Maximal Urine-Concentrating Ability: Familial Hypocalciuric HypercalcemiaVersusTypical Primary Hyperparathyroidism

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Abstract
Impairment of urine-concentrating ability is common in persons with chronic hypercalcemia. We assessed urineconcentrating ability in 40 patients withtypical primary hyperparathyroidism and 10 patients with familial hypocalciuric hypercalcemia, a disorder resembling typical primary hyperparathyroidism but lacking some of its clinical complications. Urineconcentrating ability was determined during a dehydration test of 18–22 h. The two patient groups were comparable withrespect to serum calcium concentration and creatinine clearance. In the group with familial hypocalciuric hypercalcemia, the duration of hypercalcemia was probably greater, because it commences during infancy; the urinary excretion ratefor calcium was lower [6.6 ± 5.4 (mean ±1 SD) vs. 14.8 ± 7.5 meq/day; P < 0.005]. Patients with familial hypocalciuric hypercalcemia showed higher maximal urinary osmolality (860 ± 150 vs. 664 ± 130 mosmol/kg; P < 0.0005). Among the patients with typical primary hyperparathyroidism, there was a negative association between maximal urinary osmolality and urinary cAMP (r 0 –0.40; P < 0.05), but there was no significant relation between maximal urinary osmolality and the urinary excretion rate for calcium. Among 18 patients retested within 1 month after surgical correction of typical primary hyperparathyroidism, urineconcentrating ability did not improve. In patients with typical primary hyperparathyroidism, impairment in urine–concentrating ability reflects features of the chronic disease state, as it is not rapidly reversible by correction of that stateHowever, in patients with familial hypocalciuric hypercalcemia, longstanding hypercalcemia is not associated with obvious impairment of urine-concentrating ability. Complete or partial freedom from impairment of urine-concentrating ability and from calcareous renal disease are expressions of the generally mild course in familial hypocalciuric hypercalcemia.