The thyroid gland, a butterfly-shaped endocrine organ located in the neck, is responsible for producing hormones that regulate the body’s metabolism and energy use. Tucked behind this larger gland are the parathyroid glands, typically four tiny, separate masses of tissue. These smaller glands secrete parathyroid hormone (PTH), which plays a direct and important role in regulating calcium levels within the blood. A thyroid ultrasound is a common, non-invasive imaging method used to examine the thyroid, but its ability to visualize the parathyroid glands is a more complex issue.
Focus and Procedure of Thyroid Ultrasound
A thyroid ultrasound uses high-frequency sound waves to create real-time images of the thyroid and surrounding structures in the neck. This non-invasive technique exposes the patient to no radiation, making it a safe choice for initial assessment. The primary goal of this scan is to assess the thyroid gland itself, looking for abnormalities like nodules, cysts, or overall enlargement (goiter).
The procedure involves applying a water-based gel to the neck, allowing a handheld wand, called a transducer, to glide smoothly. The transducer sends sound waves into the neck and captures the returning echoes, which a computer translates into detailed images. Because the thyroid is superficial, it is an easily accessible target for high-resolution ultrasound imaging. The scan provides information on the size, shape, composition, and blood flow (using Doppler technology) of any thyroid masses, which is essential for determining if a nodule requires further investigation.
Visibility of Normal Parathyroid Glands
While a thyroid ultrasound captures the anatomy of the entire central neck, a normal parathyroid gland is typically not visible. Most individuals have four parathyroid glands, and each is extremely small, usually measuring only 3 to 6 millimeters in its longest dimension. Their tiny size is the main reason they often go undetected during a routine thyroid scan.
These small glands are usually nestled against the posterior (back) surface of the thyroid gland. They are also often isoechoic, meaning their appearance on the ultrasound screen is similar to the surrounding normal thyroid tissue, which makes them difficult to distinguish. The superior parathyroid glands are often found near the back of the upper thyroid lobes, while the inferior glands are frequently located near the lower poles. In the absence of disease, their flat structure and subtle visual characteristics allow them to blend into the background tissue.
Identifying Enlarged Parathyroid Tissue
The situation changes when one or more parathyroid glands become diseased, most commonly in primary hyperparathyroidism. This condition typically causes one gland to overproduce parathyroid hormone, leading to its enlargement into a benign tumor known as a parathyroid adenoma. This pathological enlargement makes the gland visible on a neck ultrasound.
Once enlarged, typically exceeding one centimeter, the abnormal tissue becomes distinct from the surrounding structures. An adenoma usually appears as a discrete, oval or bean-shaped nodule that is homogeneously hypoechoic, meaning it is noticeably darker than the adjacent thyroid tissue. A thin, bright line of tissue is often visible, separating the adenoma from the edge of the thyroid gland.
Another feature is hypervascularity, or increased blood flow, visualized using Color Doppler ultrasound. A defining characteristic is the “polar vessel” sign, where an enlarged feeding artery or vein enters the adenoma at one of its poles. This distinct appearance allows the enlarged gland to be reliably localized before surgical intervention.
Confirmatory and Alternative Diagnostic Tests
While ultrasound can localize an enlarged parathyroid gland, it is not the final step in diagnosis. Parathyroid disease, such as primary hyperparathyroidism, is first confirmed by blood tests showing elevated levels of calcium and parathyroid hormone. Imaging is then used to find the specific location of the abnormal gland before surgery.
If a thyroid ultrasound is negative or inconclusive, specialized imaging is required, especially if the gland is small, in an unusual location (ectopic), or obscured by thyroid nodules. A common secondary test is the Sestamibi scan, a nuclear medicine procedure where a radioactive tracer is injected. The tracer is taken up by the overactive parathyroid tissue and retained longer than in the normal thyroid, highlighting the hyperactive gland.
The four-dimensional computed tomography (4D CT) scan is a specialized CT that tracks how contrast dye moves through the neck tissues over time. This method is useful for identifying glands located in difficult areas, such as the chest or deep in the neck, and is often employed when ultrasound and Sestamibi scans fail to provide clear localization.