Does the plasma concentration of 25-hydroxyvitamin D determine the level of 1,25-dihydroxyvitamin D in primary hyperparathyroidism?

In man, parathyroid hormone (PTH) is one of the main regulators of 1,25-dihydroxyvitamin D (1,25(OH)2D) production. However, conflicting results have been reported concerning the regulatory significance of PTH on 1,25(OH)2D in primary hyperparathyroidism. We measured the plasma concentrations of intact PTH, ionized calcium (pH 7.4), phosphate, creatinine, 25-hydroxyvitamin D (25(OH)D), 1,25(OH)2D and urine creatinine in a standardised regime in 17 patients with primary hyperparathyroidism. The nephrogeneous cyclic adenosine-3,5-monophosphate (NcAMP) was measured and so provides an "in vivo receptor assay' for biologically active PTH in the circulation. Multiple regression analysis was used to identify variables with a possible influence on the plasma concentration of 1,25(OH)2D. The only variable showing a significant correlation to the plasma concentration of 1,25(OH)2D was 25(OH)D (r=0.6, p < 0.03). Our results indicate that the plasma concentration of 25(OH)D may be very central for the regulation of 1,25(OH)2D production in primary hyperparathyroidism.