A thyroid scan is a nuclear medicine examination that uses a radioactive iodine tracer to see how well the thyroid gland is working.

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 the iodine pill is taken. Another scan may be taken 24 hours later.

Other scans may be done using a substance containing technetium.

After the radioactive iodine has been absorbed by the thyroid, 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.

During this part of the procedure, you must lie still to let the scanner get a clear image. A computer displays images of the thyroid gland.

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 medications that may need to be adjusted, 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.

This test is done to check for:

• Hyperthyroidism
• Thyroid cancer
• Thyroid nodules or other growths

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

A thyroid that is enlarged or pushed off to one side 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 a possible 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 cause hypothyroidism or hyperthyroidism.

Other 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 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 far outweigh the risks.

Thyroid scans using radionuclides 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.