Endocrine Diagnosis of Infertility in the Stallion

April 12, 2018

Endocrine Diagnosis of Infertility in the Stallion - head shot Diagnosis of infertility in stallions usually starts with a complete reproductive history and then collection of semen to evaluate seminal parameters, testis size and the potential presence of bacteria in the semen. A more detailed evaluation might also include drawing blood for hormonal analysis.

Just like measuring a stallion’s testicles on a frequent basis can alert the stallion owner of some changes occurring in testicular function, so could the measurement of hormones in stallions on an annual basis detect potential problems. However, very few stallion managers go to the effort of obtaining hormone levels in stallions. In general, most endocrine diagnostics are only done in stallions once a problem has been identified. These reproductive problems usually include one or more of the following: a decrease in motility and/or morphology, change in testis size and /or testicular consistency, decreased sperm numbers, higher number of immature germ cell in the ejaculate and/ or dramatic change in the stallion’s behavior. These clinical findings could involve alterations in hormonal secretion from the hypothalamus, pituitary and /or testis. The endocrine lab can provide the protocol for blood collection and what hormones to measure in order to diagnosis the problem.

The Stallion's Endocrine System

Here is a brief review of the endocrine system in the stallion, that may hopefully provide a fundamental understanding for the basis of the endocrine tests described below. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in short, pulsatile bursts. In response to stimulation by GnRH, the pituitary produces luteinizing hormone (LH) and follicle stimulating hormone (FSH). LH and FSH are referred to as gonadotropins, because they act on the gonads; their release is also pulsatile in nature. LH promotes the production of testosterone (and estrogens) by the leydig cells of the testes, and in turn an increase in testosterone in peripheral blood feeds back on the hypothalamus to suppress discharge of GnRH and LH. FSH acts directly on sertoli cells in the testes, which produce inhibin and activin, they in turn feedback on the pituitary to suppress or stimulate FSH secretion. Adequate levels of testosterone and FSH must be present to stimulate Sertoli cells to produce an environment appropriate for spermatogenesis.


There are several published protocols for blood collection from stallions. Since some hormones have a diurnal secretion pattern (such as testosterone) one can collect hourly samples for 3-6 hours and combine before analysis. Alternatively, an approach that I prefer, is to collect blood samples each morning between 8-9 am for 3 days and either assay them separately or combine prior to analysis. Endocrine Diagnosis of Infertility in the Stallion - Testosterone Response_Stallion Endocrine From these samples baseline values for LH, FSH, testosterone and estradiol will be determined. An additional protocol to assess pituitary function is to inject IV 25 ug of native GnRH and collect blood at -30, 0 ,30, 60, 90 and 120 min after GnRH. Testosterone and LH are generally measured from these samples. This provides information as to how responsive the pituitary and testis are to a GnRH challenge (see chart right).

It is generally felt that most of the endocrine dysfunction is at the level of the testis and not at the hypothalamic/ pituitary level. To directly evaluate testicular function, 10,000 units of the hormone hCG is given and blood samples collected -30, 0 and every 30 min until 3 hr post-injection and testosterone measured (see chart below).


Obviously if an endocrine problem is diagnosed then the first question the client asks is "What can we do". Can we give supplemental hormones to stallions like we give to mares? The answer is yes but the treatments that have been used are not based on as much research as we have in mares. It is difficult to get a group of stallions all having the same endocrine dysfunction where we can assign some to treatments and some to controls. Thus, most of the reports are based more on clinical impressions i.e. "we gave the treatment and it seemed to work".

Giving GnRH to stallions is quite controversial. Most people feel that since it is not a hypothalamic / pituitary problem then why give GnRH. However, we do know that GnRH given prior to a semen collection attempt can increase libido. In a clinical case that I assisted with last year, daily administration of GnRH (50ug) improved semen quality and testicular size dramatically. This stallion had normal LH and FSH but low inhibin, testosterone and estradiol. Motility was near zero at the start of the treatment but after several weeks increased to 50-60 %. This is not to suggest that GnRH treatment will cure every problem and should only be used if one is willing to monitor semen parameters and hormonal levels.

Other therapies have included the use of injectable testosterone. This has been used to increase sexual behavior in " slow working Stallions" or in stallions were testicular function was compromised. However, there is a big word of caution when using injectable testosterone. We know from studies conducted at Colorado State University that giving large amounts of testosterone every other day Endocrine Diagnosis of Infertility in the Stallion - Testosterone Response to hCG - Stallion Endocrine for 150 days resulting in a dramatic drop in LH levels and testes size decreased from 110 mm to 80 mm by 150 days. This was a result of low LH levels. Thus, when using injectable testosterone, blood samples should be taken frequently and LH and testosterone monitored. A decease in LH would indicate that the level of testosterone injected is too high. It is best to use testosterone for only a short period of time.


Lastly, as a take home message, with testicular dysfunction the first hormones to be altered are inhibin, estradiol and FSH. Inhibin and estradiol are lower in these stallions and FSH is increased. LH and testosterone levels are the last to change in stallions with testicular dysfunction.

See Also: Measuring Hormone Levels in Mares