Purpose <p>This study aims to explore the inverse correlation between obesity and testosterone concentrations in men, with a particular focus on the role of hemodilution in this association. The research question addressed is whether hemodilution, due to increased plasma volume (PV), contributes to the lower testosterone concentrations observed in obese men.</p> Methods <p>We conducted a retrospective analysis using three datasets with sample sizes of 86, 134, and 446 participants, respectively. Multivariable linear regression models were used to examine the changes in PV, testosterone concentration, and mass across BMI categories, adjusting for age in each dataset and in the pooled data.</p> Results <p>The study demonstrated a significant inverse correlation between BMI groups and testosterone concentrations across the three datasets, as well as in the pooled data (beta coefficients: -0.024, -0.045, -0.040, -0.043; <i>P</i> &lt; 0.001). Despite this, total testosterone mass remained stable (<i>P</i> &gt; 0.05), suggesting that hemodilution, rather than a reduction in testosterone production, may account for the lower testosterone concentrations in obesity. Adjusted testosterone concentrations for obese participants were calculated using a formula that accounts for the increased PV.</p> Conclusion <p>Our findings suggest that hemodilution, associated with increased PV in obesity, is a significant factor contributing to the lower testosterone concentrations in obese men. This has implications for the diagnosis and treatment of testosterone deficiency in obesity and underscores the need for adjusted reference ranges. Further research is necessary to validate these findings and to explore their clinical implications.</p>