Normocalcemic primary hyperparathyroidism: resistance to PTH effect on tubular reabsorption of calcium

The mechanism accounting for normal calcemia in certain cases of primary hyperparathyroidism (HPT) remains unclear and more generally the relative importance of each determinant of calcemia has not been systematically studied so far. 52 primary HPT patients with stable calcemias, ranging from 95 to 137 mg/l, were investigated. They all exhibited identical ionized to total plasma calcium ratios. Values of serum iPTH and nephrogenous cyclic AMP were similarly elevated in all patients, and all displayed similar high values for net bone resorption and intestinal absorption of calcium. Tubular reabsorption of calcium was normal or slightly subnormal in the normocalcemic subgroup, and rose in proportion to the increase in serum calcium in the hypercalcemic subgroup. No correlation was found between tubular reabsorption of calcium on the one hand and serum immunoreactive calcitonin, degree of phosphate depletion, sodium urinary excretion or magnesemia on the other. Plasma acid base equilibrium was normal in all patients. Finally, after surgical removal of adenomas, fasting calciuria and intestinal absorption of calcium returned to normal. It is concluded that (1) the main determinant of plasma calcium value in stable calcemia primary HPT is tubular reabsorption of calcium and (2) the absence of correlation between the tubular reabsorption of calcium and the degree of PTH hypersecretion suggests that as yet undetermined factors interfere with the tubular action of PTH.