Abstract
J Clin Endocrinol Metab 2013;
It is well established that both elevated parathyroid hormone (PTH) concentration and insufficient 25-hydroxyvitamin D (25OHD) have a negative impact on bone integrity. However, there may be more to the relationship between these two molecules and osteoporosis than just the lack of 25OHD stimulating PTH secretion. Indeed, the correlation between the serum concentrations of 25OHD and PTH is very weak. This study examines the relationship further by comparing vertebral fracture rates between individuals with respect to their PTH and 25OHD status.
Postmenopausal women (n = 820) with normal mineral and bone metabolism were selected for the study. Laboratory measurements of serum PTH, 25OHD, C-terminal telopeptide of type I collagen (CTX) and N-terminal propeptide of type I collagen (PINP) were performed, whilst bone mineral density (BMD) was measured using dual energy X-ray absorptiometry.
The study cohort was divided into three groups based on serum PTH concentration. After adjusting for relevant factors, those in the group with serum PTH of ≥6.1 pmol/L had a higher incidence of vertebral fractures (20% compared to 9%, odds ratio [OR] 1.84, 95% confidence interval [CI] 1.16–2.90, P = 0.009) and lower BMD at several sites (e.g. 2.5%; P = 0.028 at lumbar spine). In addition, those in the group with the highest PTH concentration had higher concentrations of CTX and PINP (markers of bone resorption and formation, respectively). As expected, there was an inverse relationship between concentrations of 25OHD and PTH (r = −0.25; P = 0.009); however, both these factors were shown to be significantly associated with vertebral fractures (OR for PTH, 1.02 per ng/L* [1.01–1.03]; P = 0.004; OR for 25OHD, 0.97 per ng/L [0.94–0.99]; P = 0.025). When the women were grouped by 25OHD concentration, significant differences in BMD were seen between the group with a PTH concentration >6.1 pmol/L and the group with a PTH concentration ≤6.1 pmol/L, when the 25OHD was below 50 nmol/L (P = 0.002 and P = 0.046 at the neck and lumbar spine, respectively).
These results demonstrate that 25OHD directly protects bone from resorption when PTH concentrations are high, in addition to its previously established role of suppressing PTH secretion. The authors hypothesize a direct interaction between 25OHD and cells of the bone, in keeping with other research demonstrating such a mechanism of 25OHD action.