title: Female sex hormone replacement therapy increases serum free 1,25-dihydroxyvitamin D3: a 1-year prospective study.
abstract: OBJECTIVE
  Currently, hormone replacement therapy is applied successfully to reduce post-menopausal bone resorption. However, the exact mechanism by which oestrogen exerts its effect has not yet been fully elucidated. In order to determine whether changes in the biologically active 1,25-dihydroxyvitamin D3 may be of importance in this process, the concentrations of both total and free 1,25-dihydroxyvitamin D3 in serum were assessed.
  
  
  METHODS
  In 36 post-menopausal women the effect of hormone replacement therapy, with a combination of 17 beta-oestradiol and norethisterone acetate, on the serum levels of total and free 1,25-dihydroxyvitamin D3 was studied after 0, 3, 6 and 12 cycles.
  
  
  METHODS
  The total concentration of 1,25-dihydroxyvitamin D3 in serum was assessed using a radioreceptor assay after diethylether extraction of the samples followed by paper chromatography. The free fraction of 1,25-dihydroxyvitamin D3 was measured using symmetric dialysis. The free 1,25 dihydroxyvitamin D3 concentration was calculated by multiplying the total concentration by the free fraction.
  
  
  RESULTS
  During therapy, mean serum total 1,25-dihydroxyvitamin D3 concentrations (+/- SD) were 106.4 pmol/l (+/- 27.5), 155.0 pmol/l (+/- 49.5), 176.7 pmol/l (+/- 70.0) and 161.1 pmol/l (+/- 55.3) at 0, 3, 6 and 12 cycles, respectively. Serum free 1,25-dihydroxyvitamin D3 concentrations were 68 fmol/l (+/- 22), 107 fmol/l (+/- 35), 120 fmol/l (+/- 43) and 108 fmol/l (+/- 37), respectively. Baseline values of both total and free 1,25-dihydroxyvitamin D3 were significantly lower than those during therapy at all time (P < or = 0.001).
  
  
  CONCLUSIONS
  Both the serum total 1,25-dihydroxyvitamin D3 and the serum free 1,25-dihydroxyvitamin D3 concentrations are increased during combined 17 beta-oestradiol and norethisterone acetate therapy for a year. Assuming that the free concentration 1,25-dihydroxyvitamin D3 reflects the biologically active fraction, this rise may in part explain the preventive effect of hormone replacement therapy on osteoporosis.
authors.affiliation: Department of Chemical Endocrinology, University Hospital Nijmegen St Radboud, The Netherlands.
authors.forename: H J
authors.initials: HJ
authors.lastname: van Hoof
authors.forename: M J
authors.initials: MJ
authors.lastname: van der Mooren
authors.forename: L M
authors.initials: LM
authors.lastname: Swinkels
authors.forename: C G
authors.initials: CG
authors.lastname: Sweep
authors.forename: J M
authors.initials: JM
authors.lastname: Merkus
authors.forename: T J
authors.initials: TJ
authors.lastname: Benraad
mesh_terms: D002117:Calcitriol / Q000097:blood*; D004359:Drug Therapy, Combination; D004958:Estradiol / Q000627:therapeutic use; D015914:Estrogen Replacement Therapy; D005260:Female; D006801:Humans; D008875:Middle Aged; D009640:Norethindrone / Q000031:analogs & derivatives / Q000627:therapeutic use; D000077563:Norethindrone Acetate; D015663:Osteoporosis, Postmenopausal / Q000097:blood / Q000517:prevention & control*; D017698:Postmenopause / Q000097:blood*
chemical_list: D004958:Estradiol; D000077563:Norethindrone Acetate; D002117:Calcitriol; D009640:Norethindrone