Yes, testosterone injections can cause a reduction in the size of the testicles. When a person receives exogenous testosterone, such as through injections for Testosterone Replacement Therapy (TRT), the body detects the high level of the hormone. This triggers a natural feedback mechanism that signals the body to stop its own internal production of testosterone. The testes, whose primary function is to produce both testosterone and sperm, become less active without normal stimulating signals from the brain, leading to a decrease in their overall size.
The Hormonal Mechanism of Testicular Size Reduction
The mechanism behind testicular size reduction involves the Hypothalamic-Pituitary-Testicular (HPT) axis, which regulates the body’s testosterone levels. The hypothalamus releases Gonadotropin-Releasing Hormone (GnRH), prompting the pituitary gland to secrete Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH).
LH travels to the testes and stimulates the Leydig cells to manufacture testosterone. FSH acts primarily on the Sertoli cells to support sperm creation, known as spermatogenesis. When exogenous testosterone enters the system, it acts as a negative feedback signal to the brain, indicating that no more stimulation is required.
This signal causes the pituitary gland to reduce or stop the secretion of both LH and FSH. Without constant LH stimulation, the Leydig cells in the testes become inactive and shrink. This reduction in Leydig cell size is the direct cause of the noticeable reduction in testicular volume, often measured as a 15% to 25% decrease after several months of continuous therapy. Testicular atrophy is a predictable response to the suppression of the HPT axis.
Impact on Sperm Production and Male Fertility
The suppression of the HPT axis by testosterone injections directly impacts reproductive function, specifically sperm production. While LH controls testosterone synthesis, FSH is primarily responsible for stimulating the Sertoli cells within the seminiferous tubules, the sites of spermatogenesis. The lack of FSH stimulation causes these sperm-producing cells to become severely inhibited.
Exogenous testosterone maintains high levels in the general circulation but does not adequately replace the extremely high concentration of testosterone naturally produced inside the testes. This high internal level is necessary for healthy sperm development. Because this internal testicular testosterone level is suppressed, spermatogenesis is dramatically compromised. The result is often a significant drop in sperm count, which can lead to temporary or prolonged male infertility.
This reduction in fertility is a common consequence of testosterone replacement. For men of reproductive age, the decision to begin therapy requires careful consideration of this potential effect. Suppression can be severe, with some men experiencing a complete absence of sperm, known as azoospermia.
Strategies for Managing Testicular Atrophy
Several medical strategies exist to counteract the suppression of the HPT axis for men concerned about testicular atrophy or fertility during testosterone therapy. One common approach involves the co-administration of Human Chorionic Gonadotropin (hCG). This medication mimics the action of Luteinizing Hormone (LH), directly stimulating the Leydig cells in the testes.
Stimulating the Leydig cells with hCG encourages the testes to maintain their function and size, preventing or reversing atrophy while the patient continues therapeutic testosterone. This strategy also helps preserve the high internal testicular testosterone levels necessary for sperm production, supporting fertility preservation.
Another class of medications, Selective Estrogen Receptor Modulators (SERMs), like Clomiphene Citrate, can also be utilized. These compounds block estrogen’s negative feedback signal to the brain, increasing the natural release of LH and FSH. For those who discontinue testosterone therapy, the suppression of the HPT axis is generally reversible, and testicular volume and function can return to normal, though this recovery may take several months.