Male hormones, collectively called androgens, are a group of steroid hormones that drive the development and maintenance of male physical traits, reproductive function, and a range of metabolic processes. Testosterone is the most abundant and well-known androgen, but it is not the only one that matters. Its more potent derivative, dihydrotestosterone (DHT), plays outsized roles in specific tissues, and even hormones not typically thought of as “male” — like estrogen and prolactin — carry out important jobs in the male body. Understanding how these hormones are made, what they do, and how their levels shift over a lifetime fills in a picture that goes well beyond the popular association of testosterone with muscles and aggression.
Testosterone and DHT Are the Two Key Androgens
Testosterone is produced mainly in the Leydig cells of the testes. The raw material is cholesterol, which gets shuttled into the mitochondria of Leydig cells and converted through a series of enzymatic steps into testosterone. The process is kicked off by luteinizing hormone (LH), a signaling hormone released by the pituitary gland in the brain. When LH binds to receptors on Leydig cells, it sets the cholesterol-to-testosterone assembly line in motion.1PubMed Central. Leydig cells: formation, function, and regulation A small amount of testosterone is also produced by the adrenal glands, which sit on top of the kidneys, but the testes are responsible for the vast majority of circulating testosterone in men.2PubMed Central. Hormonal regulation of testicular steroid and cholesterol homeostasis
Dihydrotestosterone, or DHT, is not produced independently. It is created when an enzyme called 5-alpha reductase converts testosterone into DHT inside target tissues. DHT binds to the same androgen receptor that testosterone does, but it binds more tightly and activates it more strongly, making it the more potent androgen in tissues where 5-alpha reductase is active.3DigitalCommons@TMC. The Role of 5-Alpha Reductase 3 In Steroid Metabolism Which androgen does the heavy lifting depends on where in the body you look. Testosterone is the dominant signal in skeletal muscle, while DHT takes the lead in the prostate, skin, and hair follicles.4Baillière’s Clinical Endocrinology and Metabolism. Role of 5α-reductase in health and disease
The Feedback Loop That Keeps Levels in Check
Testosterone production does not run on autopilot. It is regulated by a tightly coordinated feedback loop between the hypothalamus, the pituitary gland, and the testes, sometimes called the HPG axis. The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses, which signals the pituitary to release LH and follicle-stimulating hormone (FSH). LH stimulates the Leydig cells to produce testosterone, while FSH acts on a different cell type in the testes called Sertoli cells, which support sperm production.5PubMed. Physiology of the Hypothalamic Pituitary Gonadal Axis in the Male
When testosterone levels rise high enough, the hormone feeds back to the hypothalamus and pituitary to suppress further GnRH and LH release, dialing production down. This negative feedback is what keeps testosterone within a normal range rather than climbing indefinitely or crashing to zero.6PubMed Central. The role of testosterone, the androgen receptor, and hypothalamic-pituitary–gonadal axis in depression in ageing Men The system is sensitive enough that even the brain’s own opioid-like chemicals participate in fine-tuning LH release, adding another layer of regulation.7PubMed. Endogenous opioids participate in the regulation of the hypothalamus-pituitary-luteinizing hormone axis and testosterone’s negative feedback control of luteinizing hormone
Once testosterone enters the bloodstream, most of it does not float freely. The majority is bound to proteins, mainly sex hormone–binding globulin (SHBG) and albumin. Only a small fraction circulates unbound, and that “free” testosterone is what is most readily available to enter cells and activate androgen receptors.8PubMed. Binding of testosterone and oestradiol to sex hormone binding globulin, human serum albumin and other plasma proteins This distinction between total and free testosterone becomes clinically important when evaluating whether someone actually has a hormone deficiency, because a person can have a normal total level but low free testosterone if their SHBG is unusually high.
