Plasma concentrations of testosterone and nandrolone in racing and nonracing intact male horses.

Pennsylvania (PA) State Racing Commissions regulate the endogenous androgenic steroid, testosterone (TES), in racing intact males (RIM) by quantification of TES in post-race samples. Post-race plasma samples (2209) collected between March 2008 and November 2010 were analyzed for TES, nandrolone (NAN), and other anabolic steroids (ABS). Of the 2209 plasma samples, 2098 had quantifiable TES ≥ 25 pg/mL. Plasma (mean ± SD) concentrations of TES and NAN in RIM were 329.2 ± 266.4 and 96.0 ± 67.8 pg/mL, respectively. Only 64.6% of RIM had quantifiable concentration of NAN, and there was no relationship between TES and NAN. Plasma TES concentrations were significantly (P < 0.0001) higher during the months of April, May, June, July, and August. A significantly higher (P < 0.006) plasma TES was observed in Thoroughbred (TB) (347.6 ± 288.5 pg/mL) vs. that in Standardbred (STB) (315.4 ± 247.7 pg/mL). Plasma concentrations of TES from breeding stallions (BS) were 601.6 ± 356.5 pg/mL. Statistically significant (P < 0.0001) lower plasma concentrations of the two steroids were observed in RIM horses. Based on quantile distribution of TES in the RIM and BS populations, 99.5% were at or below 1546.1 and 1778.0 pg/mL, respectively. Based on this population of RIM, the suggested upper threshold plasma concentration of endogenous TES in horses competing in PA should remain at 2000 pg/mL.