The interrelationship of serum 1,25-dihydroxyvitamin D, 25-hydroxyvitamin D and 24,25-dihydroxyvitamin D in pregnancy at term: a meta-analysis.

OBJECTIVE To assess the relationship between circulating 25(OH)D, 1,25(OH)2D, and 24,25(OH)2D in pregnancy at term, and to test the hypothesis that serum 25(OH)D and 24,25(OH)2D are repressed by high serum 1,25(OH)2D levels. DESIGN Twenty studies in pregnant women at term and 14 studies in young non-pregnant women were selected from the electronic literature. From these studies the mean serum concentrations of 25(OH)D, 1,25(OH)2D, and 24,25(OH)2D from 26 groups of pregnant and 14 groups of non-pregnant young women were analyzed. The grand mean (mean of the means) of serum 25(OH)D, serum 1,25(OH)2D, and of the ratio 1,25(OH)2D/25(OH)D were compared between pregnant and non-pregnant women. Also, the means of serum 1,25(OH)2D were regressed on the means of serum 25(OH)D. The relationship between 25(OH)D and 24,25(OH)2D was also evaluated using similar methods. RESULTS The grand mean of 1,25(OH)2D was two-fold higher, and the grand mean of the ratio 1,25(OH)2D/25(OH)D was 2.5-fold higher in pregnant than in non-pregnant women with comparable serum 25(OH)D. Regression showed no significant relationship between 1,25(OH)2D and 25(OH)D in pregnant and non-pregnant women. The ratio 24,25(OH)2D/25(OH)D was not different in pregnant and non-pregnanat women. Regression showed significant relationship between 24,25(OH)2D and 25(OH)D in both pregnant and nonpregnant women. CONCLUSIONS Serum 1,25(OH)2D is not, whereas serum 24,25(OH)2D is significantly related to serum 25(OH)D in pregnant women. Serum 24,25(OH)2D seems not to be repressed by the high 1,25(OH)2D levels during pregnancy, whereas a similar conclusion for 25(OH)D is less clear.