A thyroid scan uses a radioactive iodine tracer to examine the structure and function of the thyroid gland. This test is often done together with a radioactive iodine uptake test.
How the Test is Performed
The test is done in this way:
- You are given a pill that contains a tiny amount of radioactive iodine. After swallowing it, you wait as the iodine collects in your thyroid.
- The first scan is usually done 4 to 6 hours after you take the iodine pill. Another scan is usually done 24 hours later. During the scan, you lie on your back on a movable table. Your neck and chest are positioned under the scanner. You must lie still so that the scanner gets a clear image.
The scanner detects the location and intensity of the rays given off by the radioactive material. A computer displays images of the thyroid gland. Other scans use a substance called technetium instead of radioactive iodine.
How to Prepare for the Test
Follow instructions about not eating before the test. You may be told not to eat after midnight the night before your scan.
Tell your health care provider if you are taking anything that contains iodine because it may affect your test results. This includes some medicines, including thyroid drugs. Supplements such as kelp also contain iodine.
Also tell your provider 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
Remove jewelry, dentures, or other metals because they may interfere with the image.
How the Test will Feel
Some people find it uncomfortable to stay still during the test.
Normal test results will show that the thyroid appears to be the correct size, shape, and in the proper location. It is an even gray color on the computer image without darker or lighter areas.
What Abnormal Results Mean
A thyroid that is enlarged or pushed off to one side could be a sign of a tumor.
Nodules absorb more or less iodine and this will make them look darker or lighter on the scan. A nodule is usually lighter if it has not taken up the iodine. If part of the thyroid appears lighter, it could be a thyroid problem. Nodules that are darker have taken up more iodine. They can be overactive and may be the cause of an overactive thyroid.
The computer will also show the percentage of iodine that has collected in your thyroid gland (radioiodine uptake). If your gland collects too much iodine, it may be due to an overactive thyroid. If your gland collects too little iodine, it may be due to an underactive thyroid.
All radiation has possible side effects. 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 to shellfish may not be able to have this test. You may be able to have this test if you have an allergy to the iodine in the contrast dye.
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 may 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.
Scan - thyroid; Radioactive iodine uptake and scan test - thyroid; Nuclear scan - thyroid
Blum M. Thyroid imaging. In: Jameson JL, De Groot LJ, eds. Endocrinology: Adult and Pediatric. 7th ed. Philadelphia, PA: Elsevier Saunders; 2016:chap 79.
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, Kronenberg HM, eds. Williams Textbook of Endocrinology. 13th ed. Philadelphia, PA: Elsevier; 2016:chap 11.
Update Date 2/3/2016
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.