What testosterone actually does — and what it does not.
Before deciding whether your level is a problem, it helps to know what the hormone is for. Most of what testosterone governs is not what men are told it governs.
Most men picture testosterone as a fuel gauge. More in the tank, more masculinity, energy, drive. Less in the tank, decline.
That picture is wrong in a way that matters, because almost every bad decision in this field follows from it. Testosterone is not a fuel level. It is one signal inside a regulated feedback system, converted into two other active hormones, carried by proteins that determine how much is available, and acting through receptors whose sensitivity varies between men and between tissues.
Understanding that structure is what lets you interpret your own results instead of reacting to them.
Where testosterone comes from
Roughly 95% of a man’s testosterone is produced by the Leydig cells, which sit in the connective tissue between the seminiferous tubules of the testes. The remainder comes from the adrenal glands, where it is a minor product.
Leydig cells don’t work autonomously. They produce testosterone when told to, and the instruction comes from above.
The axis
Three organs in a loop, usually called the hypothalamic-pituitary-gonadal axis — or HPG axis.
The hypothalamus releases gonadotropin-releasing hormone (GnRH) in pulses. The pulsatility matters: continuous GnRH exposure actually shuts the system down, which is why GnRH agonists are used to suppress testosterone in prostate cancer.
The pituitary responds to GnRH pulses by releasing two hormones:
- Luteinizing hormone (LH) tells the Leydig cells to make testosterone.
- Follicle-stimulating hormone (FSH) acts on the Sertoli cells, which support developing sperm.
The testes respond. Leydig cells produce testosterone; Sertoli cells support spermatogenesis.
And then the loop closes. Testosterone — and estradiol converted from it — feeds back to the hypothalamus and pituitary, reducing GnRH and LH output. Rising testosterone turns the signal down. Falling testosterone turns it up.
This is why measuring LH and FSH alongside testosterone tells you so much. High LH with low testosterone means the pituitary is shouting and the testes aren’t answering — the problem is in the testes. Low or inappropriately normal LH with low testosterone means nobody is shouting — the problem is upstream. One test, entire differential. See Why Is Testosterone Low?.
It is also why exogenous testosterone suppresses fertility. The body cannot distinguish injected testosterone from its own. It reads the elevated level, turns down LH and FSH, and the testes stop working — including the part that makes sperm. See TRT and Fertility.
Why the number changes through the day
Testosterone is secreted in pulses on a daily rhythm, peaking in the early morning and falling through the day.
The size of that fall is larger than most men expect. In a study of 66 men, those in their thirties had total testosterone 20 to 25% lower at 4pm than at 8am on the same day, and 17 of the 66 had a normal morning value alongside a value below 300 ng/dL that same afternoon. The rhythm flattens with age, to around 10% by 70.
This is the physiological reason every guideline requires morning testing. See How Testosterone Should Actually Be Tested.
What testosterone becomes
Testosterone is not only a hormone. It is also a precursor for two others, and a good deal of what testosterone “does” is actually done by its metabolites.
Testosterone → dihydrotestosterone (DHT), via the enzyme 5-alpha-reductase. DHT binds the androgen receptor more tightly than testosterone does. It drives prostate growth, facial and body hair, and male-pattern scalp hair loss in genetically predisposed men, and it was essential for genital development before birth and at puberty. Finasteride and dutasteride block this conversion.
Testosterone → estradiol, via the enzyme aromatase, which is abundant in adipose tissue. Yes: men convert testosterone into estrogen, and this is not a malfunction. Estradiol in men is essential for bone health — it is the dominant regulator of bone resorption in men, more so than testosterone itself — and contributes to libido and to feedback regulation of the axis.
This is why aggressively suppressing estradiol in men on testosterone therapy is usually a mistake rather than a refinement. See Testosterone, Estradiol and DHT.
It is also why obesity lowers testosterone: more adipose tissue means more aromatase, more conversion to estradiol, and more feedback suppression of the pituitary.
How testosterone travels
Most circulating testosterone is not available to your tissues.
- Sex hormone-binding globulin (SHBG) binds testosterone tightly and carries 40 to 60% of it. This fraction is generally considered unavailable.
- Albumin binds testosterone loosely. It dissociates readily and is usually counted as available.
- Free testosterone — genuinely unbound — is roughly 1 to 4% of the total.
So total testosterone is everything, bioavailable testosterone is free plus albumin-bound, and free testosterone is the unbound fraction.
For most men, total testosterone is an adequate proxy. It stops being adequate when SHBG is abnormal — and SHBG is abnormal in a large share of men who come asking about testosterone.
Obesity, insulin resistance, type 2 diabetes and hypothyroidism lower SHBG, which depresses total testosterone while free testosterone may be preserved. Aging, hyperthyroidism, liver disease, estrogens and anticonvulsants raise SHBG, which props up total testosterone while free testosterone may be genuinely low.
