Testosterone is a hormone that works by entering your cells, binding to receptors inside them, and switching specific genes on or off. That single mechanism drives a surprisingly wide range of effects, from building muscle and bone to influencing mood, fat storage, and red blood cell production. Understanding how testosterone actually operates in the body means following it from where it’s made, to how it’s regulated, to what it does once it reaches your tissues.
How Your Body Produces Testosterone
Testosterone production starts in the brain, not the testes. A region called the hypothalamus releases a signaling hormone (GnRH) in small, rhythmic pulses into the bloodstream. These pulses travel a short distance to the pituitary gland, which responds by releasing two more hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
LH is the direct trigger for testosterone production. It travels through the bloodstream to specialized cells in the testes called Leydig cells, which manufacture testosterone and release it into circulation. FSH, meanwhile, works alongside testosterone to support sperm production. Women also produce testosterone, primarily in the ovaries and adrenal glands, though at much lower levels.
This entire chain operates on a feedback loop. When testosterone levels in the blood rise high enough, the hypothalamus and pituitary detect the increase and dial back their signaling. When levels drop, they ramp it up again. This is why the system is called the hypothalamic-pituitary-gonadal axis: three organs working in a continuous conversation to keep testosterone within a functional range.
What Happens Inside Your Cells
Once testosterone enters the bloodstream, most of it binds to carrier proteins that shuttle it around the body. The small fraction that’s “free” (unbound) can pass through cell membranes and enter tissues directly. What happens next is the core of how testosterone works.
Inside a cell, testosterone finds an androgen receptor sitting in the fluid-filled space outside the nucleus. That receptor is essentially dormant, held in place by protective chaperone proteins. When testosterone binds to it, the receptor changes shape, sheds those chaperone proteins, and exposes a signal that acts like a nuclear entry pass. The testosterone-receptor pair then moves into the cell’s nucleus, pairs up with a second copy of itself, and latches onto specific stretches of DNA. This turns particular genes on or off, changing what proteins the cell produces.
This process explains why testosterone’s effects are tissue-specific. Muscle cells, bone cells, brain cells, and fat cells all have androgen receptors, but each cell type has different genes available to be activated. The same hormone triggers muscle growth in one tissue and red blood cell production in another.
Conversion to Other Hormones
Testosterone doesn’t always act as testosterone. Your body converts roughly 10% of its daily testosterone into a more potent form called DHT (dihydrotestosterone) using an enzyme called 5-alpha reductase. DHT binds to the same androgen receptors but with greater strength, and it drives effects in specific tissues like the prostate, skin, and hair follicles. This is why DHT is closely linked to both male-pattern hair loss and prostate growth.
A separate enzyme called aromatase converts some testosterone into estradiol, a form of estrogen. This isn’t a malfunction. Estradiol converted from testosterone plays important roles in men, particularly in bone density maintenance and in regulating the feedback loop that controls testosterone production itself. Research in the Journal of Clinical Investigation has shown that this conversion to estradiol may actually be required for testosterone to properly signal the hypothalamus and pituitary to regulate its own production.
How Testosterone Builds Muscle
Testosterone’s muscle-building effects work through several overlapping pathways. The most direct: when testosterone activates androgen receptors in muscle tissue, it triggers a chain of molecular events that promotes the growth of both type 1 (endurance) and type 2 (power) muscle fibers. It increases the number of muscle progenitor cells by steering stem-like cells toward becoming muscle rather than fat.
At the same time, testosterone stimulates the rapid multiplication of muscle-building cells by boosting the production of compounds called polyamines. It also increases the activity of growth hormone and IGF-1 (a growth factor) both in the bloodstream and within muscle tissue itself, further accelerating protein synthesis. On the defensive side, testosterone suppresses genes that would otherwise break muscle tissue down. The net result is that higher testosterone levels shift the balance toward building and maintaining muscle mass, while lower levels make it harder to hold onto muscle even with the same exercise routine.
Effects on Red Blood Cells
Testosterone stimulates the production of red blood cells through at least two mechanisms. First, it increases levels of erythropoietin (EPO), a hormone produced by the kidneys that tells bone marrow to make more red blood cells. In clinical studies, testosterone administration raised EPO levels by about 58% within the first month, and this increase persisted at three months.
More interestingly, testosterone appears to reset the body’s threshold for EPO production. Normally, as red blood cell counts rise, the kidneys produce less EPO to prevent overproduction. After several months of elevated testosterone, researchers found that at any given level of red blood cells, EPO levels were about 30% higher than they had been before. Testosterone may also heighten bone marrow’s sensitivity to EPO, so each unit of EPO produces a stronger response. This is why men on testosterone therapy sometimes develop elevated red blood cell counts, a side effect that requires monitoring.
Fat Distribution and Metabolism
Testosterone has a region-specific effect on body fat. In abdominal fat tissue, testosterone inhibits the uptake of fat into storage, reduces the activity of enzymes that help fat cells absorb circulating fats, and increases the rate at which stored fat is broken down and released. These effects are notably absent in fat tissue around the thighs and hips, which is why testosterone levels strongly influence where your body stores fat rather than just how much.
This selectivity explains a well-documented clinical pattern: men with declining testosterone tend to accumulate fat around the abdomen, while men with higher levels carry proportionally less visceral fat. The relationship runs in both directions, since excess body fat contains more aromatase, which converts more testosterone to estrogen, further lowering available testosterone.
Brain, Mood, and Cognition
Androgen receptors are widely distributed across the brain, with particularly high concentrations in the hippocampus (involved in memory) and the prefrontal cortex (involved in decision-making and attention). Testosterone’s effects on these regions follow the same receptor-binding mechanism it uses elsewhere in the body, activating genes that influence how brain cells communicate and survive.
Men with low testosterone levels show a measurably higher risk of cognitive problems, particularly in spatial memory and sustained attention. In small clinical studies, testosterone supplementation in older men with both low levels and existing cognitive decline led to improvements in verbal memory, global cognition, and depression scores. One study of 106 men with testosterone deficiency found that after eight months of treatment, depression scores dropped significantly while cognitive function scores rose. These findings don’t mean testosterone is a cognitive enhancer in people with normal levels, but they do illustrate how the hormone supports baseline brain function.
Normal Levels and Age-Related Decline
In healthy, non-obese men aged 19 to 39, total testosterone typically falls between 264 and 916 ng/dL, with a midpoint around 531 ng/dL. These numbers come from harmonized data across four large cohort studies in the U.S. and Europe. Levels naturally vary throughout the day (peaking in the morning), between individuals, and across body compositions. Obesity significantly lowers the range: when all men are included regardless of weight, the 2.5th percentile drops from 267 to 229 ng/dL in the same age group.
After about age 40, total testosterone declines at an average rate of 0.4% per year, while free testosterone (the unbound, active fraction) drops faster at about 1.3% per year. This difference matters because as men age, more testosterone gets bound to carrier proteins, so the total number can look relatively stable while the amount actually available to enter cells shrinks more noticeably. By ages 70 to 79, the median total testosterone in non-obese men sits around 477 ng/dL, only modestly lower than the younger median, but free testosterone tells a steeper story.
The decline isn’t purely biological. Weight gain, poor sleep, chronic stress, and certain medications all accelerate it. This means some of the testosterone drop commonly attributed to aging is actually driven by lifestyle factors that are, at least partially, modifiable.