A parathyroid scan is a nuclear medicine imaging test that pinpoints overactive parathyroid glands in your neck or chest. It’s most commonly ordered when blood tests show elevated calcium and parathyroid hormone levels, pointing to a condition called hyperparathyroidism. The scan helps a surgeon know exactly where the problem gland is before operating, which can mean a smaller incision and a shorter procedure.
Why Doctors Order This Scan
You have four parathyroid glands, each about the size of a grain of rice, sitting behind your thyroid in your neck. Their job is to regulate calcium levels in your blood. When one or more of these glands grows into a benign tumor called an adenoma, it pumps out too much parathyroid hormone, pulling calcium from your bones and raising your blood calcium to unhealthy levels. This is primary hyperparathyroidism, and it’s the main reason for a parathyroid scan.
The scan can detect a range of problems: a single adenoma (the most common cause), multiple adenomas, cystic or fatty adenomas, and even glands that have migrated to unusual locations during fetal development. It’s particularly valuable when hyperparathyroidism comes back after surgery or when the first operation didn’t fully resolve symptoms. Increasingly, surgeons also request it before a first-time surgery to plan a minimally invasive approach rather than exploring the entire neck.
How the Scan Works
The standard parathyroid scan uses a small amount of a radioactive tracer injected into a vein in your arm. Both thyroid tissue and parathyroid tissue absorb this tracer initially, but here’s the key: normal thyroid tissue releases it relatively quickly, while overactive parathyroid tissue holds onto it longer. By taking images at two different time points, the radiologist can see which spots “light up” on the later images, revealing the problem gland.
An early set of images is typically taken about 10 minutes after the injection, followed by delayed images around 2 hours later. The best results come from comparing these two time points. Some facilities also take images at 4 hours, though the 2-hour delayed view is generally considered the most useful. Depending on the protocol, you may be free to leave the imaging area between the early and delayed scans.
Types of Parathyroid Imaging
Not all parathyroid scans use the same technology. The simplest version is planar imaging, which produces flat, two-dimensional pictures. Adding SPECT (a type of 3D nuclear imaging) improves accuracy considerably. In one comparative study, SPECT detected abnormal glands with about 92% sensitivity, compared to roughly 69% for the standard planar approach. Combining SPECT with a CT scan in the same session gives both functional and anatomical detail, helping surgeons see not just that a gland is overactive but precisely where it sits relative to surrounding structures.
European nuclear medicine guidelines recommend combining the nuclear scan with a neck ultrasound performed by an experienced sonographer as the standard first-line strategy. Together, these two tests achieve a sensitivity of 81 to 95% for locating abnormal glands.
4D CT Scans
A newer alternative called 4D CT uses a rapid series of contrast-enhanced CT images taken at different phases to identify parathyroid adenomas based on how they absorb and release contrast dye over time. A large meta-analysis of nearly 4,700 patients found 4D CT had an overall accuracy of about 88%, compared to 71% for the nuclear scan with SPECT/CT. The difference is even more dramatic when multiple glands are involved: 4D CT detected those cases with 58% sensitivity versus 31% for the nuclear approach. Doctors often turn to 4D CT when the standard nuclear scan comes back negative or unclear, when a patient has had previous neck surgery, or when multiple glands are suspected.
Choline PET/CT
PET/CT scans using a choline-based tracer represent the newest option and have shown superior results in early studies, particularly for patients whose standard imaging was inconclusive. This technology is not yet widely available and is still being evaluated for cost-effectiveness, but some centers already offer it as an alternative first-line test.
What to Expect on Scan Day
Preparation is straightforward but does require some advance planning. You’ll need to stop thyroid medications about a month before the scan, and you cannot have received IV contrast dye (from a CT scan or other test) within the prior month, as it can interfere with results. You’ll also be asked to fast before your appointment.
The tracer injection feels like a standard blood draw. Once injected, you’ll lie still on a scanning table while a camera positioned close to your neck and upper chest captures images. The camera doesn’t touch you. Each imaging session takes roughly 15 to 30 minutes. With the gap between early and delayed images, plan for the entire appointment to take about 2 to 3 hours, though much of that time is spent waiting rather than being scanned.
Side effects from the tracer are rare. When they do occur, they’re typically mild: brief nausea, a warm flushing sensation, or slight dizziness. Serious allergic reactions are extremely uncommon. The radiation dose from a standard-protocol scan is about 8 millisieverts, roughly equivalent to a few years of natural background radiation. Low-dose protocols can reduce this to under 2 millisieverts.
What the Results Can Show
A positive scan shows one or more “hot spots” that persist on the delayed images, indicating overactive parathyroid tissue. This gives the surgeon a roadmap for a focused, minimally invasive operation that targets only the affected gland.
One of the scan’s most important roles is finding glands that aren’t where they’re supposed to be. During fetal development, the parathyroid glands migrate from higher in the neck down to their final position, and sometimes they don’t stop where expected. The lower parathyroid glands are especially prone to ending up in unusual spots, most commonly embedded in the thymus gland in the front of the chest. Upper glands, when displaced, tend to drift into the back of the upper chest. In rarer cases, ectopic glands have been found near the aorta, in the tissue surrounding the heart, or even near the diaphragm. Without a scan, a surgeon might explore the neck and find nothing, not realizing the problem gland is several inches lower in the chest.
A negative or inconclusive result doesn’t rule out hyperparathyroidism. It simply means the scan couldn’t pinpoint the location. This happens more often with smaller adenomas or when multiple glands are mildly enlarged rather than one gland being dramatically overactive. In these cases, your doctor may recommend a 4D CT, a choline PET/CT, or a combination of imaging methods to get a clearer picture before proceeding with surgery.