Secondary hyperparathyroidism in idiopathic renal hypercalciuria: fact or theory?

Secondary hyperparathyroidism (HP) has been reported to be characteristic of idiopathic hypercalciuria (IHC) of the renal type. Out of the 155 patients with urinary stone disease and a normal plasma calcium level, only 1 had a distinctively increased plasma parathyroid hormone (PTH) level; however, he was found to be hypercalcemic in a second control study. In 33 patients with renal IHC, none had an elevated PTH level measured under normal, low, and high calcium intake. The mean basal PTH level of these patients was identical to that of 86 normal controls; it was significantly lower than that of the patients with primary or secondary HP who had normal PTH levels. During EDTA infusion, seven of eight patients with primary HP and a normal basal PTH level showed PTH responses greater than those observed in six patients with renal IHC, all of the latter responses being in the normal range. Chlorthalidone, given to seven patients with renal IHC during 4 weeks (100 mg/day), did not decrease the PTH levels. Measurement of nephrogenous cAMP performed in only a few patients revealed a slightly increased value in one of them who had an exceptionally severe hypercalciuria. Beside this latter result, no evidence for secondary HP could be found. Although secondary HP is tempting pathophysiological explanation for many characteristics of renal IHC, it seems to be quite rare.