The effects of age and gender on parathyroid hormone dynamics

BACKGROUND AND AIMS Hyperparathyroidism is a risk factor for bone loss. An age-related increase in parathyroid hormone (PTH) level has been demonstrat ed in several studies. It has been suggested that the type II osteoporotic syndrome, a condition of increased prevalence among elderly women, may be a t least partially caused by elevations in intact parathyroid hormone (iPTH) levels. To date, however, the effects of age and gender per se on PTH dyna mics 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) i nfusions to characterize the impact of age and gender on PTH dynamics in no rmal subjects. SUBJECTS AND METHODS 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 w ith 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 seru m levels of ionized Ca (Ca-i) were characterized by maximal and minimal iPT H levels, the set-points (levels of Ca-i causing half-maximal suppression o f iPTH), and the slopes of the curves at the set-points. RESULTS Baseline serum Ca, Ca-i, 25 hydroxyvitamin D [25(OH)D] and 1,25 dih ydroxyvitamin D [1,25(OH)(2)D-3] levels, as well as the set-points, slopes and minimal values of the sigmoidal curves relating Ca-i to iPTH, did not d iffer 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 l evel was 25% higher in the older women than in the younger women, although this difference was not significant (P = 0.29). However, the integrated iPT H responses calculated from the areas under the curves (AUC) of iPTH levels vs. time during the calcium and citrate infusions were significantly highe r 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 effec t of gender on serum iPTH levels. CONCLUSIONS In both women and men, ageing per se, independent of changes in vitamin D economy or renal function, is associated with an increase in int egrated PTH secretory response to changes in serum calcium. No alterations in the Ca-i/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.