Associations between thyroid hormones and sperm parameters in men undergoing fertility consultation

Thyroid hormones (THs) are critical in male reproductive physiology, but their associations with sperm parameters, particularly within the euthyroid range, remain controversial. This study aimed to investigate the relationships between free triiodothyronine (FT3), free thyroxine (FT4), and sperm quality, and evaluate the potential influence of thyroid autoimmunity (TAI).

This retrospective cohort study included 8478 men undergoing fertility consultation at the Third Affiliated Hospital of Zhengzhou University (April 2023-October 2024). Serum thyroid-stimulating hormone (TSH), FT3, FT4, thyroid antibodies, and semen parameters (volume, concentration, motility, morphology) were assessed. Participants were stratified into quartiles (Q1-Q4) based on FT3 and FT4 levels. Binary logistic and linear regression models, adjusted for potential confounding variables were used.

In non-azoospermic men, higher FT3 quartiles associated with improved sperm motility (Q4 vs. Q1: progressive motility 39.2% vs. 35.0%, p < 0.001) and reduced midpiece defects (Q4:13% vs. Q1:15%, p < 0.001). Elevated FT3 levels were associated a decreased risk of asthenozoospermia (adjusted odds ratio [OR] for Q1:1.23, p = 0.010), whereas FT4 showed only weak associations with sperm concentration (p = 0.008). Among TAI-negative individuals, the prevalence of asthenozoospermia significantly decreased in the highest FT3 quartile (Q4:34.7% vs. Q1:42.6%, p < 0.001), but this trend was not observed in TAI-positive men. Linear regression revealed dose-dependent trends between FT3 and motility parameters (straight-line velocity [VSL], average path velocity [VAP], wobble [WOB]; all p < 0.05).

Although limited by its retrospective design, which could affect causal inference between THs and sperm parameters, this study demonstrated that TH levels, particularly FT3, showed significant associations with sperm motility and morphology, even within the euthyroid range. Furthermore, TAI status may potentially modulate TH effects on semen quality by some unidentified mechanisms. These findings supported that thyroid function screening should be provided for men with infertility and could help guide personalized endocrine interventions.