The relationship between testosterone and the volume of fluid ejaculated is complex and depends entirely on whether the hormone is produced naturally within the body or introduced externally. Endogenous, or naturally produced, testosterone is necessary to initiate and maintain the creation of sperm cells. Conversely, high doses of external testosterone, such as those used in replacement therapy, typically have the opposite effect, significantly suppressing the body’s natural sperm output. Understanding this dual nature requires separating the hormone’s local action in the testes from its systemic effect on the entire reproductive system.
The Essential Role of Endogenous Testosterone
Sperm production, or spermatogenesis, is highly dependent on a specific, high concentration of testosterone within the testes. This local concentration, known as intratesticular testosterone (iTT), is maintained at levels 50 to 100 times greater than the amount circulating in the bloodstream. Specialized Leydig cells within the testes produce this testosterone under the direction of Luteinizing Hormone (LH) from the pituitary gland.
The testosterone then acts on neighboring Sertoli cells, which reside in the seminiferous tubules where sperm development occurs. These Sertoli cells possess androgen receptors that must be activated to provide the structural support and signaling required for germ cells to mature into sperm.
Why External Testosterone Lowers Sperm Production
Introducing external testosterone, such as injections or gels, triggers a negative feedback loop that shuts down the body’s natural reproductive signaling system. This suppression occurs along the Hypothalamic-Pituitary-Testicular (HPT) axis, the communication pathway between the brain and the testes. When the brain detects high levels of circulating testosterone, it interprets this as an overproduction signal.
In response, the hypothalamus and pituitary gland reduce the release of Gonadotropin-Releasing Hormone (GnRH), Luteinizing Hormone (LH), and Follicle-Stimulating Hormone (FSH). The reduction in LH causes Leydig cells to stop producing the high concentrations of iTT necessary for sperm maturation, while the drop in FSH impairs the Sertoli cells’ ability to support developing sperm. This systemic suppression halts spermatogenesis, leading to a drastically reduced sperm count or, often, a complete absence of sperm (azoospermia). The external testosterone fails to reach the necessary high concentration inside the testes to sustain production, resulting in infertility while the treatment is active. This mechanism is so reliable that it has been studied for male contraception.
Defining Sperm Volume Versus Sperm Count
It is important to differentiate between sperm volume and sperm count, as the two terms are often incorrectly used interchangeably. Sperm count refers to the total number of sperm cells present in the ejaculate, a metric directly affected by spermatogenesis in the testes. Sperm volume, however, refers to the total amount of fluid released during ejaculation.
The vast majority of the total volume—about 95%—is composed of seminal fluid, not sperm cells. This seminal fluid is secreted primarily by the seminal vesicles, which contribute 50% to 65% of the total volume, and the prostate gland, which contributes 20% to 30%. Since sperm cells account for less than 5% of the total volume, external testosterone’s severe impairment of sperm count has a less pronounced effect on total ejaculate volume.
Non-Hormonal Factors Affecting Ejaculate Volume
Since the total volume is predominantly composed of accessory gland secretions, non-hormonal factors tend to have a greater influence on the final amount of fluid. Dehydration levels can significantly affect ejaculate volume, as seminal fluid is largely water-based. Also, the frequency of ejaculation plays a major role; a longer period of latency typically correlates with a higher volume in the next release.
Physiological issues, such as obstruction of the ejaculatory ducts or seminal vesicles, can reduce volume by blocking the flow of fluid contributors. Retrograde ejaculation, where the fluid moves backward into the bladder instead of forward out of the penis, also dramatically lowers the visible volume. Lifestyle factors, including chronic stress, excessive alcohol consumption, and smoking, have been associated with a lower total ejaculate volume.