The aim of this study was to determine the association between total triiodothyronine (T3), free fraction of thyroxin (FT4), and thyrotropin (TSH) levels with prostate cancer histopathological features. Blood samples from 140 patients with prostate cancer were analyzed preoperatively and stratified according to postoperative histopathological differentiation. The first group (N=62) included patients with prostate cancer Grade Groups (GG) 1-2, while the second group (N=63) included patients with prostate cancer GG 3-5. T3 levels were significantly higher in patients with prostate cancer GG 3-5 (p=0.047). There was no significant difference in the FT4 and TSH levels between the two groups (p=0.680 and 0.801, respectively). T3 levels were positively correlated with tumor percentage involvement (TPI) (p=0.002), and pT stage (p=0.047) on definitive pathology. Higher T3 levels are associated with several indicators of prostate cancer histopathological aggressiveness.