The effects of age and gender on parathyroid hormone dynamics

Abstract

Hyperparathyroidism is a risk factor for bone loss. An age-related increase in parathyroid hormone (PTH) level has been demonstrated in several studies. It has been suggested that the type II osteoporotic syndrome, a condition of increased prevalence among elderly women, may be at least partially caused by elevations in intact parathyroid hormone (iPTH) levels. To date, however, the effects of age and gender per se on PTH dynamics in healthy subjects independent of other risk factors such as vitamin D deficiency and/or impaired renal function that can impact on parathyroid function, remain unknown. In this study, we used citrate and calcium (Ca) infusions to characterize the impact of age and gender on PTH dynamics in normal subjects. Twelve young women with mean age +/- SD of 26.4 +/- 1.6 years, 12 young men with mean age of 26.6 +/- 1.3 years, 12 older women with mean age of 68.6 +/- 1.3 years and 12 older men with mean age of 67.2 +/- 1.6 years were studied. The sigmoidal curves relating serum iPTH to serum levels of ionized Ca (Cai) were characterized by maximal and minimal iPTH levels, the set-points (levels of Cai causing half-maximal suppression of iPTH), and the slopes of the curves at the set-points. Baseline serum Ca, Cai, 25 hydroxyvitamin D [25(OH)D] and 1,25 dihydroxyvitamin D [1,25(OH)2D3] levels, as well as the set-points, slopes and minimal values of the sigmoidal curves relating Cai to iPTH, did not differ among the four groups. iPTH levels at baseline were slightly but not significantly higher in the older age groups (P = 0.18). The maximal iPTH level was 25% higher in the older women than in the younger women, although this difference was not significant (P = 0.29). However, the integrated iPTH responses calculated from the areas under the curves (AUC) of iPTH levels vs. time during the calcium and citrate infusions were significantly higher in postmenopausal women than in young women during both infusions and in older men than in young men during the calcium infusion. There was no effect of gender on serum iPTH levels. In both women and men, ageing per se, independent of changes in vitamin D economy or renal function, is associated with an increase in integrated PTH secretory response to changes in serum calcium. No alterations in the Cai/iPTH set-point were present. The biological relevance of these modest increments in integrated iPTH levels during dynamic testing in older healthy men and women remain uncertain.