Testosterone affects nearly every major system in your body, from how much muscle you carry to how your brain processes emotions. It’s the primary male sex hormone, but it plays essential roles in women too, with normal levels ranging from 270 to 1,070 ng/dL in men and 15 to 70 ng/dL in women. Here’s what it actually does, system by system.
Muscle Growth and Protein Synthesis
Testosterone is one of the strongest natural signals your body has for building muscle. It binds to receptors on muscle cells and triggers protein synthesis, the process by which your body repairs and enlarges muscle fibers. This doesn’t happen instantly. Research on muscle protein dynamics shows that testosterone exerts a delayed but broad stimulatory effect, boosting the production rate of the majority of individual muscle proteins over time rather than targeting just a few.
This is why testosterone levels matter so much for body composition. People with higher levels build and retain muscle more easily, while those with declining levels often notice gradual loss of strength and muscle mass, even with consistent exercise. The hormone also helps preserve muscle during periods of calorie restriction, which is one reason it’s been studied in the context of energy deficits and recovery.
Body Fat and Where It Accumulates
Testosterone’s relationship with fat is more complex than “more testosterone, less fat.” The hormone interacts with fat-burning signals differently depending on where the fat is stored. In visceral fat (the deep abdominal fat surrounding organs), testosterone promotes faster loss of lipid deposits. Men given testosterone show a selective decrease in visceral fat compared to subcutaneous fat, the type stored just under the skin.
In subcutaneous fat cells, testosterone actually slows down fat breakdown by reducing the number of receptors that respond to adrenaline-like hormones. This is why the hormone doesn’t simply melt fat everywhere equally. It reshapes where your body stores fat rather than uniformly reducing it. In women with polycystic ovarian syndrome (PCOS), elevated testosterone levels are linked to reduced fat breakdown in subcutaneous tissue, which may partly explain the body composition changes many women with PCOS experience.
Bone Density and Strength
Testosterone directly stimulates osteoblasts, the cells responsible for building new bone. It binds to receptors on these cells, promoting their growth and activity, which increases bone mineral density over time. The hormone also amplifies the number of receptors on osteoblasts, creating a feedback loop that enhances bone formation further.
There’s an indirect pathway too. Your body converts some testosterone into estrogen through an enzyme called aromatase. That estrogen then acts on bone through a separate set of receptors, adding another layer of bone protection. This dual mechanism is why testosterone loss hits bones hard. Men with low testosterone face significantly increased fracture risk, and the effect compounds with age as levels naturally decline.
Red Blood Cell Production
Testosterone stimulates your body to produce more red blood cells, a process called erythrocytosis. This is actually the most common side effect in people receiving testosterone therapy. The hormone works by stimulating erythropoietin (EPO), the kidney hormone that tells bone marrow to make more red blood cells, and by increasing iron utilization for that production. It also suppresses hepcidin, a protein that limits iron availability.
The result is higher hemoglobin and hematocrit levels, which means your blood carries more oxygen. In people who are anemic, testosterone treatment can bring hemoglobin back into the normal range. But in people who already have normal levels, the extra red blood cells can thicken the blood, which is why hematocrit monitoring is standard practice during testosterone therapy.
Sexual Function and Libido
Testosterone drives sexual desire through both the brain and the body. In the brain, it influences dopamine signaling, a neurotransmitter system that modulates sexual motivation, arousal, and the anticipation of reward. This central effect is why low testosterone so often shows up as reduced interest in sex before any physical symptoms appear.
In the body, testosterone maintains the structural health of erectile tissue and the nerves that supply it. It regulates nitric oxide production in the blood vessels of the penis, and nitric oxide is the molecule that triggers the smooth muscle relaxation needed for an erection. Animal studies show that removing testosterone causes measurable structural changes in the nerves responsible for erections, changes that reverse when testosterone is restored.
The full range of sexual symptoms tied to low testosterone includes diminished erectile quality (especially nocturnal erections), delayed or absent orgasms, decreased genital sensation, and reduced sexual pleasure overall.
Skin, Hair, and the Androgen Paradox
Testosterone’s effects on skin and hair are some of the most visible, and also the most contradictory. Androgen receptors are concentrated in sebaceous glands, hair follicles, and sweat glands. During puberty, rising testosterone triggers the growth of terminal hair on the face, chest, underarms, and pubic area. It also ramps up sebum production, the oily substance that lubricates skin, which is why acne peaks in the teenage years when testosterone surges.
The paradox is that the same hormone that grows beard hair can shrink scalp hair follicles later in life. In androgenetic alopecia (pattern baldness), high local concentrations of testosterone in the scalp suppress a signaling pathway called Wnt that hair follicles depend on for growth. When this pathway is inhibited, follicles miniaturize and eventually stop producing visible hair. Meanwhile, the sebaceous glands in those same areas enlarge, which is why balding areas of the scalp often appear oilier. Blocking testosterone’s conversion to its more potent form can enhance Wnt pathway activity and slow hair loss, which is the basis for common hair loss treatments.
Mood and Emotional Processing
Testosterone influences how your brain handles social and emotional information, particularly in the prefrontal cortex and amygdala. These two regions work together to regulate emotional reactions and social behavior. Research using brain imaging has shown that people with higher endogenous testosterone levels have stronger communication between their prefrontal cortex and amygdala, which allows for more efficient regulation of automatic emotional responses.
People with lower testosterone, by contrast, need their prefrontal cortex to work harder to override impulsive social reactions, generating stronger inhibitory signals to keep the amygdala in check. This doesn’t mean higher testosterone equals better mood. The hormone is associated with increased social approach motivation and reactive aggression, meaning it pushes people toward engagement rather than withdrawal. Low testosterone commonly shows up as fatigue, low motivation, and a flat or irritable mood, while very high levels can tip emotional regulation in the other direction.
Testosterone’s Role in Women
Women produce testosterone in smaller amounts, primarily in the ovaries and adrenal glands, with normal levels between 15 and 70 ng/dL. Despite the lower concentration, it plays critical roles. It contributes to libido, bone and muscle maintenance, energy levels, and the regulation of the menstrual cycle and fertility.
When testosterone drops too low in women, the effects mirror many of the symptoms men experience: reduced sex drive, fatigue, weakness, difficulty concentrating, and irregular periods. Low testosterone can also disrupt other hormones in a cascade effect, since it serves as a precursor for estrogen production. On the other end, excess testosterone in conditions like PCOS causes acne, excess body hair, thinning scalp hair, and changes in fat distribution, essentially amplifying the same androgen-driven effects seen in male physiology.