The mean age of the study group was 51.1 ± 5.2 years, and the mean free T4 and median TSH were 1.05 ± 0.14 and 1.44 (0.96–2.13)ng/mL, respectively. In addition, the ratio of metabolic syndrome and low testosterone (<3.5 ng/mL) were 41.9% and 11.8%, respectively. There was a significant increase in the percentage of men with IPSS>7, Qmax<10 mL/sec, and prostate volume ≥30 mL, with increase of free T4 quartile (IPSS>7(%): Q1:57.2, Q2:56.7, Q3:60.3, Q4:62.5, P=.001; Qmax<10 mL/sec(%): Q1:3.5, Q2:3.2, Q3:4.1, Q4:4.8, P=.038; total prostate volume ≥30 mL(%): Q1:15.2, Q2:16.4, Q3:18.0, Q4:19.3, P=.002). After adjusting for age, body mass index, testosterone, and metabolic syndrome, the odds ratio for prostate volume ≥30 mL of free T4 Q3 and free T4 Q4 were significantly higher than free T4 Q1 [odds ratio; 5-95 percentile interval), P value; Q1:.000 (references); Q2:1.140(.911-1.361), P=.291; Q3:1.260 (1.030-1.541), P=.025; Q4:1.367(1.122-1.665), P=.002]. After adjusting for age, body mass index, testosterone, metabolic syndrome, and prostate volume, the odds ratio for IPSS>7 of free T4 Q4 were significantly higher than that of free T4 Q1 (odds ratio (5-95 percentile interval), P value; Q1:.000 (references); Q2:.969 (.836-1.123), P=.677; Q3:1.123 (.965-1.308), P=.133; Q4:1.221 (1.049-1.420), P=.010). However, the odds ratio for Qmax<10 mL/sec was not significantly different between free T4 quartile groups after adjusting confounding factors. In propensity score matched analysis (matched for age, metabolic syndrome, testosterone, and body mass index at a 1:1 ratio), 1362 cases (Q4 of free T4) and 1362 control subjects (Q1, Q2, and Q3 of free T4) were included for comparison. The ratio of prostate volume ≥30 mL (15.1% vs. 19.3%, P = .004) and mean prostate volume (23.7±6.7 vs. 24.6±7.3 mL, P=.001) was higher in the case group than in the control group. Qmax and IPSS were not different between case and control groups. TSH was not significantly related to IPSS, Qmax, and total prostate volume in univariate and multivariate analyses.