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What is the thyroid gland?
The thyroid is a gland that makes and stores essential hormones that help regulate the heart rate, blood pressure, body temperature, and the rate of chemical reactions (metabolism) in the body. It is located in the anterior neck just below the "Adam's apple."
The thyroid gland is the main part of the body that takes up iodine. In a thyroid scan, iodine is labeled with a radioactive tracer, and a special camera is used to measure how much tracer is absorbed from the bloodstream by the thyroid gland. If a patient is allergic to iodine, technetium can be used as an alternative.
How is the thyroid scan performed?
A thyroid scan is an outpatient procedure usually done in the nuclear medicine section of a hospital. This is usually part of the radiology department.
If you have a thyroid scan, you will be asked to take a drink, or swallow a pill containing the iodine. You will then wait for the iodine to be taken up by the thyroid - usually about 4 hours. At this time you lie under a scintography camera, and it will take pictures which correlate directly to the amount of iodine taken up by the thyroid. This is done by counting the intensity and location of the gamma rays emitted by the radioactively labeled iodine. You then leave the hospital, and return in 24 hours to have a second scan performed in the same manner.
There are no limitations during this 24 hour interval except that you will be asked to take precautions when you urinate. This is because the radioactive iodine is removed from your body by the urine, and it is safer to have others avoid contact with your urine during this time. Be sure to let the doctor performing the test know if you have an iodine allergy, or if you have ingested a lot of iodine-containing foods (like sushi) prior to the test.
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When is thyroid scanning helpful?
A thyroid scan can provide information on the size and shape of the gland as well as the overall activity of the gland (is the whole thyroid gland overactive or underactive?).
For example, thyroid scanning is used to determine how active thyroid tissue is in manufacturing thyroid hormone. This feature can determine whether inflammation of the thyroid gland (thyroiditis) is present. It can also detect the presence and degree of overactivity of the gland (hyperthyroidism ). In this case the scan reveals increased iodine uptake by the whole gland.
In addition, a thyroid scan can provide information on specific areas within the thyroid gland and can help to determine whether any localized areas are hyperfunctioning or under functioning compared to the rest of the gland. In these cases spots appear on the image corresponding to areas that take up iodine differently from the rest of the gland.
Thyroid scanning is especially helpful in evaluating thyroid nodules, particularly after a fine needle aspiration biopsy has failed to provide a diagnosis. A scan will reveal whether a thyroid nodule is "functioning" or "nonfunctioning". Doctors refer to hyper functioning nodules of the thyroid (those that are actively taking up iodine to produce thyroid hormone more so than surrounding thyroid tissue) as "hot" nodules, and these hyperfunctioning areas are seen on the image of the thyroid gland. A nonfunctioning nodule does not take up iodine and produces a localized "cold" area on the image of the thyroid gland.
In addition, thyroid scanning can be done to help determine if thyroid cancer has spread beyond the bed of the thyroid in the neck. For this purpose, usually a whole body thyroid scan is performed.
What is significant about whether a nodule is "hot" or "cold?"
Functioning or "hot" nodules only rarely are from cancer. Nearly all thyroid cancers are nonfunctioning or "cold" nodules. However, even among "cold" nodules, cancer is infrequent (less than 5 percent of cases).
Thyroid Scan Safety and Efficacy
While the thought of taking something "radioactive" is not very appealing, it is important to remember that iodine concentrates only in thyroid tissue, making this test safe and also quite specific for thyroid disorders.
A thyroid scan can provide information on both the structure as well as the function of the thyroid gland. This information can help you and your physician determine if further investigations, procedures, or medications are medically necessary.
Medically reviewed by John A. Seibel, MD; Board Certified Internal Medicine with a subspecialty in Endocrinology & Metabolism
National Institutes of Health
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