You are asked to swallow a liquid or capsule containing radioactive iodine.
After a certain period of time (usually 6 and 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.
The probe is placed over your thyroid gland along the outside of your neck. You will be asked to lie on a table while the scanner moves over your neck.
The scan takes about 30 minutes.
How to prepare for the test
Do not eat for 8 hours before the test.
Your health care provider will instruct you, if needed, to stop taking drugs that may affect the test results.
Tell your doctor if you have:
Diarrhea (may decrease absorption of the radioactive iodine)
Recent x-ray test using 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 - 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%
Note: Some laboratories 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 laboratories. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
Increased hormone levels may be due to an overactive thyroid gland caused by:
An enlarged thyroid gland that contains nodules producing too much thyroid hormone (toxic nodular goiter)
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 of iodine in a normal diet. However, this test is not recommended for women who are pregnant or breastfeeding.
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). Talk to your health care provider.
The radioactive iodine leaves your body through your urine. You may need to take special precautions, such as flushing twice after urinating, for 24 - 48 hours after the test. Ask your health care provider or the radiology/nuclear medicine team performing the scan.
Bahn RS, et al. Hyperthyroidism and other causes of thyrotoxicosis: management guidelines of the American Thyroid Association and the American Association of Clinical Endocrinologists. Endocr Pract. 2011;17(3):e1-e65.
Kim M, Ladenson P. Thyroid. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 233.
Salvatore D, Davies TF, Schlumberger MJ, Hay ID, Larsen PR. Thyroid physiology and diagnostic evaluation of patients with thyroid disorders. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 11.
Shehzad Topiwala, MD, Chief Consultant Endocrinologist, Premier Medical Associates, The Villages, FL. Review provided by VeriMed Healthcare Network. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M. Health Solutions, Ebix, Inc.