The relationship between endogenous oestradiol and vitamin D₃ metabolites in serum and follicular fluid during ovarian stimulation for in-vitro fertilization and embryo transfer

The present study was undertaken to examine the effect of circulating oestradiol on serum levels of 25-hydroxyvitamin D₃ (25-OHD3), 24,25-dihydroxyvitamin D₃[24, 25-(OH)₂D₃], and 1,25-dihydroxyvitamin D₃ [1,25-(OH)₂D₃] during gonadotrophin-induced ovarian stimulation in 10 healthy women undergoing in-vitro fertilization and embryo transfer (IVF). The presence of these metabolites in the follicular fluid was also investigated. Plasma oestradiol increased from 25 ± 3.2 (mean ± SE) pg/ml before initiation of treatment to 2563 ±328 pg/ml on the day of injection of human chorionic gonadotrophin (HCG) and 1641 ± 299 pg/ml on the day of ovum retrieval (P < 0.01). Serum levels of 1,25-(OH)₂D₃ increased from 32.0 ± 1.9 (mean ± SE) pg/ml to 46.6 ± 8.1 and 48.5 ± 7.7 pg/ml (P < 0.05) on the day of HCG and ovum retrieval, respectively. No changes in blood levels of 25-OHD3 and 24,25-(OH)2D3 were found. The presence of vitamin D metabolites in follicular fluid is documented herein for the first time. All three metabolites were present in the follicular fluid but were significantly lower than in the concurrent serum (P < 0.01). A highly significant correlation was found between serum and follicular fluid levels: r = 0.787, P < 0.001 for 1,25-(OH)₂D₃; r = 0.738, P < 0.01 for 25-OHD₃; and r = 0.751, P < 0.01 for 24, 25-(OH)₂D₃. Our results suggest that raised levels of circulating oestradiol during gonadotrophin-induced ovarian stimulation are associated with a significant increase of serum 1,25-(OH)₂D₃.