Physiological basis for absorptive and renal hypercalciurias

Idiopathic hypercalciuria constitutes 2 major variants - absorptive hypercalciuria, characterized by a primary intestinal hyperabsorption of calcium, and renal hypercalciuria, in which renal tubular reabsorption of calcium is primarily impaired. The 2 forms of hypercalciuria may be distinguished from each other, since (a) parathyroid function is stimulated in renal hypercalciuria, but normal or suppressed in absorptive hypercalciuria, (b) the renal leak of calcium is present in renal hypercalciuria, but not in absorptive hypercalciuria, (c) intestinal calcium absorption is probably increased primarily in absorptive hypercalciuria, and secondarily in renal hypercalciuria (from parathyroid hormone excess), (d) the increased calcium absorption in renal hypercalciuria probably results from the parathyroid hormone-dependent stimulation of 1,25-dihydroxyvitamin D synthesis, whereas that in absorptive hypercalciuria may be vitamin D-independent, (e) the response of the 2 conditions to certain treatments is unique, and (f) the sequelae of parathyroid hormone excess, such as low bone density and negative calcium balance, may be present in renal hypercalciuria, but not in absorptive hypercalciuria. These findings provide a physiological basis for the consideration of absorptive and renal hypercalciurias as distinct and separate entities.