Thyroid Radionuclide Scan
A thyroid nuclear medicine scan is a diagnostic imaging procedure to evaluate the thyroid gland , which is an endocrine gland consisting of two lobes located in the front of the neck anterior to the trachea. The two lobes are connected by a thin band of tissue called the isthmus. The thyroid gland is stimulated by hormones, and secretes other hormones that govern the body's metabolism . In a radionuclide scan, a radioactive tracer that is selectively absorbed by the thyroid is administered either orally or intravenously. Special equipment that can detect radioactive emissions from the thyroid is used to image the gland, or to measure the concentration of the radioactive tracer in the thyroid gland. The data collected are interpreted to evaluate thyroid function and to diagnose the presence of thyroid disease.
The radionuclides that are used in thyroid scans are two isotopes of iodine, I-131 and I-123, and an isotope of technetium known as 99m Tc. Technetium scanning is preferred for some diagnostic workups because it is relatively fast and does not require the patient to fast beforehand. Some professionals prefer to reserve I-131 for follow-up evaluations of cancer patients, and use I-123 for thyroid uptake tests and routine thyroid scans. The reason for the distinction is the higher radiation burden of I-131.
Thyroid scans are performed to determine the size, shape, location, and relative function of the thyroid gland. More specifically, a thyroid scan may be ordered by a physician to assess thyroid nodules; to diagnose the cause of thyrotoxicosis (excessive thyroid secretion); to evaluate patients with a history of radiation therapy of the head or neck; or to assess a goiter. A thyroid scan is also used to detect the presence of ectopic thyroid tissue. If the patient had abnormal results from a blood test that measures circulating thyroid hormone levels, a scan may be required to aid in diagnosis of the presence of thyroid disease. In some instances, an additional study performed in conjunction with a thyroid scan, called a radioactive iodine uptake, or RAIU, is required to determine the level of glandular functioning.
Although thyroid scans use only low doses of radioactive substances, women who are pregnant are cautioned not to have these tests unless the physician indicates that the benefit outweighs the risk. If the patient is breast feeding, she may be advised to interrupt nursing, depending upon the radionuclide used and the dose administered for the test.
Thyroid scans are most often performed in a nuclear medicine or radiology facility, either in an outpatient xray center or a hospital department. If radioactive iodine is given, it is administered either in the form of a tasteless liquid or a capsule. If radioactive technetium is used, the patient is given an intravenous injection. Images of the thyroid gland are obtained at a specified amount of time afterward, depending on the radionuclide administered.
Typically, if radioactive iodine is used, a RAIU is also performed. Uptakes are usually obtained at two and 24 hours after administration of the radioactive iodine. The patient is positioned in front of a piece of equipment that measures the concentration of radioactive substance in the thyroid gland. The uptake procedure takes only a few minutes and the scan is most often performed at twenty-four hours after administration. If technetium is administered, the scan is performed approximately 20–30 minutes after the injection.
For the thyroid scan, the patient is positioned lying down on his or her back, with the head tilted slightly backward. The radionuclide scanner, also called a gamma camera, is positioned above the thyroid area. This procedure takes 30–60 minutes. There is no discomfort involved with either the uptake test or the scan.
Some medications may interfere with thyroid studies. If a patient is taking thyroid replacement hormone or anti-thyroid medication, the medication must be discontinued for a specified period of time, usually several weeks. Other recent nuclear medicine scans can affect thyroid studies if there is any residual radiation in the patient's body. In these cases the thyroid scan is postponed for a specified period of time, depending upon the other radioactive material that was used.
X-ray studies using contrast material containing iodine that were performed within the previous 60–90 days will affect thyroid studies using radioactive iodine. Patients should tell their doctors if they have had either of these types of studies before a thyroid scan.
Some over-the-counter medications, herbal supplements and vitamins contain large amounts of iodine or such iodine-rich substances as kelp (a type of seaweed), and therefore should be discontinued for a specified time prior to a thyroid scan.
Ectopic thyroid —Congenital thyroid tissue found outside the normal location of the thyroid gland, usually under the breastbone or the tongue.
Endocrine —A type of gland that secretes internally into the blood or lymph.
Goiter —Enlargement of the thyroid gland along the front and sides of the neck.
Isotope —One of two or more forms of a chemical element having the same atomic number but having different atomic weights.
Nodule —A small, rounded lump or mass of tissue.
Radionuclide —A substance that emits radiation as it disintegrates.
Technetium —A synthetic element obtained from the fission of uranium. Technetium is used for some types of radionuclide thyroid scans.
Thyrotoxicosis —A disorder characterized by an enlarged thyroid, rapid pulse, and increased basal metabolism due to excessive thyroid secretion. Thyrotoxicosis is sometimes called Graves' disease.
Some institutions prefer that the patient have nothing to eat or drink after midnight on the day before the scan. Most departments provide detailed written instructions regarding preparation for the scan, including dietary restrictions. A normal diet can usually be resumed two hours after the radioisotope is taken. Jewelry and other metallic objects worn around the neck must be removed before the scanning. No other physical preparation is necessary. Patients should understand that there is no danger of radiation exposure to themselves or others. Only very small amounts of the radioactive tracer 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.
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. Nursing mothers should inquire about resumption of breast feeding.
There are no complications with this type of diagnostic study.
Normal findings will show a thyroid gland of normal size, shape, and position. The amount of radionuclide concentrated by the thyroid will be within established laboratory guidelines. There should be no areas where the concentration of radionuclide is increased or decreased. An area of increased radionuclide uptake may be called a hot nodule or "hot spot," and may represent a hyperfunctioning nodule. An area of decreased radionuclide uptake may be called a cold nodule or "cold spot." This finding indicates that a particular area of the thyroid gland is underactive or low-functioning. A variety of conditions, including cysts, localized inflammation, or cancer may produce a cold spot.
Abnormal findings for an RAIU would include abnormally high and abnormally low uptake of the radioactive iodine. A low RAIU suggests hypothyroidism; a high RAIU points to a hyperthyroid condition.
A thyroid scan is rarely sufficient to establish a clear diagnosis by itself. The data collected from a thyroid scan are usually combined with data from blood tests that measure circulating thyroid hormone levels to establish the diagnosis. If nodules are present, a thyroid ultra-sound may be performed.
The data collected are typically stored in a computer, and the images of the thyroid gland are made on film or paper. The results for an RAIU are expressed as a mathematical equation and are reported as a percentage.
Health care team roles
A nuclear medicine technologist administers the radioactive substance to the patient and operates the equipment that produces the scan. The nuclear medicine technologist obtains pertinent medical history from the patient and will explain the nature of the test. All data collected by the technologist are interpreted by a physician who is a specialist in nuclear medicine or a radiologist. Patients usually obtain the test results from their physician or the physician who requested the thyroid tests.
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Klingensmith, William C. III, Dennis Eshima, and John Goddard. Nuclear Medicine Procedure Manual 2000-2001. Englewood, CO: Oxford Medical Inc., 2000.
Feld, Stanley. AACE Clinical Practice Guidelines for the Diagnosis and Management of Thyroid Nodules. New York: American Association of Clinical Endocrinologists, 1996.
Christine Miner Minderovic, B.S., R.T., R.D.M.S.