Role of parathyroid hormone in mediating nocturnal and age-related increases in bone resorption

PTH has been postulated to play a role in both nocturnal and age-related increases in bone resorption. We tested this hypothesis directly in 10 young (ages 24–35 yr) and 10 elderly (ages 71–78 yr) normal women by measuring the cross-linked N-telopeptide of type I collagen (NTx), a marker for bone collagen breakdown, in 4-h urine collections before and during suppression of PTH secretion by a 24-h iv infusion of calcium. Serum ionized calcium and PTH levels were also measured every 2 h before and during the infusion. In both groups of women, serum PTH levels and urinary NTx excretion followed a circadian pattern before calcium infusion (analysis of variance, P = 0.0001) with peaks in the afternoon and at night for PTH and at night for urinary NTx. During the calcium infusion, the nocturnal urinary NTx excretion peak persisted (P = 0.0001), despite elimination of both PTH peaks. Urinary 24-h NTx excretion (nanomoles per millimoles of creatinine) at baseline was higher in the elderly women (mean ± SEM, 25.7 ± 2.1) than in the young women (19.3 ± 1.7) (P < 0.01), and the decrease during calcium infusion was greater (7.5 ± 1.9 vs. 4.1 ± 1.5, P < 0.05). Therefore, the increase in serum PTH levels with age is one of the major factors responsible for the age-related increase in bone resorption. PTH does not mediate the circadian pattern of bone resorption but does play a role in setting the absolute level of bone resorption at which this pattern occurs.