Thyroid Hormones and Male Fertility: Current Understandings and Future Directions

Infertility affects approximately 15% of couples worldwide, with male factors overall contributing to nearly half of all cases. While the role of thyroid hormones (THs) in female reproduction is well established, their influence on male fertility remains less clearly defined and continues to be an area of active investigation. This narrative review aims to provide a comprehensive and updated overview of the role of THs in male reproductive physiology and to evaluate the impact of thyroid dysfunction (TD) on testicular function, spermatogenesis, and semen quality. THs may influence testicular function through effects on Sertoli and Leydig cell activity and spermatogenesis. Both hypothyroidism and hyperthyroidism have been associated with alterations in hormonal balance and semen parameters, including reduced sperm concentration, motility, and abnormal morphology. At the molecular level, TD is associated with oxidative stress, mitochondrial dysfunction, apoptosis, and increased sperm DNA fragmentation. Importantly, restoration of euthyroid status has been shown to improve hormonal balance and partially reverse seminal abnormalities, particularly sperm motility. Emerging evidence also suggests a role of thyroid autoimmunity in compromising reproductive function through immune-mediated mechanisms. TD represents a significant but often underrecognized factor in male infertility. Also, its impact extends beyond conventional semen parameters to include alterations in sperm functional integrity and genomic stability. Further large-scale prospective studies are required to establish standardized clinical guidelines and clarify the role of TD in male reproductive health. A structured literature search was performed using major databases with relevant keywords and Boolean operators.