Follicular Klotho in the Ovarian Microenvironment: Exploring Its Role in IVF Outcome Prediction

<i>Background and Objectives:</i> Klotho (KL) is a multifunctional protein involved in reproductive physiology; however, its precise role in ovarian reserve and in vitro fertilization (IVF) outcomes remains unclear. This study aimed to evaluate the relationship between follicular fluid KL levels, ovarian reserve markers, and key IVF success parameters. <i>Materials and Methods:</i> This prospective study included a total of 150 women undergoing IVF, of whom 82 had diminished ovarian reserve (DOR) and 68 had normal ovarian reserve (NOR). All participants underwent controlled ovarian stimulation using a standard antagonist protocol. During oocyte pick-up (OPU), the first aspirated follicular fluid sample was collected, processed, and analyzed for KL concentrations using a Human Klotho ELISA kit. Hormonal profiles, ovarian reserve markers, and IVF outcomes were compared between groups. <i>Results:</i> Follicular fluid KL levels were significantly lower in the DOR group compared with the NOR group (117.07 ± 28.88 pg/mL vs. 266.13 ± 58.29 pg/mL; <i>p</i> < 0.001). Anti-Müllerian hormone (AMH) levels were reduced, whereas follicle-stimulating hormone (FSH), luteinizing hormone (LH), and estradiol (E2) levels were significantly higher in the DOR group (all <i>p</i> < 0.001). Implantation and clinical pregnancy rates were also significantly lower in the DOR group compared with the NOR group (<i>p</i> < 0.001 and <i>p</i> = 0.003, respectively). KL levels showed a strong positive correlation with the number of fertilized oocytes in both groups (DOR: r = 0.690; NOR: r = 0.552). Each one-unit increase in KL was associated with a 3.7% increase in implantation probability and a 3.2% increase in clinical pregnancy probability in the DOR group, and with corresponding increases of 4.4% and 1.2% in the NOR group (all <i>p</i> < 0.05). <i>Conclusions:</i> This study demonstrates significant associations between follicular fluid KL levels and fertilization, implantation, and clinical pregnancy outcomes. These associations appear to be more pronounced than those observed with traditional ovarian reserve markers such as AMH and antral follicle count. Reduced KL levels are associated with fewer fertilized oocytes, whereas higher KL concentrations correspond to increased implantation and clinical pregnancy probabilities. Nevertheless, similar to other non-invasive biomarkers, current evidence is insufficient to support routine clinical use of KL. Large-scale, well-designed, multicenter studies are therefore required to validate its clinical relevance and to determine whether KL can serve as a reliable and practical predictor of IVF success.