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Abstract
The aim of the study was to investigate the interrelation between induced hypercalcaemia and serum intact parathyroid hormone (S-PTH(1–84)) in normal man and in patients with primary hyperparathyroidism (PHPT) by measuring blood ionized calcium (B-Ca++) and S-PTH(1–84) before and during a controlled calcium infusion. Guided by frequent measurements of B-Ca++, we adjusted the calcium infusion rate continuously, thereby keeping B-Ca++ in a steady state at a pre-determined level approximately 0.25 mmol 1−1 above baseline values. This calcium clamp technique (CCT) applied to 14 normal volunteers for 120 min established a standardized reference for parathyroid suppression and the renal physiological PTH response. The reproducibility of the method and the results obtained by the CCT were satisfactorily assessed in six of the 14 normal subjects. In normal subjects B-Ca++ was raised from 1.25 ± 0.3 mmol 1−1 (mean ± SD) to 1.49 ± 0.02 mmol 1−1 suppressing S-PTH (1–84) to 264 ± 9.9% of pre-infusion levels. We applied the CCT to 10 patients with PHPT for 120 min raising B-Ca++ from 1.41 ± 0.09 mmol 1−1 to 1.69 ± 0.08 mmol 1−1, thereby suppressing S-PTH(1–84) to 47.9 ± 16.3% of pre-infusion levels. The renal handling of calcium and phosphate during CCT demonstrates the biological effects of suppressed activity of PTH on the renal tubules showing increments in the maximal tubular phosphate reabsorption in relation to the glomerular filtration rate (TmP/GFR) and decreased tubular reabsorption fraction of calcium. The described CCT is a safe and reliable dynamic test. The observed difference in sensitivity of parathyroid secretory activity to increased extracellular concentration of calcium might be useful in evaluating suspected primary hyperparathyroidism in patients with S-PTH within the formal reference range.