A thyroid scan is a radiographic study that is used to generate images of the thyroid gland. Radioactive isotopes are special forms of elements that undergo a process called radioactive decay that involves a change from a higher energy state to a lower energy state. As radioactive isotopes undergo this change, they release small bursts of energy in the form of radiation. The tissue that makes up the thyroid gland is unique in that it is able to take up and trap certain radioactive isotopes called tracers. Different tracers may be ingested and absorbed through the gastrointestinal tract or injected directly into the bloodstream. Once a tracer has been taken up by the tissue that makes up the thyroid gland, a special scanner can be used to capture the energy released by the radioactive isotope to generate a picture of the thyroid gland. The two radioactive isotopes most commonly used to perform thyroid scans are called 123-Iodine and 99m-Technetium Pertechnetate.
What information can it provide?
A thyroid scan can be used to generate images that reflect the relative activity of different parts of the thyroid gland. The amount of tracer taken up in the course of performing a thyroid scan reflects the amount of thyroid hormone produced and secreted by tissue distributed in different regions of the thyroid gland. When 123-Iodine is used as a tracer, it is taken up and trapped through the same pathways that process the nonradioactive iodine circulating in the bloodstream that is used to produce thyroid hormone. Overactive regions of the thyroid gland may take up increased amounts of tracer, releasing increased amounts of energy that produce darker spots when images are generated. Underactive regions of the thyroid gland may take up lesser amounts of tracer, releasing lesser amounts of energy that produce lighter spots when images are generated.
How is it performed?
Most thyroid scans are performed by radiologists or radiology technicians who generate images that are interpreted by radiologists. Before a thyroid scan is performed, it is important to make sure that the patient undergoing the study is not taking any medications that contain iodine. It is also important to make sure that the patient has not undergone any recent radiographic studies involving intravenous dyes that contain iodine. In either situation, previous exposure to iodine may block the entry of radioactive tracer into the thyroid gland, making it difficult to obtain clear images. If the patient is taking either methimazole or propylthiouracil for treatment of hyperthyroidism, the medication may need to be held for a few days before a thyroid scan is performed. If the patient is a woman of childbearing age, it is important to make sure that there is no chance that she may be pregnant before performing a thyroid scan.
When a thyroid scan is performed using 123-Iodine, the patient is usually told to begin fasting after midnight the evening before the study begins. Once the patient has reported for the study, the amount of radioactivity present in the dose of 123-Iodine to be used as a tracer is carefully measured. After the amount of radioactivity has been documented, the patient swallows the tracer in the form of a liquid or capsule. A first set of images is usually acquired about 4-6 hours after the tracer is swallowed. The patient lies on a flat table and is positioned so that the neck and upper part of the chest lie beneath the surface of the scanner. The patient is told to lie still while the scanner acquires images of the thyroid gland that can be viewed as pictures. This process is usually repeated about 24 hours after the tracer is swallowed to acquire a second set of images.
When a thyroid scan is performed using 99m-Technetium Pertechnetate, there may not be any special preparation that is required. Once the patient has reported for the study, an intravenous line is inserted into one of the forearms. After the line has been inserted, the tracer is injected with a syringe. The patient waits for about 15-30 minutes, and then lies on a flat table and is positioned so that the neck and upper part of the chest lie beneath the surface of the scanner. The patient is told to lie still while the scanner acquires images of the thyroid gland that can be viewed as pictures. In some cases, daubs of tracer may be placed on the chest or the neck to provide landmarks to help locate the position of the thyroid gland.