the thyroid gland is normal. An RAIU test measures the amount of tracer taken up by the thyroid gland at certain times after the tracer is given. The measured amount of radioactive tracer in the thyroid gland at each one of these times is at normal levels.
The test shows either more or less uptake of tracer than normal in the thyroid gland. If hyperthyroidism is present, abnormal test results may mean certain conditions are present.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Taking thyroid medicine.
- Eating foods with iodine, such as shellfish, iodized salt, or kelp.
- Having other tests using contrast materials in the past 4 weeks.
What To Think About
- Blood tests may be done before a radioactive iodine uptake (RAIU) to measure the amount of thyroid hormones (TSH, T3, and T4) in your blood.
- A thyroid scan may be done at the same time as an RAIU test. To learn more, see the topic Thyroid Scan.
Other Works Consulted
- Fischbach FT, Fischbach MA (2018). Fischbach's Manual of Laboratory and Diagnostic Tests, 10th ed. Philadelphia: Wolters Kluwer.
Current as of: July 28, 2019
Author: Healthwise Staff
Medical Review: E. Gregory Thompson MD - Internal Medicine
Kathleen Romito MD - Family Medicine
Alan C. Dalkin MD - Endocrinology