Testosterone Map: Where It’s Made and What It Does
Understand testosterone's systemic role in the body. This guide traces its biological pathway, key functions, and the intricate systems that regulate it.
Testosterone is an androgen, a type of hormone that contributes to growth and reproduction in both men and women. In men, testosterone is produced primarily in the testes and is responsible for the development of male secondary sexual characteristics. In women, it is produced in the ovaries and adrenal glands, although in much smaller amounts. This hormone influences a wide range of bodily functions, from muscle mass and bone density to mood and energy levels.
Where Testosterone is Made
In males, about 95% of testosterone is synthesized in the testes by specialized endocrine cells known as Leydig cells. These cells are located in the tissue between the sperm-producing tubules, and their production is directed by signals from the brain.
In females, testosterone originates from two main sources: the ovaries and the adrenal glands. The ovaries synthesize androgens, including testosterone, as precursors for estrogen. The adrenal glands, situated on top of the kidneys, also contribute to the testosterone pool in both sexes by producing androgens that can be converted elsewhere in the body.
How Testosterone Travels and Transforms
Once produced, testosterone enters the bloodstream to travel to various tissues. Most testosterone, around 98%, is bound to transport proteins. The two main proteins for this are sex hormone-binding globulin (SHBG) and albumin, with about 65% of testosterone tightly bound to SHBG and a smaller portion attached to albumin.
The small fraction not bound to proteins is known as free testosterone. This unbound testosterone, along with the portion bound to albumin, is considered bioavailable, meaning it can easily enter cells and exert its effects. Testosterone attached to SHBG is unavailable to tissues, acting as a reservoir for the hormone.
Testosterone does not always bind to a receptor in its original form. In certain target tissues, it transforms into other active hormones, such as dihydrotestosterone (DHT) via the enzyme 5-alpha reductase. DHT is a more potent androgen important for male external genitalia and hair growth. In other tissues, testosterone can be converted into estradiol, a form of estrogen, through a process called aromatization.
Testosterone’s Impact Zones
Testosterone’s influence extends to many areas of the body where it binds to androgen receptors inside cells. This binding triggers changes in gene expression, leading to a wide array of physiological effects. These effects depend on the tissue and whether testosterone acts directly or is converted into DHT or estradiol.
In skeletal muscle, testosterone promotes growth and strength by stimulating protein synthesis, which increases muscle mass. For the skeletal system, testosterone contributes to bone density and maturation. It helps stimulate growth during puberty and maintain bone strength throughout adulthood.
The brain is another significant target for testosterone. It can influence mood, cognitive function, and libido in both sexes. In the skin, testosterone affects hair growth, contributing to patterns of body and facial hair. It also stimulates sebaceous glands, which can lead to increased oil production and acne.
In the reproductive system, testosterone’s effects are extensive. In males, it drives the development of primary sexual characteristics during fetal development and is necessary for spermatogenesis, or sperm production. It also causes changes during puberty, such as a deepening voice. In females, androgens like testosterone are involved in ovarian function.
Navigating Testosterone Levels
The body maintains a balance of testosterone through the hypothalamic-pituitary-gonadal (HPG) axis. This system involves the hypothalamus and pituitary gland in the brain, and the gonads (testes in males, ovaries in females). The process begins when the hypothalamus releases gonadotropin-releasing hormone (GnRH).
The release of GnRH signals the pituitary gland to secrete luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH is the primary signal that stimulates the gonads to produce and release testosterone. In males, FSH supports the process of sperm production in the testes.
This system operates on a negative feedback loop. When testosterone levels in the blood rise, it signals the hypothalamus and pituitary gland to reduce their production of GnRH and LH. This decreases stimulation of the gonads, leading to lower testosterone production and ensuring levels remain stable.
Several factors can influence this regulatory system. Age is a significant factor, with levels naturally declining in men as they get older. The time of day also affects testosterone, with levels being highest in the morning. Overall health and lifestyle can also impact the functioning of the HPG axis.