Session: OR45-What Is New in Vitamin D?
Room 29 (San Diego Convention Center)
Comparison of bioavailable 25-hydroxy vitamin D (25OH-D) and bioavailable calcitriol (1,25 dihydroxy vitamin D) with PTH and CTX in pregnancy serum.
Pregnancy is associated with major changes in calcium homeostasis. Vitamin D and PTH play important roles during pregnancy. Currently, serum 25OH-D is used in monitoring vitamin D status during pregnancy. Calcitriol is the active form of vitamin D. The level of 25OH-D and calcitriol vary due to changes in maternal serum vitamin D binding protein(DBP). Serum concentrations of bioavailable and free vitamin D are not influenced by DBP. Therefore, we measured total, bioavailable, and free fractions of 25OH-D or calcitriol in pregnant serum samples and correlated them with serum calcium, PTH and CTX (C-terminal collagen degradation product) .
Bioavailable 25OH-D or bioavailable calcitriol are fractions not bound to DBP. They are the combined fractions of albumin bound and the free fractions of 25OH-D or calcitriol. To obtain the bioavailable fraction, total vitamin D (25OH-D or calcitriol) was quantitated using immunoassays (IDS reagents ). DBP was quantitated by an ELISA using reagents from R&D systems. Using the affinity constants of 25OH-D and calcitriol for DBP, and the affinity constants of 25OH-D or calcitriol for serum albumin, bioavailable 25OH-D, bioavailable calcitriol, free 25OH-D and free calcitriol were calculated. PTH and CTX assays were performed in pregnancy serum samples using IDS kits. Pregnant serum samples (n=51) were collected between 27 to 38 weeks of pregnancy.
Total 25OH-D was significantly lower in pregnant women( 24.7 ± 1.1 ng/ml) despite a significant increase in DBP (410 ± 30 ug/ml vs 276 ± 15 in non-pregnant serum). Bioavailable and free 25OH-D levels during pregnancy were lower than non-pregnant women(n=53)although the levels of PTH and CTX were in the normal range.The correlation between PTH with total 25OH-D was poor (r2= 0.3).There was also poor correlation between PTH and bioavailable or free 25OH-D (r2= <0.5). Calcitriol was high in the pregnancy samples (127.5 ± 15.5 pg/ml) compared to non-pregnant samples (36.2 ± 5.6 pg/ml), and the DBP-corrected bioavailable and free calcitriol was twofold higher than non-pregnant controls. Calcitriol and its fractions (bioavailable and free of calcitriol) correlated well with serum PTH and CTX (r2 = >0.9).
The current practice of assessing vitamin D in pregnancy consists of measurement of 25OH-D. However, 25OH-D as an indicator of sufficiency does not correlate well with calcium homeostasis markers in pregnancy. Calcitriol correlates better with PTH and CTX in pregnancy. It appears that bioavailable and free fractions of calcitriol are the best markers for determining vitamin D status in pregnant women.
Nothing to Disclose: MRP, JP, ZS, ANE
*Please take note of The Endocrine Society's News Embargo Policy at https://www.endocrine.org/news-room/endo-annual-meeting