Endocrine and metabolic profile of peripubertal Standardbred colts

The objectives of this study were to determine the concentrations of reproductive and metabolic hormones during the peripubertal period and to assess their relationship with testicular development and body fat deposition. Blood samples were collected from 23 healthy Standardbred colts every four weeks for twelve months. Colts were weighed monthly, and percent of body fat and testicular volume estimated by ultrasound. Onset of puberty was determined as the month when testosterone was two standard deviations above the previous mean. Plasma FSH, LH, leptin, estradiol-17β, androstenedione, IGF-1, insulin, inhibin-A, and inhibin-B were analyzed for a seven month peripubertal period. Spring born Standardbred colts underwent puberty at 13 months of age; onset of puberty coincided with exponential testicular growth but did not coincide with an increase in cutaneous body fat deposition or leptin (p > 0.05). Plasma inhibin-B concentrations were significantly increased in the postpubertal period (p < 0.05), but no increase was seen in inhibin-A, androstenedione, FSH, LH, or estradiol-17β. In conclusion, the rise in testosterone and subsequent onset of puberty coincides with rapid testicular growth but is not correlated with an increase in gonadotropins, IGF-1, cutaneous body fat or leptin in the horse.