A thyroid scan is a nuclear medicine test that 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 the following way:
- You are given a pill that contains radioactive iodine. After swallowing it, you wait as the iodine collects in the thyroid.
- The first scan is usually done 4 to 6 hours after you take the iodine pill. Another scan is usually taken 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 to let the scanner get 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
You may be told not to eat after midnight the night before the exam.
Tell your health care provider if you are taking any medicines, including thyroid drugs and anything with iodine in it. The dosage of these drugs may need to be changed.
Remove jewelry, dentures, or other metals because they may interfere with the image.
How the Test will Feel
Some patients find it uncomfortable to stay still during the test.
The thyroid appears the correct size, shape, and in the proper location. It appears 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 will look darker or lighter on the scan (usually lighter if there is a tumor). If part of the thyroid appears lighter, it could be a thyroid problem. Nodules that are darker 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. 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. There is a very small amount of radiation in the tracer swallowed during this test. Women who are nursing or pregnant should not have this test with radioactive iodine, but can sometimes have a test with different tracers.
The health care provider will usually consider the concerns regarding radiation side effects when the test is ordered. But the benefits of the test usually outweigh the extremely small risks in non-pregnant patients.
Scan - thyroid; Radioactive iodine uptake and scan test - thyroid; Nuclear scan - thyroid
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, 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.