A recent study showed that paternal subclinical hypothyroidism adversely affects the clinical outcomes of assisted reproductive technologies (ARTs). The aim of this study was to determine whether paternal serum free thyroxine (FT4) concentrations within the reference range are associated with ART outcomes.This retrospective cohort study included 4,066 couples who received 4,894 ART treatment cycles in our clinic between April 1, 2016 and August 31, 2021. The differences in sperm parameters and ART outcomes across the paternal FT4 concentration tertiles were compared using generalized linear models or generalized estimation equation models. The primary outcomes were clinical pregnancy rate (CPR) and live birth rate (LBR) per oocyte retrieval following the first embryo transfer cycle.The mean ages of the males and their female partners were 32.8 (standard deviation, 5.0) and 30.7 (standard deviation, 4.1) years, respectively. No significant differences were observed in the sperm parameters or ART outcomes between the paternal FT4 concentration tertiles of the overall population. However, a stratified analysis of men aged ≥ 35 showed an adjusted CPR of 0.36 (95% confidence interval [CI]: 0.27-0.45) for the lower paternal FT4 concentration tertile relative to the middle (adjusted rate: 0.45, 95% CI: 0.38-0.53) and upper (adjusted rate: 0.43, 95% CI: 0.36-0.51) tertiles (p for trend > 0.05). The adjusted LBRs were 0.21 (95% CI: 0.15-0.30) for men aged ≥ 35 in the lower FT4 concentration tertile (p = 0.024, with reference to the upper tertile), 0.27 (95% CI: 0.21-0.35) for those in the middle tertile, and 0.30 (95% CI: 0.23-0.38) for those in the upper tertile. No differences in these outcomes were observed in men aged < 35. The nonlinear smoothing curve obtained using FT4 concentration as a continuous variable further supported these findings.Men of older reproductive age (≥ 35 years old) with low-normal FT4 concentrations within the reference range are associated with a decreased LBR. Future prospective studies are warranted to confirm the detrimental effects of low-normal paternal FT4 concentrations on ART outcomes.