Patients with primary hyperparathyroidism--why do some form stones?

PURPOSE: Primary hyperparathyroidism is a common cause of hypercalcemia with resulting hypercalciuria and a subset of these patients with primary hyperparathyroidism present with urolithiasis. We evaluated the metabolic stone profile in patients presenting for parathyroid surgery and determined whether there were differences between those with and without a history of urinary stones.
MATERIALS AND METHODS: A total of 60 patients with primary hyperparathyroidism presenting for parathyroid surgery were prospectively enrolled in the study, of whom 54 completed a preoperative 24-hour urine collection. Of these 54 patients 27 completed an additional postoperative 24-hour urine collection. We compared preoperative metabolic urinary profiles and serum calcium in patients with primary hyperparathyroidism with and without a history of urolithiasis. In addition, we compared changes in urine metabolic profiles that occurred after successful parathyroid surgery.
RESULTS: There were no significant differences in preoperative 24-hour urine metabolic profiles or serum calcium between patients who had primary hyperparathyroidism with and without a history of urolithiasis. Compared to preoperative levels after successful parathyroid surgery there were significant decreases in serum calcium (10.8 to 9.3 mg/dl, p <0.001), urinary calcium (319 to 156 mg per day, p <0.01) calcium oxalate supersaturation (8.6 to 5.7, p = 0.016) and calcium phosphate supersaturation (1.6 to 0.9, p = 0.002) in the 27 patients who completed a postoperative 24-hour urine collection.
CONCLUSIONS: Other etiological factors must exist that cause some patients with primary hyperparathyroidism to form stones, while most never have stones despite profound hypercalcemia and hypercalciuria. Routine 24-hour urine evaluation cannot predict which patients with primary hyperparathyroidism will have kidney stones.