Familial hypocalciuric hypercalcaemia I: Renal handling of calcium, magnesium and phosphate.

Ten hypercalcaemic members from three generations of a family with familial hypocalciuric hypercalcaemia (FHH) were compared with age and sex matched healthy subjects. Two of the former had undergone unsuccessful subtotal parathyroidectomy. Our results showed that the hypercalcaemia was mainly attributable to an increased capacity for tubular reabsorption of calcium, but in part also to an increased release of calcium from bone. The relative hypermagnesaemia had a similar dual origin. The serum phosphate concentration was low and this could be accounted for in full by a decrease in renal tubular reabsorption of phosphate, as assessed by the renal threshold phosphate concentration (TmPO4/GFR). The results of PHT measurements with two radioimmunoassays were equivocal. Most patients had normal serum PTH values, but with one assay mean serum PTH was significantly higher in the hypercalcaemic group. We conclude that the abnormalities of the divalent cation and phosphate metabolism cannot be accounted for in full by increased circulating PTH activity, and are predominantly due to an intrinsic renal abnormality.