Parathyroid function in persistent hyperparathyroidism: relationship to gland size.

The release of parathyroid hormone in experimental animals is related inversely to the plasma calcium concentration. The relevance, though, of these observations to the dynamics of parathyroid function in normal and hyperparathyroid humans is uncertain. We assessed the in vivo parathyroid hormone response to changes in extracellular calcium in 8 normal subjects and 15 patients with persistent hyperparathyroidism following renal transplantation. In 12 hyperparathyroid patients, the hormone response was related to their total gland size measured at the time of their parathyroidectomy. Plasma ionized calcium, magnesium, and parathyroid hormone concentrations were measured in the basal state and during a 2-hr infusion of EDTA (50 mg/kg), and a 4-hr calcium infusion (15 mg/kg). The parathyroid function curves of both groups of subjects (P less than 0.001) fit a log-linear relationship. The slopes of the respective parathyroid function curves were similar, although the hyperparathyroid curve was shifted to the right (P less than 0.0001). Gland size was not predicted by basal PTH levels; however, it did correlate with changes in parathyroid hormone induced by EDTA (P less than 0.001) and calcium (P less than 0.001). We conclude that the in vivo sensitivity of hyperplastic glands to changes in plasma calcium is maintained. The excessive secretion of immunoreactive parathyroid hormone in chief cell hyperplasia primarily reflects total gland mass. Our results indicate that the assessment of the dynamics of parathyroid response, rather than measurements of static plasma parathyroid hormone and calcium concentrations, should be further investigated as a more rational application of radioimmunoassays in the evaluation of the parathyroid axis.