A thyroid nuclear medicine scan is a diagnostic procedure to evaluate the thyroid gland, which is located in the front of the neck and controls the metabolism of the body. A radioactive substance that concentrates in the thyroid is taken orally or injected into a vein (intravenously), or both. There are three types of radioactive iodine used in these scans. A special camera is used to take an image of the distribution of the radioactive substance in and around the thyroid gland. This is interpreted to evaluate thyroid function and to diagnose abnormalities. Although other imaging methods exist for evaluating thyroid disease, thyroid scanning is the most commonly used and is the most cost-effective.
Purpose
A thyroid scan can help assess the overall structure and function of the thyroid. It can be used to identify benign cancers, to assess nodules, to evaluate masses, to locate the source of a painful gland, to assess gland size, to find differentiated carcinomas, and to identify thyroid tissue. A thyroid scan may be ordered by a physician when the gland becomes abnormally large, especially if the enlargement is greater on one side, or when hard lumps (nodules) are felt. The scan can be helpful in determining whether the enlargement is caused by a diffuse increase in the total amount of thyroid tissue or by a nodule or nodules. The thyroid scan plays a critical role in the diagnosis of thyroid cancer.
Women who are pregnant should not have this test. Any person with a history of allergy to iodine, such as those with shellfish allergies, should notify the physician before the procedure is performed.
Description
This test is performed in a radiology facility, either in an outpatient x-ray center or a hospital department. Most often, the patient is given the radioactive substance in the form of a tasteless liquid or capsule. It may be injected into a vein (intravenously) in some instances. Generally, the patient lies on an examination table as the scanning is performed. Images will be taken at a specified amount of time after this, depending on the radioisotope used. Most often, scanning is done 24 hours later, if the radioisotope is given orally. If it is given intravenously, the scan is performed approximately 20 minutes later.
For a thyroid scan, the patient is positioned lying down on his or her back, with the head tilted back. The radionuclide scanner, also called a gamma camera, is positioned above the thyroid area as it scans. This takes 30-60 minutes.
The uptake study may be done with the patient sitting upright in a chair or lying down. The procedure is otherwise the same as described for the thyroid scan. It takes approximately 15 minutes. There is no discomfort involved with either study.
A thyroid scan may also be referred to as a thyroid scintiscan. The name of the radioactive substance used may be incorporated and the study called a technetium thyroid scan or an iodine thyroid scan. The radioactive iodine uptake scan may be called by its initials, an RAIU test, or an iodine uptake test.
Preparation
Certain medications can interfere with iodine uptake. These include certain cough medicines, some oral contraceptives, non-steroidal anti-inflammatory drugs, epilepsy drugs, and thyroid medications. The patient is usually instructed to stop taking these medications for a period of time before the test. This period may range from several days up to three to four weeks, depending on the amount of time the medicine takes to clear from the body.
Other nuclear medicine scans and x-ray studies using contrast material performed within the past 60 days may affect this test. Therefore, patients should tell their doctors if they have had either of these types of studies before the thyroid scan is begun, to avoid inaccurate results.
Some institutions prefer that the patient have nothing to eat or drink after midnight on the day before the radioactive liquid or capsule is to be taken. A normal diet can usually be resumed two hours after the radioisotope is taken. Dentures, jewelry, and other metallic objects must be removed before the scanning is performed. No other physical preparation is needed.
The patient should understand that there is no danger of radiation exposure to themselves or others. Only very small amounts of radioisotope are used. The total amount of radiation absorbed is often less than the dose received from ordinary x rays. The scanner or camera does not emit any radiation, but detects and records it from the patient.
Aftercare
No isolation or special precautions are needed after a thyroid scan. The patient should check with his or her physician about restarting any medications that were stopped before the scan.
Risks
There are no risks with this procedure.
Normal results
A normal scan will show a thyroid of normal size, shape, and position. The amount of radionuclide uptake by the thyroid will be normal, according to established laboratory figures. There will be no areas where radionuclide uptake is increased or decreased.
Abnormal results
An area of increased radionuclide uptake may be called a hot nodule or "hot spot." This means that a benign growth is overactive. Despite the name, hot nodules are unlikely to be caused by cancer. Increased radionuclide uptake is indicative of hyperthyroidism and may suggest Graves' disease or an active pituitary adenoma.
An area of decreased radionuclide uptake may be called a cold nodule, or "cold spot." This indicates that this area of the thyroid gland is underactive. A variety of conditions, including cysts, hypothyroidism, nonfunctioning benign growths, localized inflammation, or cancer, may produce a cold spot. Single nodules that are not functioning are malignant in about 10-20% of cases. Completely nonfunctioning nodules have a higher probability of being malignant than those that have some degree of function.
A thyroid nuclear medicine scan is rarely sufficient to establish a clear diagnosis. A majority of nonfunctioning nodules are not malignant, but their presence increases the probability of a malignancy. Nodules that are functioning are rarely malignant. Frequently, the information revealed will need to be combined with data from other studies to determine the problem.
Resources
BOOKS
Goroll, Allan H., et al., eds. "Screening for Thyroid Cancer." In Primary Care Medicine: Office Evaluation and Management of the Adult Patient. Philadelphia: Lippincott, Williams & Wilkins: 2000.
Kronenberg, Henry M., et al., eds. "Imaging Techniques." In Williams Textbook of Endocrinology. Philadelphia: W.B. Saunders, 1998.
Ravel, Richard, ed. "Thyroid Function Tests." In Clinical Lab oratory Medicine: Clinical Applications of Laboratory Data. St. Louis: Mosby, 1995.
Siberry, George K., et al., eds. The Harriet Lane Handbook: A Manual for Pediatric House Officers. St. Louis: Mosby, 2000.
PERIODICALS
Nusynowitz, M. L. "Thyroid Imaging." Lippincotts Primary Care Practice 3 (November-December, 1999): 546-55.
Sandhu, A., et al. "Subclinical Thyroid Disease after Radiation Therapy Detected by Radionuclide Scanning." Interna tional Journal of Radiation Oncology, Biology, Physics 48 (August, 2000): 181-8.
Mark A. Mitchell, M.D.
Adenoma
—A type of non-cancerous tissue that emanates from glands.
Benign
—A type of tissue overgrowth that is not progressive, unlike malignant tissue.
Graves' disease
—A condition characterized by bulging eyeballs, among other symptoms, that is synonymous with hyperthyroidism.
Radioisotope
—A radioactive or radiation-emitting form of an element.
Radionuclide
—A substance that emits radiation as it disintegrates.
