CLINICAL STUDY 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 ^ 0:43mmol=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