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Thyroid Scan

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What is the test?

There are two types of thyroid nuclear medicine tests. Both assess the health of your thyroid, a gland in your neck that controls metabolism. A thyroid scan produces a picture of the gland to help evaluate any lumps or inflammation, or to investigate the cause of an overactive thyroid. 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

A thyroid scan is usually ordered when a physical examination or laboratory finding suggests that the thyroid is enlarged. If laboratory tests show an overactive thyroid, a radioactive iodine uptake test may be ordered at the same time.

A radioactive iodine uptake test measures the amount of radioactivity in your thyroid after you've been given a relatively small dose of radioactive iodine in pill form. Your thyroid gland absorbs iodine and uses it to make hormones. Therefore, the amount of radioactive iodine detected in your thyroid gland corresponds with the amount of hormone your thyroid is producing.

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How do I prepare for the test?

If there is any chance that you are pregnant, or if you are breast-feeding, let your doctor know. Radionuclides could harm your nursing baby or a developing fetus, so your doctor will use an alternative method to diagnose the problem, such as ordering additional blood tests or monitoring your symptoms over several weeks to see if they resolve.

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 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.

The preparation for a radioactive iodine uptake test is almost the same as for a thyroid scan. However, because you are given radioactive iodine in pill form for an uptake test, you need to wait four to six hours, and possibly as long as a day, before having the scan. (This gives the radioactive iodine time to reach your thyroid.)

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What happens when the test is performed?

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, 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.

An uptake test only takes several minutes and is performed while you are sitting up. Using a device that resembles a Geiger counter, the doctor or technician places a probe several inches in front of your neck, where the thyroid gland is located, and measures the percentage of radioactivity that is retained by the thyroid gland. You return the next day for follow-up testing to obtain a second set of uptake readings, which are then compared with the first set to determine how much hormone has been formed and secreted in the interim.

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What risks are there from the test?

The risk from both varieties of 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.

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Must I do anything special after the test is over?

No. 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.

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How long is it before the result of the test is known?

For the thyroid scan, 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.

Uptake test results are available immediately, but because the initial and follow-up readings must be compared, it may take your doctor a day or two to get back to you. To obtain results, your doctor determines an uptake value, which is the net result of how much iodine is picked up by the thyroid, how much is converted to hormone since the time of administration, and how much is either leaked or secreted into the bloodstream. (The thyroid normally secretes hormone in an orderly fashion based on physical needs; leakage is less controlled and indicates that the gland is damaged.) A low reading of radioactivity suggests that your thyroid gland has retained only a small amount of iodine. This generally indicates that the thyroid gland is not producing excess thyroid hormone, but has become inflamed and is unable to properly store the hormone, which then leaks into the bloodstream. A high reading suggests that your thyroid is overactive, producing an excessive amount of thyroid hormone.

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