SPECIFICITY OF URINARY-EXCRETION OF CROSS-LINKED N-TELOPEPTIDES OF TYPE-I COLLAGEN AS A MARKER OF BONE TURNOVER

Urinary excretion of cross-linked N-telopeptide of type I collagen (NT X) has been reported to be a specific indicator of bone resorption. We studied the utility of a new immunoassay for NTX as an indicator of c hanges in bone resorption caused by treatment with pamidronate (APD) f ollowed by T-3. Twenty-two male subjects received either placebo (Grou p 1) or APD on study days 1-2 (Group 2). One week later all subjects r eceived T-3 100 mu g/day (days 8-15). Urinary NTX, pyridinoline (PYD), hydroxyproline (HYP), and creatinine (cr) were measured on 2-hour fas ting urine samples at baseline (day 1), after APD/placebo (day 8), aft er T-3 (day 16), and at days 30 and 58. NTX/cr excretion fell 85% afte r treatment with APD (P < 0.001 versus baseline), but not after placeb o. The fall in mean urinary NTX after receiving APD was greater than t he fall in PYD (25%) or HYP (31%) (P < 0.001 NTX versus PYD and HYP). After treatment with APD, NTX excretion remained suppressed below base line until day 58, whereas PYD and HYP excretion returned to baseline by study day 16. Persistence of APD's effect on bone until day 58 was suggested by the fact that serum calcium and parathyroid hormone level s had not returned to baseline by day 58. On day 16, after all subject s were treated with T-3, urinary NTX/cr rose significantly (P < 0.01) in Group 1 (-bisphosphonate) but not in Group 2 (+bisphosphonate). We conclude that urinary NTX is responsive to acute thyroid hormone-induc ed increases and bisphosphonate-induced decreases in bone resorption, and may reflect these changes more accurately than PYD or HYP.