What Testosterone Does Across the Body
Testosterone’s influence is remarkably broad. During fetal development, it drives the formation of the male reproductive tract. Testosterone stabilizes and promotes the development of the structures that become the epididymis, vas deferens, and seminal vesicles. In peripheral tissues, its conversion to DHT drives the formation of the prostate, scrotum, and penis.9PubMed. Physiology, Testosterone At puberty, the surge in testosterone triggers the familiar cascade of changes: genital maturation, deepening of the voice, growth of body hair, increased bone density, and the linear growth spurt.10ScienceDirect. Mapping the serum testosterone landscape in boys aged 1–14 years: A population-based study
In adulthood, testosterone continues to maintain muscle mass, bone density, and red blood cell production. In skeletal muscle, it enhances protein synthesis and activates satellite cells, which are the repair-and-growth units that allow muscle fibers to get larger.11PubMed Central. Cellular and molecular mechanisms responsible for the action of testosterone on human skeletal muscle Its effect on red blood cells is substantial: testosterone stimulates the production of erythropoietin (the hormone that tells bone marrow to make more red blood cells) and suppresses hepcidin, a protein that restricts the availability of iron. The result is roughly a 7–10 percent increase in hemoglobin and hematocrit when testosterone is administered to men with low levels.12PubMed Central. Testosterone Induces Erythrocytosis via Increased Erythropoietin and Suppressed Hepcidin: Evidence for a New Erythropoietin/Hemoglobin Set Point This is also why one of the monitored side effects of testosterone therapy is polycythemia, a condition where red blood cell counts climb too high and thicken the blood.13PubMed Central. Risks of testosterone replacement therapy in men
Testosterone also plays a role in correcting certain types of anemia. Beyond simply boosting red cell numbers, it appears to improve red cell survival and promote the maturation of early-stage red blood cell precursors, which may help men whose anemia stems from chronic inflammation.14JAMA Network Open. Efficacy of Testosterone Replacement Therapy in Correcting Anemia in Men With Hypogonadism: A Randomized Clinical Trial
Where DHT Takes the Lead
While testosterone handles the heavy lifting in muscle and the reproductive system at large, DHT dominates in a few specific tissues where its stronger receptor-binding matters. The prostate gland is one such tissue. DHT drives prostate growth throughout life, which is why drugs that block the enzyme converting testosterone to DHT are a standard treatment for benign prostate enlargement.15PubMed Central. Can we predict prostate size by scoring baldness? The relationship of androgenic alopecia and lower urinary tract symptoms
The scalp is another. In men genetically susceptible to pattern baldness, DHT activity in scalp hair follicles causes those follicles to miniaturize over time: each growth cycle produces a thinner, shorter hair until eventually the follicle stops producing visible hair altogether.16PubMed Central. SRD5A2 and emerging therapies in androgen-driven disorders The connection between prostate growth and hair loss is not a coincidence — both are DHT-driven processes. That shared mechanism is also why the same class of drugs (5-alpha reductase inhibitors) can treat an enlarged prostate and slow hair loss.
An interesting wrinkle is that testosterone itself, without conversion to DHT, is sufficient to stimulate red blood cell production and alter iron metabolism. Research has shown that blocking 5-alpha reductase does not prevent the blood-boosting effects of testosterone, confirming that the erythropoiesis pathway is driven by testosterone directly rather than by DHT.17PubMed Central. Testosterone alters iron metabolism and stimulates red blood cell production independently of dihydrotestosterone
Hormones You Would Not Expect to Matter in Men
Estrogen is typically thought of as a female hormone, but men produce it too, and it turns out to be surprisingly important for male bone health. In the body, an enzyme called aromatase converts a portion of testosterone into estradiol (a form of estrogen). Case reports of men born with genetic mutations that eliminated either the estrogen receptor or aromatase function showed that these individuals developed weak bones, unfused growth plates, and accelerated bone turnover — problems you would associate with estrogen deficiency in women.18PubMed. Estrogens and bone health in men So some of the bone-protecting effects often attributed to testosterone are actually carried out by the estrogen it gets converted into.
Prolactin is another hormone worth knowing about. It is produced by the pituitary gland and is best known for its role in breast milk production in women, but men produce it as well. At normal levels, prolactin has a supportive role in male reproductive organs — prolactin receptors have been found on Leydig cells, Sertoli cells, and the lining of the ducts that transport sperm.19PubMed Central. Hyperprolactinaemia in male infertility: Clinical case scenarios However, when prolactin levels climb too high (a condition called hyperprolactinemia, often caused by a benign pituitary tumor), it suppresses the pulsatile release of GnRH, which in turn reduces LH, FSH, and testosterone production. The downstream effects can range from reduced sperm quality to complete arrest of sperm production.
