The role of la,25-dihydroxyvitamin D in the mediation of intestinal hyperabsorption of calcium in primary hyperparathyroidism and absorptive

absorption of calcium (Ca) in various forms of hyper-calciurias was explored by a careful measurement of plasma la,25-dihydroxycholecalciferol [la,25-(OH)2D] and by an assessment of intestinal Ca absorption and of parathyroid function. In 18 cases of primary hyperparathyroidism (PHPT), the mean plasma con-centration of la,25-(OH)2D was significantly increased (4.9+2.2 SD ng/dl vs. 3.4+0.9 ngldl for the control group), and was significantly correlated with fractional Ca absorption (a) (r = 0.80, P < 0.001). Plasma la,25-(OH)2D was also correlated with urinary Ca (P < 0.05), but not with serum Ca or phosphorus (P), P clearance, urinary cyclic AMP, or serum immunoreactive parathy-roid hormone. In 21 cases of absorptive hypercalciuria (AH), plasma la,25-(OH)2D was elevated in one-third of cases, and the mean value of 4.5±+1.1 ng/dl was significantly higher than that of the control group (P < 0.01). Since relative hypoparathyroidism may be present, the normal absolute value of plasma la,25-(OH)2D, found in two-thirds of cases of AH, may be considered to be inappropriately high. Moreover, in the majority of cases of AH, the data points relating plasma la,25-(OH)2D and a fell within 95 % confidence limits of values found in non-AH groups (including PHPT). The results suggest that the intestinal hyper-absorption of Ca in PHPT and AH may be vitamin D dependent. However, the disturbance in vitamin D metabolism may not be the sole cause for the high Ca absorption in AH, since in some patients with AH, the intestinal Ca absorption appears to be Received for publication 29 April 1976 and in revised form 4 January 1977. inappropriately high for the level of plasma la-25-(OH)2D