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) 2 D) production. However, conflicting results have been reported concerning the regulatory significance of PTH on 1,25(OH) 2 D 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) 2 D 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) 2 D. The only variable showing a significant correlation to the plasma concentration of 1,25(OH) 2 D 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) 2 D production in primary hyperparathyroidism.