A thyroid ultrasound, or sonogram, is a diagnostic imaging technique used to evaluate the structure of the thyroid gland. The thyroid is an endocrine gland, which means that it releases its secretions directly into the bloodstream or lymph. It consists of two lobes located in the front of the neck that are connected by a thin band of tissue called the isthmus, which lies in front of the trachea (windpipe). Ultrasound procedures utilize high frequency sound waves to obtain images of various anatomical structures. Ultrasonography is the most common imaging technique used to evaluate the thyroid because it is not invasive, does not expose patients to radioactive materials, is less expensive than CT scans or MRI, and is more effective in detecting small lesions on the thyroid.
An ultrasound of the thyroid is performed to evaluate thyroid nodules discovered during a physical examination or revealed by a radionuclide study (thyroid scan). A sonogram is most useful when the physician must distinguish between cystic lesions and solid ones, or evaluate any mass in the neck. In many cases the ultrasound examination identifies additional nodules in the thyroid that are too small for the doctor to feel during the external physical examination.
Most thyroid cysts are benign; however, ultrasound imaging cannot be used to differentiate between benign cysts or nodules and cancer. Specialized thyroid sonograms, such as color Doppler flow studies, can add valuable information. By showing an image of the blood circulation in the gland, this study can assess some ambiguous masses in greater detail. The shade and intensity of the color indicate the direction and the velocity of the flow. The physician may insert a needle in order to remove some tissue for laboratory evaluation (needle biopsy or fine needle aspiration). Ultrasound is used during this procedure to help the physician guide the needle into the mass under evaluation. The use of color Doppler flow helps the physician to avoid puncturing a blood vessel while collecting the tissue sample.
Thyroid ultrasound can measure the size of the gland with great precision, and may be done periodically to assess the results of treatment. An enlarged thyroid gland or a benign nodule should decrease in size with appropriate medication. In addition, patients who have had radiotherapy of the head or neck may be monitored at regular intervals using thyroid ultrasound. Patients who had radiation treatment in these areas in childhood or adolescence have a 30% risk of developing thyroid cancer or other glandular abnormalities in adult life. In the early stages, these conditions may not cause symptoms or be discovered during a physical examination. They may, however, be detected by ultrasound.
Thyroid ultrasound is safe for people of all ages. It is the preferred procedure to evaluate suspected disease in pregnant women because no radioactive materials are involved.
Thyroid ultrasonograms may be performed in an outpatient facility or in a hospital department. The patient usually lies on his or her back, although the procedure can also be done with the patient in a sitting position. A pillow or rolled towel is placed under the shoulders and upper back, allowing the head to tilt back (hyperextend). A gel that enhances sound transmission is spread over the thyroid area. The technologist then gently places a transducer, an instrument that both emits and receives sound waves, against the skin. The transducer is about the size of an electric shaver and is moved over the thyroid area. The most common frequencies used for thyroid ultrasound are between 7.5 and 10 megahertz (mHz). The patient should not experience any discomfort from the procedure. The examination takes 15–30 minutes.
The high-frequency sound waves emitted by the transducer are transmitted or reflected differently by different body tissues and structures. Bone and cartilage block the passage of the sound waves, producing a very bright signal. The windpipe, which is filled with air, does not transmit ultrasound waves. Most tissues do, however, transmit the sound waves to a greater or lesser extent. Fluid-filled structures, such as cysts, have a uniform
appearance. Muscles, organs, and other fleshy structures have a ground-glass appearance; that is, they appear to diffuse light.
Some facilities recommend limiting food and drink for one hour before the study to prevent discomfort. No other preparation is needed.
No special restrictions or procedures are needed after a thyroid ultrasound.
There are no risks or complications with this procedure.
A normal study will demonstrate a thyroid gland of normal size, shape, position, and uniform echotexture. A thyroid gland that measures outside of the normal limits suggests a goiter. If the overall echotexture or pattern of reflected sound waves is mottled and uneven, the pattern may indicate the presence of thyroiditis or other inflammatory disease. Lesions, both solid and cystic, are easily visualized on ultrasound examination.
Health care team roles
The ultrasound examination is performed by an ultrasound technologist, or diagnostic medical sonographer. The sonographer will review any medical history provided and may need to obtain additional information from the patient. All information obtained from the ultrasound is interpreted by a physician who is a radiologist or, in some cases, an endocrinologist. Patients typically receive the results of the examination from the doctor who ordered the test. This physician will correlate the results of the sonogram with the patient's history as well as findings from the physical examination, thyroid function tests, and nuclear medicine tests.
Endocrine—A type of gland that secretes internally into the blood or lymph.
Fine needle aspiration (FNA)—A technique for diagnosing thyroid nodules by withdrawing, or aspirating, a sample of thyroid tissue cells through a 22–29-gauge needle.
Goiter—Enlargement of the thyroid gland along the front and sides of the neck.
Nodule—A small, rounded lump or mass of tissue.
Sonogram—Another word that is sometimes used for an ultrasound examination.
Transducer—A device that converts a signal from one form of energy to another. In an ultrasound examination, the transducer converts an electrical current to sound waves and echoes from the sound waves back into electrical current.
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Feld, Stanley, M.D. 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.