Idiopathic juvenile osteoporosis: evidence of normal osteoblast function by 1,25-dihydroxyvitamin D3 stimulation test.

Idiopathic juvenile osteoporosis (IJO) is a rare form of bone demineralization that occurs during childhood. The mechanism of bone loss is unclear. Some bone hystomorphometric studies have found osteoblast failure and decreased bone formation in the affected patients whereas others have reported increased bone resorption. To elucidate this issue, we studied osteoblast function in six patients with IJO (five males, one female; aged 2.3-14.6 years) and five healthy sex- and age-matched subjects (four males, one female; aged 2.0-15.1 years) measuring serum values of osteocalcin under basal condition and during an osteoblast stimulation test performed by oral 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] administration (1.8 micrograms/1.73 m2/daily). After a baseline day (day 0), all the subjects (patients and controls) received 1,25(OH)2D3 in four divided doses for 6 days (days 1-6). Fasting blood samples were obtained every morning (0800 h) for the determination of serum osteocalcin. Baseline osteocalcin levels were not significantly different between IJO and controls (13.58 +/- 6.05 ng/ml versus 16.04 +/- 5.09 ng/ml, respectively) even if two patients had low osteocalcin values. During 1,25(OH)2D3 administration, serum osteocalcin values significantly increased (P less than 0.001) from baseline in both children with IJO and controls, reaching peak values not significantly different in the two groups. Our results do not support the hypothesis that defective osteoblast function is the primary factor of bone demineralization in IJO.