Thyroid scan

Definition

A thyroid scan is a nuclear medicine test that uses a radioactive iodine tracer to examine the structure and function of the thyroid gland.

Alternative Names

Scan - thyroid; Radioactive iodine uptake and scan test - thyroid; Nuclear scan - thyroid

How the test is performed

You will be given a pill that contains radioactive iodine, and then you will wait as the iodine collects in the thyroid. The first scan is usually done 4 - 6 hours after you take the iodine pill. Another scan may be taken 24 hours later.

During the scan, you will lie on your back on a movable table with your neck and chest under the scanner. The scanner detects the location and intensity of the rays given off by the radioactive material.

You must lie still to let the scanner get a clear image. 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 must sign a consent form. 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 that may need to be adjusted. These may include thyroid medication and anything with iodine in it. 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.

Why the test is performed

This test is done to:

• Check for thyroid cancer
• Evaluate thyroid nodules or goiter
• Find the cause of an overactive thyroid gland

Normal Values

The thyroid appears the correct size, shape, and in the proper location. It appears an even gray color on the computer image.

What abnormal results mean

A thyroid that is enlarged or pushed off to one side could be a sign of a tumor.

Nodules will 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.

The computer will also show the percentage of iodine that has collected in your thyroid gland. If your gland collects too much or too little of the radiotracer, this can be due to an underactive or overactive thyroid gland.

What the risks are

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 discuss the risks to the fetus or infant with their health care providers before taking this test.

The health care provider will usually consider the concerns regarding radiation side effects when the test is ordered, but the benefits of taking the test usually outweigh the risks.

Special considerations

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References

Kim M, Ladenson P. Thyroid. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 233.

Schlumberger M-J, Filetti S, Hay ID. Nontoxic diffuse and nodular goiter and thyroid neoplasia. In: Melmed S, Polonsky KS, Larsen PR, Kronenberg HM, eds. Williams Textbook of Endocrinology. 12th ed. Philadelphia, Pa: Saunders Elsevier; 2011:chap 14.