Diagnostics: DEXA Scan (Bone Density Test)

A DEXA scan is a quick, painless, low-radiation test that measures bone density, used to find osteoporosis before a fracture and to follow bone health in conditions such as hyperparathyroidism.

What a DEXA scan is

A DEXA scan (also spelled DXA, short for dual-energy X-ray absorptiometry) is a quick, painless imaging test that measures the strength of the bones. It does this by estimating bone mineral density — essentially, how much calcium and other minerals are packed into the bones. Denser bones are stronger and less likely to break.

The scan uses two low-energy X-ray beams. Because bone and soft tissue absorb these two beams differently, a computer can separate them and calculate how dense the bones are. The radiation dose is very low — far less than a standard chest X-ray and only a small fraction of the natural radiation people are exposed to in daily life.

DEXA is considered the "gold standard" test for measuring bone density. It is accurate, widely available, inexpensive, and reproducible, which also makes it an excellent tool for tracking changes in the bones over time.

Why a doctor may order one

The most common reason is to screen for osteoporosis — a condition in which bones become thin and fragile and break more easily. Osteoporosis usually causes no symptoms until a bone breaks, so the scan is a way to find a problem before a fracture happens.

DEXA is commonly recommended for:

• All women age 65 and older

• Younger postmenopausal women who have risk factors for weak bones

• Men age 70 and older, or younger men with risk factors

• Anyone who has broken a bone easily (from a minor fall or bump)

• People taking steroid medications long term, or other medicines that can weaken bone

• People with certain medical conditions that affect bone, including hyperparathyroidism, overactive thyroid, and others

The scan is also used to check whether a bone-strengthening treatment is working, by repeating it after an interval of time.

What happens during the scan

The patient lies on their back on a padded table while a scanning arm passes above. The test is painless, patients stay fully clothed (avoiding clothing with metal zippers or buttons), and it usually takes only about 10 to 20 minutes. Most scans measure the hip and the lower spine, because these central bones best predict the risk of a serious fracture. Sometimes the forearm is measured as well.

Understanding the results: the T-score

Results are reported mainly as a T-score, which compares a person's bone density with that of a healthy young adult:

• Normal: T-score of −1.0 or above

• Low bone mass (osteopenia): T-score between −1.0 and −2.5

• Osteoporosis: T-score of −2.5 or lower

A more negative number means thinner, weaker bone. A report may also show a Z-score, which compares bone density with others of the same age and sex; this is used mainly in younger adults.

Bone density is only part of the picture. The doctor may combine the results with a tool called FRAX, which estimates the chance of breaking a bone over the next 10 years by also factoring in age, prior fractures, family history, smoking, and other risks. Most fractures actually occur in people whose bone density is not yet in the osteoporosis range, which is why this bigger-picture assessment matters.

How hyperparathyroidism fits in

The parathyroid glands are four tiny glands in the neck that make parathyroid hormone (PTH), which controls the calcium level in the blood. In hyperparathyroidism, one or more of these glands becomes overactive and makes too much PTH. Over time, the excess hormone pulls calcium out of the bones and into the blood, which can gradually weaken the skeleton. For this reason, a DEXA scan is a standard part of evaluating anyone with hyperparathyroidism.

Hyperparathyroidism affects the skeleton in a characteristic way. It tends to thin the outer, dense (cortical) bone more than the inner, spongy bone. Because the forearm (the lower third of the radius) is made mostly of this cortical bone, it is often the site most affected — so in people with hyperparathyroidism the forearm is scanned in addition to the hip and spine, even though it is not routinely measured in standard osteoporosis screening.

The encouraging news is that the bone loss is often reversible. When an overactive parathyroid gland is removed by surgery (parathyroidectomy), bone density typically improves at all sites over the following years, and the risk of future fractures goes down. When surgery is not appropriate, medications can also help protect the bones. A baseline DEXA and repeat scans help the doctor decide whether treatment is needed and whether it is working.

Key points to remember

• DEXA is a fast, painless, low-radiation scan that measures bone strength.

• The T-score shows whether bone density is normal, low, or in the osteoporosis range.

• Bone density is combined with other risk factors to estimate real-world fracture risk.

• In hyperparathyroidism, the forearm is scanned because that bone is affected the most, and bone density often recovers after the problem gland is treated.