Changes of Vitamin D-Binding Protein, and Total, Bioavailable, and Free 25-Hydroxyvitamin D in Transgender People

CONTEXT: Total 25-hydroxyvitamin D [25(OH)D] is mainly bound to vitamin d-binding protein (DBP). Bioavailable 25(OH)D consists of albumin-bound and free 25(OH)D and is available for metabolic processes. As sex steroids influence DBP, hormonal treatment (HT) in transgender people might affect DBP and consequently the available 25(OH)D. Total 25(OH)D might therefore not well represent bioavailable and free 25(OH)D. OBJECTIVE: To investigate the effects of HT on DBP, and total, bioavailable, and free 25(OH)D, and to assess whether total 25(OH)D well represents bioavailable and free 25(OH)D. DESIGN: A prospective study. SETTING: A university hospital. PARTICIPANTS: Twenty-nine transwomen and 30 transmen. INTERVENTION: Estradiol and cyproterone acetate in transwomen, and testosterone in transmen. MAIN OUTCOME MEASURES: DBP, total 25(OH)D, free 25(OH)D, and albumin were measured at baseline and after 3 months of HT, and deseasonalized total 25(OH)D and bioavailable 25(OH)D were calculated. RESULTS: DBP changed with +5% (95% CI, -0% to 10%; P = 0.06) in transwomen and with -3% (95% CI: -9% to 3%; P = 0.34) in transmen. No significant changes were found in total 25(OH)D, free, and bioavailable 25(OH)D concentrations. Total 25(OH)D was well correlated with bioavailable (R2, 0.75) and free (R2, 0.76) 25(OH)D. CONCLUSIONS: DBP tended to increase in transwomen, but did not change in transmen. HT did not influence free 25(OH)D, total 25(OH)D, and bioavailable 25(OH)D concentrations in transwomen and transmen. As total 25(OH)D represents bioavailable and free 25(OH)D well, HT in transgender people does not interfere with the assessment of vitamin D status.