Same molecule, same amount produced, completely different reading — because of a carrier protein. See SHBG.
How testosterone acts
Testosterone works by binding the androgen receptor, a protein inside the cell that, once bound, moves to the nucleus and changes which genes are transcribed. This is why effects take time: testosterone changes what cells build, and building takes weeks to months.
Receptor sensitivity is not identical between men — it varies with androgen receptor gene polymorphisms — and it varies between tissues. Two men at the same testosterone level do not necessarily experience the same androgen effect.
This is a genuine and underappreciated source of individual variation. It is also, at present, not something we can measure usefully in the clinic. Anyone offering to assess your androgen sensitivity commercially is ahead of what the science supports.
What testosterone does
Muscle. Increases muscle protein synthesis and lean mass. Real, dose-dependent, and the effect at replacement doses is moderate — not the effect seen with supraphysiologic dosing.
Bone. Maintains bone mineral density, substantially through conversion to estradiol. Note that raising bone density did not translate into fewer fractures in the largest trial — see Testosterone, the Prostate and the Heart.
Red blood cells. Stimulates erythropoiesis, largely by suppressing hepcidin and increasing iron availability. This is why testosterone corrects some anemia, and why it causes erythrocytosis.
Sexual desire. Testosterone’s clearest and most consistent effect.
Erectile function. Supports the tissue, but erection is largely a vascular event. Testosterone helps markedly deficient men and does little for men with vascular disease. See Testosterone, Libido and Sexual Function.
Fertility. Intratesticular testosterone — roughly 80 to 100 times higher than blood levels — is required for spermatogenesis. Blood testosterone tells you nothing about it.
Body composition. Reduces fat mass and increases lean mass over months.
Mood and cognition. The relationship is weaker and more confounded than commonly presented. The European Male Ageing Study screened 32 symptoms in 3,369 men and found only three sexual symptoms independently associated with testosterone; physical and psychological symptoms tracked age and accumulating comorbidity instead.
Development. Before birth and at puberty, testosterone and DHT drive genital development, voice change, hair growth and the pubertal growth spurt. These are developmental effects. They do not reverse or re-run in adults, which is why testosterone does not change adult penis size or voice.
What testosterone doesn’t do
It does not determine personality. It does not reliably cause aggression. It does not make men more intelligent. It does not reverse aging. It does not, above the normal range, produce more of any good effect — the dose-response curve flattens, and past it the risks rise while the benefits don’t.
And it is not the explanation for most of how a man feels. That is the hardest thing on this page to accept and the most useful.
What remains uncertain
How much androgen receptor sensitivity varies between men, and whether it matters clinically. Real at a genetic level, not measurable usefully in practice.
Whether non-sexual symptoms have any direct testosterone relationship. EMAS found no independent association, which is not the same as no relationship.
What the optimal level is for any individual. Reference ranges describe populations. Nobody has established a personal optimum, and the concept may not be coherent.
Why raising bone density didn’t reduce fractures. Unexplained.
Questions patients ask
Testosterone is the male version of estrogen.
No — and men need estrogen too.
What the evidence showsBoth are steroid hormones and men produce both. Testosterone converts into estradiol via aromatase, and estradiol in men is essential for bone health, contributing to libido and to feedback regulation of the axis.
What remains uncertainThe optimal estradiol level in men is not established.
Bottom lineNot opposites. Testosterone is partly the raw material for estrogen in men.
Strong
Testosterone makes you more masculine.
It drives specific physical traits. It doesn't determine masculinity.
What the evidence showsTestosterone and DHT drive genital development, body hair, voice change and muscle mass, mostly during development. Within the normal adult range, differences in testosterone correlate poorly with the traits people mean by "masculine."
What remains uncertainHow androgen receptor sensitivity modifies individual response.
Bottom lineA hormone with specific biological functions, not a personality setting.
Moderate
More testosterone is always better.
No. The benefit curve flattens and the risk curve doesn't.
What the evidence showsAbove the normal range, additional testosterone produces diminishing returns on the effects men want while erythrocytosis, fertility suppression and cardiovascular concerns continue to accumulate. No trial evidence supports treating men above mid-normal.
What remains uncertainWhere exactly the flattening occurs, which likely varies.
Bottom lineReplacement aims for normal. "More" is a marketing position, not a clinical one.
Moderate
Testosterone falls off a cliff after 30.
It declines gradually at about 1.0 to 1.5% a year, and most older men stay in range.
What the evidence showsAround 63% of men over 70 remain within the reference interval. Longitudinal data from the European Male Ageing Study attribute the decline "overwhelmingly" to accumulating comorbidity rather than to aging itself.
What remains uncertainHow much decline is unavoidable in a man who stays genuinely healthy.
Bottom lineGradual, health-dependent, and not a cliff.