The picture with prolactin is nuanced, though. In mouse studies, knocking out the prolactin receptor entirely did not significantly alter LH, FSH, or testosterone levels, and the mice remained fertile.20PubMed. Male reproductive function is not affected in prolactin receptor-deficient mice This suggests that while excess prolactin clearly disrupts the system, the hormone may not be strictly necessary for normal male reproductive function — at least in mice. In humans, the clinical evidence points more clearly to prolactin having a modulatory role that becomes a problem only when levels swing to extremes.
How Testosterone Levels Change With Age
Testosterone levels peak during the late teens to early twenties and then gradually decline with age.21PubMed. Benefits and Risks of Testosterone Treatment in Men with Age-Related Decline in Testosterone The decline is not steep — it is typically estimated at roughly one to two percent per year starting around age 30 — but it accumulates over decades. By the time a man is in his 60s or 70s, his testosterone level may be substantially lower than it was in his twenties.
The decline is not purely a clock problem. The age-related drop in testosterone is amplified by weight gain (particularly abdominal fat), chronic illness, certain medications, and genetic factors.22PubMed Central. Age-related testosterone decline: mechanisms and intervention strategies This means that two men of the same age can have very different testosterone levels depending on their overall health. A 55-year-old man who is lean and physically active may maintain testosterone levels well within the normal range, while a peer carrying significant excess weight may test well below it. The practical implication is that some of what gets labeled “age-related” testosterone decline is partly lifestyle-related and potentially modifiable.
When testosterone drops low enough to cause symptoms — reduced libido, fatigue, loss of muscle mass, depressed mood, anemia — and blood tests confirm consistently low levels, the diagnosis is hypogonadism. This can originate from problems with the testes themselves (primary hypogonadism) or from the hypothalamus and pituitary failing to send the right signals (secondary hypogonadism), or a combination of both.23PubMed Central. Diagnosis of hypogonadism: clinical assessments and laboratory tests
Effects on Mood and Cognition
The link between testosterone and the brain extends beyond the stereotypical association with aggression. Research has found that testosterone therapy can improve depressive symptoms in men with treatment-resistant depression, and it may enhance specific cognitive abilities — particularly verbal memory and spatial processing — in older or hypogonadal men.24PubMed Central. Psychiatric and Cognitive Effects of Testosterone Therapy in Adult Men: A Systematic Review of Clinical Evidence and Mechanistic Insights
Observational data supports this connection from the other direction as well. In a large cross-sectional study, older men with higher total and free testosterone levels performed better on tests of memory and processing speed compared to men with lower levels. The association held after adjusting for factors like age, education, and body weight.25PubMed Central. Low Serum Testosterone Concentrations Are Associated With Poor Cognitive Performance in Older Men but Not Women Interestingly, the same pattern was not found in women, suggesting something specific about testosterone’s role in the male brain rather than a general hormonal effect on cognition.
It is worth being careful about extrapolating from these findings. Observational studies cannot prove that low testosterone causes cognitive decline — it could be that poor health drives both lower testosterone and worse cognition. And while testosterone therapy appears to help mood and some cognitive domains in men who are genuinely deficient, it is not a cognitive enhancer for men with normal levels.
Sleep, Environment, and Other Lifestyle Factors
Day-to-day habits have a measurable impact on testosterone levels. Sleep restriction is one of the clearest examples. A study of young healthy men found that cutting sleep to under five hours per night for just one week dropped daytime testosterone levels by 10 to 15 percent, with the lowest levels occurring in the afternoon and evening. The men also reported decreased vigor and well-being.26PubMed Central. Effect of 1 week of sleep restriction on testosterone levels in young healthy men Given that many people routinely sleep fewer than six hours, this finding suggests that chronic sleep debt could be a meaningful contributor to lower-than-expected testosterone levels in otherwise healthy men.
Environmental chemicals are another concern. A class of compounds known as endocrine-disrupting chemicals (EDCs), found in some plastics, pesticides, and industrial products, can interfere with the synthesis and action of sex hormones. Long-term exposure has been linked to changes in reproductive tract development and declines in semen quality.27PubMed Central. Endocrine disrupting chemicals and impact on male reproductive health The degree to which typical environmental exposures affect testosterone levels in an individual man remains an area of active research, but the biological plausibility is well established, and population-level trends in sperm counts have drawn attention to these exposures as a public health question.
