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

OBJECTIVE

A novel assay was employed to study the free 1,25-dihydroxyvitamin D (1,25(OH)2D) concentrations in various populations with different levels of 1,25(OH)2D 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 free 1,25(OH)2D, DBP and albumin were assessed.

METHODS

The total concentration of 1,25(OH)2D in serum was assessed using a radioreceptor assay. The free fraction of 1,25(OH)2D 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)2D, free 1,25(OH)2D and DBP were 132+/-19 pmol/l, 92+/-30 fmol/l and 5.59+/-0.43 micromol/l. After 3 months of estrogen/progestagen treatment, total 1,25(OH)2D and DBP levels rose significantly to 175+/-51 pmol/l and 8.32+/-1.59 micromol/l (P< or =0.05 and P< or =0.001); the free 1,25(OH)2D remained unchanged (105+/-39 fmol/l; not significant). Pregnant women had significantly higher levels of total 1,25(OH)2D and DBP (239+/-68 pmol/l and 11.32+/-1.77 micromol/l; both P</=0.001); the free 1,25(OH)2D 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 micromol/l) relative to that in controls. Despite this, levels of both total (69+/-26 pmol/l, P< or =0.001) and free 1,25(OH)2D (53+/-28 fmol/l; P< or =0.001) were significantly lower than in controls. Albumin levels were lowered from 628+/-38 micromol/l to 300+/-84 micromol/l (P < or = 0.001).

CONCLUSIONS

Higher estrogen levels result in higher DBP levels, with a parallel rise in total 1,25(OH)2D 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)2D concentration cannot be maintained.