Inter-relations between the calcium set-points of Parfitt and Brown in primary hyperparathyroidism: a sequential citrate and calcium clamp study.

The objective of the present study was to compare the calcium set-points of E. M. Brown and A. M. Parfitt obtained by sequential citrate and calcium clamp in patients with primary hyperparathyroidism and healthy controls. Twenty-six patients with primary hyperparathyroidism were investigated and compared to 22 healthy volunteers. All participants were investigated by sequential calcium lowering and raising comprising the following four phases: Phase (1) blood ionized calcium lowering of about 0.20 mmol l-1; phase (2) steady-state (relative) hypocalcaemia of blood ionized calcium 0.20 mmol l-1 below baseline; phase (3) blood ionized calcium is raised to about 0.20 mmol l-1 above baseline; and phase (4) (relative) hypercalcaemia of blood ionized calcium 0.20 mmol l-1 above baseline. Serum parathyroid hormone (1-84) was measured by an immunoradiometric assay. Blood ionized calcium was measured by a calcium selective electrode. We found the calcium set-points of Parfitt to be 1.42 mmol l-1 (SD 0.12, n = 52) vs. 1.25 mmol l-1 (SD 0.04, n = 44) in patients and controls, respectively (P < 0.001). The calcium set-points of Brown were 1.32 mmol l-1 (SD 0.10, n = 26) vs. 1.13 mmol l-1 (SD 0.04, n = 22), respectively (P < 0.001). By comparing the calcium set-points of Parfitt and Brown, a strikingly good correlation was observed, in patients (r = 0.91, P < 0.001) and in controls (r = 0.85, P < 0.001). We demonstrate in this paper in vivo that Brown's and Parfitt's calcium set-points are raised in primary hyperparathyroidism and return to normal following parathyroidectomy.(ABSTRACT TRUNCATED AT 250 WORDS)