Radioactive iodine uptake (RAIU) tests thyroid function. It measures how much radioactive iodine is taken up by your thyroid gland in a certain time period.
A similar test is the thyroid scan. The 2 tests are commonly performed together, but they can be done separately.
Thyroid uptake; Iodine uptake test; RAIU
The test is done in this way:
The test takes less than 30 minutes.
Follow instructions about not eating before the test. You may be told not to eat after midnight the night before your test.
Your health care provider will tell you if you need to stop taking medicines before the test that may affect your test results. DO NOT stop taking any medicine without first talking to your provider.
Tell your provider if you have:
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 after the test.
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.
These are normal results at 6 and 24 hours after swallowing the radioactive iodine:
Some testing centers measure only 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 provider about the meaning of your specific test results.
Higher-than-normal uptake may be due to an overactive thyroid gland. The most common cause is Graves disease.
Other conditions can cause some areas of higher-than-normal uptake in the thyroid gland. These include:
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). This can only be determined if the scan is done along with the uptake test.
Lower-than-normal uptake may be due to:
All radiation has possible side effects. The amount of radiation in this test is very small, and there have been no documented side effects.
Women who are pregnant or breastfeeding should not have this test.
Talk to your provider if you have concerns about this test.
The radioactive iodine leaves your body through your urine. You should not need to take special precautions, such as flushing twice after urinating, for 24 to 48 hours after the test. Ask your provider or the radiology/nuclear medicine team performing the scan about taking precautions.
Guber HA, Farag AF. Evaluation of endocrine function. In: McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 23rd ed. St Louis, MO: Elsevier; 2017:chap 24.
Mettler FA, Guiberteau MJ. Thyroid, parathyroid, and salivary glands. In: Mettler FA, Guiberteau MJ, eds. Essentials of Nuclear Medicine and Molecular Imaging. 7th ed. Philadelphia, PA: Elsevier; 2019:chap 4.
Salvatore D, Cohen R, Kopp PA, Larsen PR. Thyroid pathophysiology and diagnostic evaluation. In: Melmed S, Auchus RJ, Goldfine AB, Koenig RJ, Rosen CJ, eds. Williams Textbook of Endocrinology. 14th ed. Philadelphia, PA: Elsevier; 2020:chap 11.
Weiss RE, Refetoff S. Thyroid function testing. In: Jameson JL, De Groot LJ, de Kretser DM, et al, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 78.