Effects of gonadotropin-releasing hormone administration on the pituitary-gonadal axis in male and female dogs before and after gonadectomy

GnRH-stimulation tests were performed in 14 female and 14 male client-owned dogs of several breeds, before and 4 to 5 mo after gonadectomy. The aim of the study was to obtain more insight into the pituitary-gonadal axis in intact and neutered dogs and to establish reference values. Basal plasma luteinizing hormone (LH) and follicle-stimulating hormone (FSH) concentrations were increased significantly after gonadectomy in both bitches and male dogs. In both males and females ranges of the basal plasma FSH concentrations, before and after gonadectomy, did not overlap as opposed to the overlap in ranges of the basal plasma LH concentrations. Before gonadectomy basal plasma LH concentrations were lower and basal plasma FSH concentrations were higher in bitches than in male dogs. After gonadectomy these basal values did not differ significantly. GnRH administration before gonadectomy resulted in an increase in plasma LH and FSH concentrations in both genders. GnRH administration after gonadectomy produced an increase only in plasma LH concentrations in both genders, and a just significant increase in plasma FSH in castrated male dogs. GnRH administration before gonadectomy resulted in a significant increase in plasma testosterone concentration in both genders. In males ranges of basal and GnRH-stimulated plasma testosterone concentrations before and after gonadectomy did not overlap. Basal plasma estradiol concentrations were significantly higher in intact males than in castrated males and their ranges did not overlap. The basal estradiol concentrations in bitches before and after ovariectomy were not significantly different. At 120 min after GnRH administration, ranges of plasma estradiol concentration of intact and ovariectomized bitches no longer overlapped. In conclusion, basal plasma FSH concentration appears to be more reliable than basal plasma LH concentration for verification of neuter status in both male and female dogs. The basal plasma testosterone concentration appears to be reliable for verification of neuter status in male dogs. The plasma estradiol concentration at 120 min after GnRH administration can be used to discriminate between bitches with and without functional ovarian tissue.