A thyroid scan is a nuclear medicine examination that uses the emissions of gamma rays from radioactive iodine to help determine whether a patient has thyroid problems, including hyperthyroidism, cancer, or other growths.

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 4 - 6 hours after the iodine has been ingested, and another scan may be taken 24-hours later. Additional or substitute imaging may be performed using a compound containing technetium.

After the radioiactive iodine has been absorbed by the thyroid, you will lie on your back on a movable table with your neck and chest positioned under the scanner. The scanner detects the location and intensity of the gamma rays emitted. During this part of the procedure, you must lie still to let the scanner get a clear image.

Next, the information is sent to a computer that displays images of the thyroid and any possible nodules that have absorbed the iodine.

You must sign a consent form. You may be told not to eat after midnight the night before the exam. Consult the health care provider if you are taking any medications that may need to be regulated, such as thyroid medication and anything with iodine in it. Remove jewelry, dentures, or other metals, because they may interfere with the image.

Some patients find remaining still during the test uncomfortable.

When thyroid cancer or nodules are suspected.

The thyroid appears the correct size, shape, and in the proper location. It appears a uniform gray on the computer.

If the thyroid is enlarged or pushed off to one side, this could indicate a tumor. Nodules will absorb more or less iodine and will appear darker or lighter on the scan (usually lighter if tumor). If part of the thyroid appears lighter, it may indicate there is possible thyroid dysfunction.

Additional conditions under which the test may be performed:

• Anaplastic carcinoma of the thyroid
• Colloid nodular goiter
• Goiter
• Medullary carcinoma of thyroid
• Multiple endocrine neoplasia (MEN) II
• Papillary carcinoma of the thyroid
• Toxic nodular goiter

All radiation has possible side effects. There is a very small amount of radiation in the radioisotope ingested during this test, but 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 concerns regarding radiation side effects are taken into consideration when the test is ordered, but the benefits of taking the test usually far outweigh the risks.

Thyroid scans using radinuclides are used with other studies, such as blood tests and ultrasound, to evaluate the thyroid. Your doctor may send you for more than one type of test.