BDNF and polycystic ovary syndrome: In this context, several studies have been published on the relationship between vitamin D and ovarian reserve markers, BDNF, and neurotrophic factors [14,15], and there have also been many clinical reports that vitamin D deficiency is associated with reproductive hormonal decline or imbalances such as menopause, polycystic ovarian syndrome (PCOS), primary ovarian insufficiency (POI), and depression in women [16,17,18,19].