The pathogenesis of idiopathic hypercalciuria: evidence for parathyroid hyperfunction

We studied the effect of calcium deprivation and loading in 17 healthy subjects and 76 patients with renal calculi. Five had primary hyperparathyroidism with an elevated plasma ionised calcium and detectable plasma parathyroid hormone. Forty-nine had idiopathic hypercalciuria, defined by a urine calcium greater than 7 mmol/day on a free diet. Twenty-two were normocalciuric. Fasting plasma calcium, corrected for albumin, was higher in the patients with idiopathic hypercalciuria (2.40 +/- 0.10 mmol/l) than in controls (2.28 +/- 0.05 mmol/l, p less than 0.005). Plasma calcium was intermediate in the normocalciuric stone formers (2.35 +/- 0.08 mmol/l) and elevated in the patients with primary hyperparathyroidism (2.62 +/- 0.07 mmol/l). Nephrogenous cyclic adenosine monophosphate (cAMP) and parathyroid hormone levels were highest in the primary hyperparathyroid group and did not differ significantly within the other groups. Nephrogenous cAMP correlated positively with plasma calcium in the patients with primary hyperparathyroidism and negatively in controls; there was no correlation in the idiopathic hypercalciuria group. Following an oral calcium load, plasma calcium rose and nephrogenous cAMP fell similarly in all groups. Fasting urinary calcium and its increase after load were greatest in the idiopathic hypercalciuria and primary hyperparathyroid groups, with intermediate results in the normocalciuric patients. Neither the initial metabolic patterns nor the response to thiazide fitted with the previously described patterns of absorptive and renal hypercalciuria. Increased parathyroid gland activity is the most probable cause of idiopathic hypercalciuria.