Both scores appear on a DEXA report, and both measure bone density. But they compare your bones to different groups of people, and they answer different questions.
The T-score asks: how do your bones compare to a young adult at peak density?
As covered in the previous article, your T-score compares your bone density to a healthy young adult woman at her lifetime best. It is the number used to diagnose osteoporosis and osteopenia, and it is the score your doctor will focus on when making treatment decisions.
The Z-score asks: how do your bones compare to other women your age?
Your Z-score compares your bone density to women of the same age, body size, and ethnicity. It tells you whether your bone density is unusual for someone at your stage of life, not whether it is low in an absolute sense.
A Z-Score of zero means your bones are exactly average for your age group. A Z-score of -1.0 means your density is lower than most women your age. A Z-score of +1.0 means it is higher.
Why the Z-score matters, but differently
Because bone density naturally declines with age, a woman of 65 will almost always have a lower T-score than a woman of 35 even if both are perfectly healthy for their age. The Z-score corrects for this. It asks whether your bone loss is tracking normally for your age, or whether something else may be contributing to lower-than-expected density.
A Z-score below -2.0 is considered below the expected range for age. When this happens, your doctor may investigate whether a secondary cause (a medical condition or medication) is accelerating bone loss beyond what menopause alone would explain.
Secondary causes worth knowing about
Conditions and medications that can cause bone loss beyond the expected rate include: long-term corticosteroid use (such as prednisolone), coeliac disease, inflammatory bowel disease, rheumatoid arthritis, hyperthyroidism, and certain cancer treatments. If your Z-score is significantly low, your doctor may order blood tests to rule these out.
A Z-score below -2.0 is a signal, not a verdict. It simply means your doctor should look a little deeper. That is exactly what good care looks like. If yours is significantly low, ask your doctors whether any blood tests are worth doing to rule out a secondary cause.
Which score should you focus on?
For most women, the T-score is what drives the conversation about diagnosis and treatment. The Z-score is a supporting piece of information most useful when it prompts further investigation into why bone density is lower than expected for your age. If your Z-score is within the normal range, that is reassuring context. If it is significantly low, it is a signal worth exploring with your doctor.
Both numbers are on your report for a reason. Neither tells the whole story alone.
Bone is a very dynamic tissue. Every single day, your skeleton breaks down old bone and replaces it with new. This process is called remodelling. In a healthy adult, your entire skeleton is effectively replaced over the course of about ten years. Your Z-score is a snapshot of how that ongoing process is tracking for your age