Thyroid Nuclear Medicine Tests (Thyroid Scan and Uptake)
What Is It?
There are two types of thyroid nuclear medicine tests. Both assess the health of your thyroid, a gland in your neck. The first type, a thyroid scan, produces a picture of the gland. It can spot lumps or inflammation, or to investigate the cause of an overactive thyroid. The second type, a radioactive iodine uptake test, is performed to see if your thyroid is functioning normally and to determine why thyroid hormone levels may be elevated. For both types of test, a small amount of a weakly radioactive substance, known as a radionuclide, is either injected into a vein or given to you as a pill.
What It's Used For
A thyroid scan is usually ordered when a physical examination or laboratory finding suggests that the thyroid is enlarged or has a lump (called a thyroid nodule). If laboratory tests show an overactive thyroid, a radioactive iodine uptake test may be ordered at the same time to evaluate thyroid function.
If there is any chance that you are pregnant, or if you are breast-feeding, let your doctor know: radionuclides could harm a developing fetus or your nursing baby. Your doctor has other ways of diagnosing the problem, such as ordering additional blood tests or a thyroid ultrasound.
For about a week before a thyroid scan, your doctor may ask you to avoid certain foods and medicines that can interfere with the results, including thyroid hormones and shellfish (which contain iodine). You should inform your doctor of any vitamins or herbal supplements you are taking. You might have to fast entirely for several hours beforehand if you'll be given a radioactive iodine pill for the test. You might also need to have blood tests that check thyroid function.
You'll be asked to sign a consent form before having the test done, indicating that you understand the risks of the test and agree to have it done. As with x-rays, metal can interfere with the imaging during a thyroid scan, so you'll need to remove all jewelry and dentures.
How It's Done
A radionuclide is either injected into a vein or given to you as a pill. Timing of the test then depends on the type of radionuclide your doctor uses, and whether you will also have an uptake test.
If you are having only a thyroid scan and your doctor prefers to give a radionuclide by intravenous injection, the scan can be done within 30–60 minutes. If you are given radioactive iodine in pill form, you need to wait four to six hours, and possibly as long as a day after taking the pill, before having the scan. (This gives the radioactive iodine time to reach your thyroid.)
If you're having both a scan and the uptake test, you are likely to receive radioactive iodine in pill form. This allows one radionuclide to be used for both the scan and uptake test, instead of two, and eliminates the need for an injection.
After you've received the radionuclide and have waited the appropriate amount of time, a technician places a radioactivity detector — a camera specially designed to take pictures of radioactive objects — against your neck and takes several images. The camera itself doesn't expose you to any radiation. This portion of the test usually takes about half an hour.
The vast majority of the weakly radioactive substances used in these tests are cleared from your body within a day or two. But even before then, you can interact normally with other people because there's no risk of exposing them to significant amounts of radiation from your body.
It takes an hour or more for the pictures to be developed and additional time for a radiologist to examine them. Your doctor will probably receive a report within a day or two.
The scan will show the outline, shape, and position of your thyroid so that the doctor can determine whether it is enlarged and whether there are any suspicious growths or nodules. The scan also provides a rough measure of thyroid activity, although this has to be confirmed with a radioactive iodine uptake test.
Risk of a thyroid scan is minimal. The amount of radioactivity you are exposed to is comparable to that from a routine x-ray. The amount of radionuclide used is so small that it's unlikely to cause side effects or allergic reactions, and the benefits of having the test done usually outweigh any risks.
Women who are nursing or pregnant should discuss potential risks to the fetus or infant with a healthcare provider before having a thyroid scan and should consider alternatives.
When to Call a Professional
Harmful side effects are not expected; people typically need to call their doctors only for scan results.
American Thyroid Association (ATA)
6066 Leesburg Pike, Suite 550
Falls Church, VA 22041
Toll Free: 1–800–THYROID (1–800–849–7643)