RELATIVE HYPOPARATHYROIDISM AND CALCITRIOL UP-REGULATION IN HYPERCALCIURIC CALCIUM RENAL STONE FORMERS - IMPACT OF NUTRITION

The issue of secondary hyperparathyroidism in idiopathic hypercalciuri a (IH) was addressed in 61 male idiopathic calcium stone formers (SF) who underwent metabolic evaluation on a free-choice diet as well as bo ne mineral density (BMD) measurements by dual-energy X-ray absorptiome try. They were divided into hypercalciurics (HCSF, n = 30, U(Ca) X V > 7.5 mmol/day) and normocalciurics (NCSF, n = 31, U(Ca) X V < 7.5 mmol /day). At identical blood Ca2+ levels, parathyroid hormone (PTH) was l ower in HCSF (25.3 +/- 1.8 pg/ml) than in NCSF (31.4 +/- 1.8 pg/ml, p = 0.017). Since neither fasting urinary hydroxyproline nor pyridinolin e/deoxypirdinoline excretions nor BMD values were different between HC SF and NCSF, chronic bone dissolution as the cause of relative hypopar athyroidism in HCSF could be excluded. Despite lower PTH in the face o f similar phosphate, Ca2+ and IGF-1 blood levels, however, serum 1,25- dihydroxyvitamin D3 (calcitriol) concentrations were slightly (though not significantly) higher in HCSF than in NCSF (52.8 +/-3.2 vs. 47.3 /- 2.9 pg/ml, p = NS), and calcitriol/PTH ratio was elevated in HCSF ( 2.52 +/- 0.29) vs. NCSF (1.66 +/- 0.15, p = 0.001). Creatinine clearan ce, significantly higher in HCSF than in NCSF (113 +/- 4 vs. 92 +/- 3 ml/min/1.73 m2, p = 0.000 1), was positively correlated with excretion rates of urinary markers of both protein and NaCl intake. Since serum calcitriol levels were positively correlated with creatinine clearanc e (r = 0.350, slope = 0.288, p = 0.006), up-regulation of calcitriol s ynthesis with subsequent relative hypoparathyroidism in HCSF is - at l east partly - explained by exaggerated protein and sodium consumption