Changes in serum osteocalcin associated with parathyroid hormone infusion in X-linked hypophosphatemic rickets.

Osteocalcin is a protein unique to bone that can be quantitated in serum by radioimmunoassay. While its function remains unknown, it appears to be a sensitive marker of changes in bone activity. To determine its relationship to parathyroid hormone action, we measured serum osteocalcin in blood samples obtained from patients with vitamin D-resistant rickets before and after administration of exogenous parathyroid hormone. Serum osteocalcin was decreased by 35% at 15 min after infusion and gradually returned toward normal levels by 75 min. We suggest that the acute decline in serum concentrations after infusion is an indication of inhibition of osteoblast activity. Thus, osteocalcin may be a useful means of assessing bone responsiveness to parathyroid hormone.