Bone Density is Not Related to Blood Calcium Level

Q:  I'm a bit confused over a calcium blood test that I had done. I'm 54 and have been on both estrogen and progesterone for the last few years. Because I have a small frame and have always been concerned about osteoporosis, I've been very religious about calcium intake and have worked out with weights for about 27 years. I had my first bone density test in fall and it already showed some thinning even though my blood test came back showing a high calcium count. How can this be? Could you explain please?

A:  The level of calcium in your blood does not reflect how much calcium is in your bones or how "dense" and strong they are. On bone density tests, they compare your bones to women your age ("Z score") and to young women ("T score"). Every woman over 50 will show bone density loss compared to 20 year-old women, so that may be the "thinning" that your test showed.

You've been doing three important things for your bones: good calcium intake, exercise/weight-training, and estrogen. Even hormones are not absolute protection though. In a recent study of over 800 women, about 10% of those on hormones lost some density in their spine and hips. This is still much better than the 60% of women not taking hormones who lost bone density.

"Dear Doctor" is a compilation of patient questions answered by doctors from the Medical College of Wisconsin.