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Inter‐relations between the calcium set‐points of Parfitt and Brown in primary hyperparathyroidism: a sequential citrate and calcium clamp study
Author(s) -
SCHWARZ P.,
SØRENSEN H. A.,
TRANSBØL I.
Publication year - 1994
Publication title -
european journal of clinical investigation
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.164
H-Index - 107
eISSN - 1365-2362
pISSN - 0014-2972
DOI - 10.1111/j.1365-2362.1994.tb01106.x
Subject(s) - calcium , primary hyperparathyroidism , endocrinology , medicine , calcium metabolism , hypercalcaemia , parathyroid hormone , chemistry , hyperparathyroidism
. 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. The values for Brown's and Parfitt's calcium set‐points are significantly different, but strikingly well correlated, supporting the view that Brown and Parfitt describe two different points on the same sigmoidal curve, corresponding to 50% and about 85% inhibition of PTH maximum, respectively. The mathematical form of the sigmoidal curve between blood ionized calcium and parathyroid hormone is very similar in primary hyperparathyroidism and normal humans.