The Harvard Medical School Family Health Guide
Diagnostic Tests - Thyroid Scan
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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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