RACIAL-DIFFERENCES IN PARATHYROID-HORMONE DYNAMICS

Elevations in PTH levels have been reported in black subjects. Such ob servations have not been consistent, however, and seem paradoxical in view of the known bone-resorptive action of this hormone and the fact that black subjects have a higher bone mineral density and fewer fract ures than their white counterparts. In this study, we used dynamic sti mulation of the calcium-PTH axis to fully characterize potential racia l differences in PTH dynamics. We, therefore, defined the inverse sigm oidal curve that describes the relationship between serum ionized calc ium concentration and intact PTH levels in six normal white and six no rmal black volunteers and determined the four parameters that characte rize this relationship. An elevation in any one of these parameters ca n result in hyperparathyroidism. Black subjects had higher maximal and minimal PTH responses to hypo- and hypercalcemia (mean intact PTH lev els of 9.2 +/- 13 and 0.7 +/- 0.1 pmol/L respectively) than white subj ects (6.9 +/- 0.6 and 0.3 +/- 0.1 pmol/L, respectively). There were no differences in the set-points or slopes of the curves. Despite the hi gher baseline and stimulated endogenous PTH levels in black subjects, their baseline and stimulated osteocalcin levels were lower. Our dynam ic studies, therefore, document mild hyperparathyroidism in black subj ects and suggest mild skeletal resistance to PTH.