Clinical Studies on Phosphate Handling in Hypercalcaemia

Abstract

Phosphate indices (serum phosphate, tubular reabsorption of phosphate, renal threshold phosphate concentration (TmP/GFR) and index of phosphate excretion) were studied in 88 hypercalcaemic subjects: 64 with primary hyperparathyroidism (HPT) and 24 with hypercalcaemia from other causes, predominantly malignant disease. HPT patients as a group could easily be separated from normal subjects (n = 16) and patients with functional hypoparathyroidism (n = 7) by use of the phosphate variables but these indices were of little discriminating value for the differential diagnosis between HPT and hypercalcaemia from other causes. There was no difference in the urinary cyclic adenosine monophosphate (cAMP) excretion between the two hypercalcaemic patient groups, but HPT patients had clearly elevated serum parathyroid hormone (PTH) levels compared with normal PTH concentrations in patients with other causes of hypercalcaemia. A positive correlation between cAMP and serum calcium and an inverse relationship between cAMP and TmP/GFR were found in patients with hypercalcaemic malignant disease. These findings suggest the existence of a humoral factor with PTH-like effects in malignant disease. Since PTH levels were low, the physiological actions were apparently not mediated by circulating PTH. No difference in the values for phosphate variables, PTH, cAMP or serum calcium was found between renal stone-forming and stone-free patients with HPT.