The use of testosterone and its pro-drugs, such as dehydroepiandrosterone (DHEA), is currently regulated in horseracing by the application of international testosterone thresholds. However, additional steroidomic approaches, such as steroid ratios, to distinguish overall adrenal stimulation from drug administrations and an equine biological passport for longitudinal steroid profiling of individual animals could be advantageous in equine doping testing. Thus, DHEA concentrations and related ratios (testosterone [T] to DHEA and DHEA to epitestosterone [E]) were assessed in the reference population by quantitative analysis of 200 post-race gelding urine samples using liquid chromatography–tandem mass spectrometry. DHEA concentrations ranged between 0.9 and 136.6 ng/mL (mean 12.8 ng/mL), T:DHEA ratios between 0.06 and 1.85 (mean 0.43), and DHEA:E ratios between 0.21 and 13.56 (mean 2.20). Based on the reference population statistical upper limits of 5.4 for T:DHEA ratio and 48.1 for DHEA:E ratio are proposed with a risk of 1 in 10 000 for a normal outlier exceeding the value. Analysis of post-administration urine samples collected following administrations of DHEA, Equi-Bolic® (a mix of DHEA and pregnenolone) and testosterone propionate to geldings showed that the upper limit for T:DHEA ratio was exceeded following testosterone propionate administration and DHEA:E ratio following DHEA administrations and thus these ratios could be used as additional biomarkers when determining the cause of an atypical testosterone concentration. Additionally, DHEA concentrations and ratios can be used as a starting point to establish reference ranges for an equine biological passport.