Low Testosterone and Sperm Quality Alterations: A Prospective Study of Sperm DNA Fragmentation and Chromatin Condensation in Infertile Men

(1) Background: Testosterone plays a key role in spermatogenesis and in maintaining semen quality and sperm DNA integrity. Consequently, reduced testosterone levels may disrupt these processes and contribute to male infertility. This study aimed to evaluate the impact of low testosterone levels on semen parameters, sperm DNA fragmentation, and chromatin condensation; (2) Methods: This was a prospective study that included 214 men aged 25-45 years undergoing infertility evaluation. Participants were classified into two groups according to serum testosterone levels: low testosterone and normal testosterone. Total testosterone was determined using electrochemiluminescence immunoassay. Semen analysis was carried out according to the WHO 2021 guidelines. The DNA fragmentation index was assessed using the TUNEL assay. The sperm decondensation index was evaluated by aniline blue staining; (3) Results: Men with low serum total testosterone levels (<2.64 ng/mL) exhibited significantly impaired semen parameters compared with those with normal testosterone levels. Serum total testosterone was positively correlated with sperm concentration (rs = 0.43, <i>p</i> < 0.001), total motility (rs = 0.20, <i>p</i> = 0.005), normal morphology (rs = 0.25, <i>p</i> < 0.001), and sperm vitality (rs = 0.173, <i>p</i> = 0.014). In contrast, testosterone levels were negatively correlated with the DNA fragmentation index (rs = -0.221, <i>p</i> = 0.0017) and the chromatin decondensation index (rs = -0.19, <i>p</i> = 0.0086). A higher proportion of pathological DFI (>15%) was observed in the low testosterone group. (4) Conclusions: These findings support the essential role of testosterone in sustaining spermatogenesis, semen quality, and sperm DNA integrity and highlight the crucial importance of testosterone assessment in the diagnosis and pathophysiological understanding of male infertility.