Radioactive iodine uptake (RAIU) is a test of thyroid function. It measures how much radioactive iodine is taken up by the thyroid gland in a certain time period.
A similar test is the thyroid scan. The two tests are commonly performed together.
How the Test is Performed
You are asked to swallow a liquid or capsule containing radioactive iodine.
After a certain period of time (usually 4 to 6 hours and again 24 hours later), you must return to the testing center so that the amount of radioactivity in the thyroid gland can be measured. This is done using a device called a gamma probe.
You lie on a table. The gamma probe is moved back and forth over the area of your neck where the thyroid gland is located.
The test takes about 30 minutes.
How to Prepare for the Test
Follow instructions about not eating before the test.
Your health care provider will tell you if you need to stop taking medicines that may affect the test results.
Tell your doctor if you have:
- Diarrhea (may decrease absorption of the radioactive iodine)
- Had recent CT scans using intravenous or oral iodine-based contrast (within the past 2 weeks)
- Too little or too much iodine in your diet
How the Test will Feel
There is no discomfort. You can eat beginning about 1 to 2 hours after swallowing the radioactive iodine. You can go back to a normal diet when the test is finished.
Why the Test is Performed
This test is done to check thyroid function. It is often done when blood tests of thyroid function show that you may have an overactive thyroid gland.
- 6 hours: 3 - 16%
- 24 hours: 8 - 25%
Some testing centers only measure at 24 hours. Values may vary depending on the amount of iodine in your diet. Normal value ranges may vary slightly among different labs. Talk to your doctor about the meaning of your specific test results.
What Abnormal Results Mean
Higher-than-normal uptake may be due to an overactive thyroid gland caused by:
- Graves disease
- An enlarged thyroid gland that contains nodules producing too much thyroid hormone (toxic nodular goiter)
- A single thyroid nodule that is producing too much thyroid hormone
These conditions often result in normal uptake, but the uptake is concentrated into a few (hot) areas while the rest of the thyroid gland does not take up any iodine (cold areas).
Lower-than-normal uptake may be due to:
The amount of radioactivity is very small, and there have been no documented side effects. The amount of iodine used is less than the amount in a normal diet.
People with an allergy to iodine in the diet or shellfish may not be able to have this test. You may be able to have this test if you have an allergy to iodine (contrast dye). Women who are pregnant or breastfeeding should not have this test. Talk to your health care provider if you have concerns about this test.
The radioactive iodine leaves your body through your urine. You may need to take special precautions, such as flushing twice after urinating, for 24 to 48 hours after the test. Ask your health care provider or the radiology/nuclear medicine team performing the scan.
Iodine uptake test; RAIU
Salvatore D, Davies TF, Schlumberger MJ, et al. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, et al., eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, PA: Elsevier Saunders; 2011:chap 11.
Update Date 5/10/2014
Updated by: Brent Wisse, MD, Associate Professor of Medicine, Division of Metabolism, Endocrinology & Nutrition, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Isla Ogilvie, PhD, and the A.D.A.M. Editorial team.