Suppression by calcium of serum levels of intact parathyroid hormone in patients with primary hyperparathyroidism

Primary hyperparathyroidism (pHPT) is associated with a right-shifted relation between parathyroid hormone (PTH) secretion and calcium. However, it is also possible that a decreased suppressibility of PTH secretion by calcium is important for maintaining hypercalcemia in pHPT. We therefore compared the suppression of serum levels of intact PTH induced by a 1.5-gram oral calcium load in patients with mild pHPT with that in healthy subjects. The calcemic response to the oral calcium load was the same in the two groups and did not correlate with the degree of PTH suppression or to serum levels of vitamin D metabolites. It was found that serum levels of intact PTH were less suppressed by the oral calcium load in patients than in healthy subjects (p < 0.01), but with a considerable overlap between the two groups. The suppression of serum levels of intact PTH was correlated both to baseline serum total calcium levels (r = -0.55; p < 0.05) and osteocalcin levels (r = -0.69; p < 0.05) in the patients, but no such correlations were seen in the controls. We conclude that patients with pHPT have a decreased suppressibility of PTH secretion by calcium. Although this reduced suppressibility could be important for maintaining hypercalcemia in some patients with pHPT, it does not aid in the differential diagnosis between patients with mild pHPT and healthy subjects.