Measuring Testosterone Is Trickier Than It Sounds
If you get a testosterone blood test at a standard clinical lab, the number you receive may not be as precise as you would expect. Most routine labs use immunoassay methods, which are fast and inexpensive but have notable accuracy problems, especially at lower concentrations. One study found that a widely used commercial immunoassay returned values about 20 percent lower than those obtained by the more accurate mass spectrometry method, with poor correlation between the two below 100 ng/dL.28PubMed Central. Using mass spectrometry to overcome the longstanding inaccuracy of a commercially-available clinical testosterone immunoassay Another comparison found that a different immunoassay platform gave values that were significantly higher than the mass spectrometry reference, with greater scatter at low concentrations.29Arq Bras Endocrinol Metab. The importance of methodology in serum testosterone measurement
The direction of the error (higher or lower) depends on which assay platform a lab uses, which is why two tests from different labs can return meaningfully different numbers for the same blood sample. This matters because treatment decisions for hypogonadism often hinge on whether a man’s level falls above or below a specific threshold. If you are borderline, the lab’s choice of methodology could tip the result either way. Endocrine guidelines increasingly recommend mass spectrometry–based testing as the standard, but it is not yet universally available in every clinical lab.
Timing also matters. Testosterone follows a circadian rhythm and is typically highest in the early morning. Blood draws done later in the day will generally return lower values, and the difference can be clinically significant. For this reason, testing is usually recommended in the morning, and a diagnosis of hypogonadism should be based on at least two separate morning measurements showing low levels.
Testosterone and the Immune System
One of the more fascinating aspects of male hormones is their relationship with immune function. Research among the Tsimane, a forager-horticultural population in Bolivia, found that men with higher endogenous testosterone showed selectively reduced immune responses — specifically, lower cytokine responses following stimulation of T-cells, while responses triggered through other immune pathways were unaffected.30PubMed Central. Associations between male testosterone and immune function in a pathogenically stressed forager-horticultural population In other words, testosterone appears to dampen some branches of the immune response while leaving others intact. This selective suppression, rather than a blanket shutdown, fits with a broader evolutionary framework suggesting that high testosterone levels represent a trade-off: the hormone supports reproduction and physical performance but comes with a cost to certain immune defenses.
Research among the Shuar people of Amazonian Ecuador adds a developmental dimension to this trade-off. In adolescents (but not adults), higher testosterone was associated with a greater initial risk of infection by one species of intestinal parasite, while heavier parasite loads were in turn associated with lower testosterone, suggesting the body may dial testosterone down during serious infection to redirect resources toward fighting it off.31PubMed Central. Amazonian Shuar testosterone levels are related to parasite infection: immunocompetence handicap hypothesis implications The relationship was specific to one parasite species and one age group, so it is far from a universal rule. But the finding illustrates that testosterone’s effects reach into parts of biology that rarely make it into popular discussions of male hormones, and the trade-offs are real, context-dependent, and still being mapped out.
Testosterone Replacement Therapy in Practice
For men with diagnosed hypogonadism, testosterone replacement therapy (TRT) can provide clear benefits: improved libido, better energy levels, gains in bone density and muscle strength, correction of anemia, and sometimes improved mood.13PubMed Central. Risks of testosterone replacement therapy in men TRT comes in several delivery forms — injections, gels, patches, and implanted pellets — each with its own absorption profile and convenience trade-offs.
The monitoring side of therapy is where things require attention. Because testosterone stimulates red blood cell production, men on TRT need regular blood counts to watch for polycythemia. Fluid retention and changes in liver function markers are also tracked. TRT suppresses the HPG axis feedback loop, which means the body’s own LH and FSH production drops. For men who want to preserve fertility, this is a significant concern, because reduced FSH signaling to Sertoli cells can substantially decrease or halt sperm production. Clinicians sometimes use alternative medications that stimulate the body’s own testosterone production without shutting down the feedback loop, or they add fertility-preserving agents alongside testosterone.
An underappreciated issue in testosterone therapy discussions is the distinction between treating genuine deficiency and chasing a number on a lab report. Testosterone levels naturally vary by time of day, season, stress, sleep, and body composition. A single low reading does not necessarily mean a man is hypogonadal, and starting testosterone in someone who does not truly need it introduces risks — including cardiovascular concerns that are still being studied — without the same benefit-to-risk ratio seen in genuinely deficient men. The current medical consensus is that TRT should be reserved for men with both consistently low levels and clear symptoms, not used as a general anti-aging strategy.