Thyroid Radionuclide Scan Health Article

Definition

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.

Purpose

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.

Precautions

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.

Description

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.

Preparation

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.