Objective: To identify the normal reference ranges of serum testosterone and gonadotropins in Thai fertile men, compare these levels among different age groups, and assess their relationships with body mass index (BMI). Material and Methods: One hundred and twenty men aged 18-65 years whose wives had conceived within the past year were recruited at Songklanagarind Hospital and stratified into 3 age groups: 18-29, 30-39, and 40-65 years. Serum total testosterone (TT) was measured by electrochemiluminescence immunoassay. Bioavailable testosterone (BT) and free testosterone (FT) were calculated using a validated formula. Results: The mean serum TT levels of the 18-29, 30-39, and 40-65 age groups were 520±200, 456±160, and 478±166 nanograms per deciliter (ng/dL), respectively. There were no significant differences in TT levels among the age groups. The overall mean TT was 481±174 ng/dL and the normal TT reference range was 211-970 ng/dL. Both calculated FT and calculated BT significantly decreased in line with increased follicular stimulating hormone with increased age, while luteinizing hormone levels remained similar. BMI had a weakly negative correlation with TT levels (correlation coefficient=-0.33, p-value<0.001) and small correlations with other measures. Conclusion: Normal ranges of serum testosterone and gonadotropins determined in Thai fertile men with various ages are useful as a reference for clinical practice. Serum TT levels which are negatively correlated with BMI should be interpreted cautiously in obese men.