A circadian rhythm of serum osteocalcin levels in postmenopausal osteoporosis.

The serum levels of osteocalcin, a 49 amino acid bone matrix protein, have been found to be a specific biochemical parameter of bone formation. The aim of our study was to assess the variability of serum osteocalcin and parathyroid hormone levels in postmenopausal osteoporosis. In 16 patients with postmenopausal osteoporosis, serum levels of osteocalcin and parathyroid hormone were determined in 4-hourly intervals by radioimmunoassay. Whereas the serum parathyroid hormone levels were similar throughout the day, the serum osteocalcin levels showed a circadian rhythm, with lowest levels in the morning and maximal levels during the night. These findings might suggest a circadian variation of bone formation in patients with postmenopausal osteoporosis.