Influence of calcium infusion on urinary cyclic AMP and phosphate in hyperparathyroidism

Urinary cyclic AMP (cAMP) and phosphate were measured before and after calcium infusion in 12 patients with operatively verified primary hyperparathyroidism (PHP) and in 12 healthy persons. In normal subjects infusion of calcium caused a reduction in urinary cAMP directly correlated to the preinfusion values and inversely correlated to the serum calcium concentration determined as albumin-corrected serum calcium. In normal subjects with high normal albumin-corrected serum calcium the infusion of calcium caused no or only a small depression in the urinary excretion of cAMP. Changes in phosphate excretion were not correlated to the calcium concentration. Four of the 12 hyperparathyroid patients showed normal relative suppression in urinary cAMP after the infusion of calcium, and 5 had normal suppression of phosphate excretion. It is concluded that some patients with PHP retain calcium-sensitive secretion of PTH, and that the classical calcium infusion test is of doubtful value in the diagnosis of PHP.