Moderate
Low testosterone is inevitable with aging.
It isn't. Most older men don't have it.
What the evidence showsAbout 63% of men over 70 remain within the reference range, and the decline that does occur tracks comorbidity more than chronology.
What remains uncertainWhether aggressive health optimization prevents the decline entirely.
Bottom lineCommon with age. Not inevitable, and not automatically a disease.
Moderate
Testosterone causes aggression.
Far weaker and more confounded than the cultural belief suggests.
What the evidence showsWithin the normal physiological range, the relationship between testosterone and aggression in men is inconsistent. Supraphysiologic doses used in anabolic steroid regimens are a different situation, with documented psychiatric effects.
What remains uncertainThe direction of causation, since behaviour and context also alter testosterone.
Bottom lineReplacement-dose testosterone is not a rage drug.
Limited
Testosterone makes men smarter.
No good evidence.
What the evidence showsCognitive symptoms were not independently associated with testosterone in the European Male Ageing Study, tracking age and comorbidity instead. Cognitive outcomes in testosterone trials have generally been unimpressive.
What remains uncertainWhether any cognitive domain benefits in markedly deficient men.
Bottom lineNot a nootropic.
Limited
Testosterone automatically increases sex drive.
It's the effect with the best evidence — in men who are actually deficient.
What the evidence showsLow sexual desire was one of three symptoms independently associated with testosterone in EMAS, and both the Testosterone Trials and TRAVERSE found significant improvement in desire.
What remains uncertainHow much desire is hormonal versus relational, psychological or medication-related in any individual.
Bottom lineThe most reliable benefit — in deficient men. Not a libido enhancer for men with normal levels.
Strong
Free testosterone is more important than total.
More informative in specific situations, not universally better.
What the evidence showsFree testosterone is roughly 1 to 4% of total and is what's immediately available to tissues. It matters most when SHBG is abnormal — obesity, aging, thyroid disease, liver disease. Societies disagree about when to measure it.
What remains uncertainThe threshold for low free testosterone is less well established than for total.
Bottom lineNot a replacement for total. Essential when SHBG is off.
Moderate
Testosterone turns into estrogen.
Yes, via aromatase — and that's supposed to happen.
What the evidence showsAromatase, abundant in adipose tissue, converts testosterone to estradiol. Estradiol is the dominant regulator of bone resorption in men and contributes to libido and axis feedback.
What remains uncertainWhat estradiol level is optimal in men on treatment.
Bottom lineNot testosterone being "wasted." It's testosterone doing a different necessary job.
Strong
DHT is just bad testosterone.
It's a more potent androgen with a different tissue profile — necessary in development, implicated in hair loss and prostate growth.
What the evidence shows5-alpha-reductase converts testosterone to DHT, which binds the androgen receptor more tightly. DHT drives prostate growth, body and facial hair, and male-pattern scalp hair loss in predisposed men, and was essential for genital development.
What remains uncertainWhether measuring DHT clinically adds anything in most men. Usually it doesn't.
Bottom lineNot bad. Different.
Strong
Men have a hormonal cycle like women.
A daily one, not a monthly one.
What the evidence showsTestosterone is secreted in pulses with a clear daily rhythm, peaking in the early morning and falling 20 to 25% by late afternoon in younger men. There is no established monthly cycle.
What remains uncertainLonger-period rhythms, if any, are poorly characterized.
Bottom lineDaily variation is large and real. Monthly cycling is not established.
Strong
Two men with the same testosterone level feel the same.
Not necessarily, and receptor sensitivity is part of why.
What the evidence showsAndrogen receptor sensitivity varies with gene polymorphisms and between tissues, so identical circulating levels don't guarantee identical androgen effect. SHBG differences also mean identical totals can reflect very different free fractions.
What remains uncertainThere is no clinically useful way to measure receptor sensitivity. Commercial offers to do so are ahead of the science.
Bottom lineReal biological variation, currently unmeasurable. Which is why symptoms matter alongside numbers.
Moderate
Testosterone only matters for muscle and sex.
It also affects bone, red blood cells, fat distribution and fertility.
What the evidence showsTestosterone maintains bone density largely through estradiol, stimulates erythropoiesis by suppressing hepcidin, influences body composition, and is required at very high intratesticular concentrations for sperm production.
What remains uncertainHow much these matter clinically at the margins of the normal range.
Bottom lineBroader than the marketing, and narrower than the mythology.
Strong
Where to go next
What counts as low: What Low Testosterone Actually Means How it should be measured: How Testosterone Should Actually Be Tested The conversion pathways in depth: Testosterone, Estradiol and DHT Individual lab values: The Lab Library
Ready for testosterone care built on a diagnosis?
The first step is a full endocrine evaluation, not a prescription. We see patients across San Diego County and welcome referrals from other physicians.