What is a thyroid scan?

A thyroid scan makes use of the thyroid gland's ability to concentrate certain radioactive isotopes to generate pictures of it. Radioactive isotopes are special forms of elements that undergo a process called decay in which they change from higher energy states to lower energy states. As they undergo this change, they release small bursts of energy in the form of radiation that can be detected by special cameras. The tissue that makes up the thyroid gland is unique in that it is able to take up and trap iodine and certain other molecules of similar size. When radioactive isotopes of these substances (tracers) are swallowed or injected into the bloodstream, they are taken up by the thyroid gland. As they decay, a special camera can detect the energy that is released, creating a picture of the thyroid gland. The radioactive isotopes that are most commonly used as tracers to perform thyroid scans are called 123-Iodine, 99m-Technetium pertechnetate and 131-Iodine.

What information can it provide?

A thyroid scan provides information about the size, shape, location, and relative activity of different parts of the thyroid gland. Certain regions of the thyroid gland are sometimes more or less active than normally functioning tissue. Overactive regions may take up more radioactive isotope, releasing more energy that produces a darker spot when a picture is generated. Underactive regions may take up less radioactive isotope, releasing less energy to produce a lighter spot when a picture is generated. The level of activity that a thyroid nodule displays may provide clues as to whether it may or may not be thyroid cancer.

When a thyroid scan is performed, the amount of radioactive isotope taken up by the thyroid gland can also be measured. While tracer uptake reflects certain aspects of gland activity, it does not reliably determine how much thyroid hormone is actually being made by the gland.

How is it done?

Before a thyroid scan is performed, it is important to make sure that a patient has not been taking any medications that could interfere with the study. Iodine-containing drugs (such as amiodarone) or radiocontrast dyes (such as those used to perform special cardiac, vascular, brain, and kidney X-rays) may block the entry of radioactive tracer into the thyroid gland, making it difficult to obtain pictures. Thyroid hormone may suppress the function of the thyroid gland, limiting the uptake and concentration of radioactive tracer. If a patient is taking Methimazole or Propylthiouracil for treatment of an overactive thyroid gland, it usually needs to be held for a few days before a thyroid scan is performed. It is also important to make sure that there is no chance that a patient of childbearing age may be pregnant.

Most thyroid scans are performed by nuclear medicine physicians, radiologists, or by technicians who capture images that are later interpreted.

For a thyroid scan using 99m-Technetium pertechnetate, the patient needs to have an intravenous line placed for administration of the tracer, which is injected with a syringe. The patient waits for about 15-30 minutes, and then lies back on a flat table and is positioned so that the neck and upper part of the chest are beneath the scanner. The patient is instructed to lie still while the scanner acquires images of the thyroid. In some cases, tracer markers may be placed on the chest or neck to provide landmarks that help locate the thyroid gland.

For a thyroid scan using 123-Iodine or 131-Iodine, the patient swallows the tracer in the form of a liquid or a capsule. A first set of images is usually acquired about 4-6 hours after the tracer is swallowed. The patient lies back on a flat table and is positioned so that the neck and upper part of the chest are beneath the scanner. The patient is instructed to lie still while the scanner acquires images of the thyroid gland. This process is usually repeated about 24 hours after the tracer is swallowed to acquire a second set of images. If uptake measurements are to be made, they are usually checked at the times that the images are acquired with a special probe that is held over the thyroid gland.

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