The Effect of Deslorelin on Stallion Hormone Levels 

A study evaluated deslorelin's effect on testosterone concentrations, follicle-stimulating hormone, and luteinizing hormone in stallions.
Holsteiner stallion, candidate for deslorelin or hCG
Deslorelin does increase testosterone, LH, and FSH, but hCG is associated with a prolonged and higher magnitude of testosterone response than either dose of deslorelin. | Getty Images

When stallions have low libido, testicular degeneration, poor semen quality, or suspected failure of testicular descent, veterinarians might reach for the long-acting gonadotropin-releasing hormone deslorelin acetate. While widely used in mare management, deslorelin is not well-studied in stallions, and its effects on the hypothalamic-pituitary-testis axis are largely unknown.  

So, Igor Canisso, DVM, MSc, PhD, Dipl. ACT, Dipl. ECAR, associate professor of theriogenology at the University of Illinois, evaluated deslorelin’s effect on testosterone concentrations, follicle-stimulating hormone (FSH), and luteinizing hormone (LH). He presented his results during the 2023 American Association of Equine Practitioners Convention Nov. 29 to Dec. 3 in San Diego.  

Study on Deslorelin and hCG for Stallions

Eleven breeding stallions randomly received one of two testosterone-stimulating protocols: deslorelin or human chorionic gonadotropin (hCG). Human chorionic gonadotropin was included because it is a well-known hormone with LH-like activity but does not stimulate LH and FSH. The stallions that received deslorelin were administered either a standard 1.8-milligram dose or a 3.6-milligram double dose. Canisso’s team collected serial blood samples and measured testosterone, FHS, and LH levels.  


“Testosterone concentrations increased 294% one hour after hCG administration, and a 608% increase was noted by 48 hours,” said Canisso.  

After low- and high-dose deslorelin, peak testosterone concentrations occurred at two hours and returned to baseline within 24 hours. The mean percent increase in testosterone was 84% after low-dose deslorelin and 168% following high-dose deslorelin.  

“So, if you’re using a testosterone-stimulating protocol in a horse with low libido and want to stimulate him before collecting, deslorelin needs at least two hours to increase testosterone levels, and hCG increases testosterone more than deslorelin,” said Canisso.  

Following deslorelin administration, both LH and FSH increased dose-dependently. Those hormone levels peaked at two hours, and both returned to baseline levels by 24 hours.  

Final Thoughts

Deslorelin does increase testosterone, LH, and FSH, but hCG is associated with a prolonged and higher magnitude of testosterone response than either dose of deslorelin.  

“Once you give a GnRH agonist like deslorelin, you are testing the pituitary response by measuring LH and FSH and the testis by measuring testosterone,” said Canisso. “Once you give hCG, you are actually only testing the testicular response.” 

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