Relationship between free and total 1,25-dihydroxyvitamin D in conditions of modified binding

Objective: A novel assay was employed to study the Gee 1,5-dihydroxyvitamin D (1,25(OH)(2)D) concentrations in various populations with different levels of 1,25(OH)(2)D and vitamin D binding protein (DBP), Design: In 12 healthy women before and after 3 months of oral estrogen/progestagen treatment. 10 pregnant women, and 16 patients with a nephrotic syndrome and normal renal function, the concentrations of total and Gee 1,25(OH)(2)D, DBP and albumin were assessed. Methods: The total concentration of 1,25(OH)(2)D in serum was assessed using a radioreceptor assay. The free fraction of 1,25(OH)(2)D was measured using symmetric dialysis. DBP was assessed using single radial immunodiffusion. Serum albumin concentrations were measured on an automated analyzer. Results: In healthy women, the concentrations of total 1.25(OH)(2)D, free 1,25(OH)(2)D and DBP were 132 +/- 19 pmol/l, 92 +/- 30 fmol/l and 5.59 +/- 0.43 mu mol/l. After 3 months of estrogen/progestagen treatment, total 1,25(OH)(2)D and DBP levels rose significantly to 175 +/- 51 pmol/l and 8.32 +/- 1.59 mu mol/l (P less than or equal to 0.05 and P less than or equal to 0.001); the free 1,25(OH)(2)D remained unchanged (105 +/- 39 fmol/l; not significant). Pregnant women had significantly higher levels of total 1,25(OH)(2)LD and DBP (239 +/- 68 pmol/l and 11.32 +/- 1.77 mu mol/l; both P less than or equal to 0.001); the free 1.25(OH)(2)D level, however, was not different (104 +/- 27 fmol/l; not significant), Unexpectedly, in patients with nephrotic syndrome, no lower DBP levels were found (5.36 +/- 0.84 mu mol/l) relative to that in controls. Despite this, Levels of both total (69 +/- 26 pmol/l, P less than or equal to 0.001) and free 1.25(OH)(2)D (53 +/- 28 fmol/l; P less than or equal to 0.001) were significantly lower than in controls. Albumin levels were lowered from 628 +/- 38 mu mol/l to 300 +/- 84 mu mol/l (P less than or equal to 0.001), Conclusions: Higher estrogen levels result in higher DBP levels, with a parallel rise in total 1,25(OH)(2)D levels but without a change in the biologically active free fraction, The results in patients with nephrotic syndrome show that, with increasing glomerular protein leakage, the free 1,25(OH)(2)D concentration cannot be maintained.