Nine patients with
primary hyperparathyroidism were studied to investigate the renal tubular reabsorption of
calcium and
sodium. Fasting serum and urine samples were analysed, and the glomerular filtration rate and the renal plasma clearance of
lithium were determined simultaneously. Comparison was made with 9 age- and sex-matched normocalcemic controls. In the proximal tubule, there was a significantly higher absolute reabsorption of
calcium in patients than in controls, whereas the fractional reabsorption rate of
calcium did not differ between the two groups. In the distal tubule, the absolute
calcium reabsorption rate was significantly higher in the patients, whereas the fractional reabsorption rate of
calcium was significantly lower than in controls. In the patient group there was a significantly positive linear correlation between the increased tubular capacity for
calcium reabsorption and the absolute proximal
calcium reabsorption rate, but not between the increased capacity and the absolute distal
calcium reabsorption rate. No significant differences were found in the renal tubular handling of
sodium between patients and controls. Our results suggest that the increased capacity for tubular
calcium reabsorption in
primary hyperparathyroidism mainly is localized in the proximal tubule, and that the renal tubular handling of
calcium and
sodium in this disease differs from that in
familial hypocalciuric hypercalcemia.