Testosterone is the dominant androgen in the human body and the most heavily mythologized hormone in modern wellness culture. It is a steroid made from cholesterol, produced mainly by the Leydig cells of the testes in men and in smaller amounts by the ovaries and adrenal cortex in women. Its job is not to make you aggressive — it is to tune a long list of tissues that evolved to behave differently in the presence of androgen.
At a glance
What it does
In men, testosterone drives the entire male developmental program — in utero genital differentiation, the puberty cascade (voice, hair, muscle, height, genital maturation), and lifelong maintenance of spermatogenesis, libido, erectile function, muscle and bone mass, and red blood cell production. Remove testosterone from an adult man and tissues depending on continuous androgen stimulation begin to atrophy within weeks to months.
In women, testosterone is not a bystander hormone. It is the most abundant active androgen in the female body and contributes to libido, muscle maintenance, energy, and mood, as well as feeding into local conversion to estradiol in peripheral tissues. Women's testosterone falls with age and crashes with bilateral oophorectomy; symptomatic low-T in women is a real condition and often missed.
Across sexes, testosterone acts on the androgen receptor (AR), which is expressed in muscle, bone, brain, skin, and many other tissues. It is also converted in target tissues to dihydrotestosterone (DHT, via 5-alpha-reductase) for stronger androgen signaling in skin, prostate, and hair follicles, or to estradiol (via aromatase) for estrogen signaling. That local conversion is why "just add testosterone" is a much messier intervention than it sounds.
How it works
Production starts in the brain. The hypothalamus releases GnRH in 60-90 minute pulses. GnRH hits the anterior pituitary, which releases LH and FSH. LH tells Leydig cells in the testes (or theca cells in the ovaries) to convert cholesterol through pregnenolone, progesterone, and androstenedione into testosterone. Free testosterone in the blood feeds back to the hypothalamus and pituitary to shut down GnRH and LH — a classic negative feedback loop.
Once in blood, testosterone travels bound. Roughly 40-60% is bound tightly to sex hormone-binding globulin (SHBG), about 38-58% is bound loosely to albumin, and only 1-3% circulates as truly free hormone. SHBG-bound testosterone is essentially unavailable to tissues; free plus albumin-bound is considered bioavailable. This is why total testosterone alone is a noisy signal — if SHBG is high (aging, alcohol, hyperthyroidism, certain meds), a "normal" total T can hide a genuinely low free T.
Inside target tissues, testosterone either binds the androgen receptor directly, gets converted to DHT by 5-alpha-reductase for more potent AR signaling, or gets aromatized to estradiol for estrogen receptor signaling. The same molecule therefore does different work in different tissues depending on which enzymes are expressed there. Block 5-alpha-reductase (finasteride, dutasteride) and you shrink scalp hair loss and benign prostate enlargement while barely touching systemic testosterone. Block aromatase (anastrozole, letrozole) and you slash estradiol while leaving testosterone intact.
Levels & ranges
Total testosterone reference ranges in healthy adult men typically run 300-1000 ng/dL, with considerable lab-to-lab variation. Free testosterone usually falls between 5-25 ng/dL or expressed as a percentage of total (roughly 1.5-2.5%). In women, total T is typically 15-70 ng/dL with free T below 1 ng/dL.
Testosterone is pulsatile and diurnal. It peaks in the early morning and dips in the late afternoon or evening, which is why lab protocols specify drawing before 10 a.m. for accurate results. A random afternoon draw can read 100-200 ng/dL lower than a morning draw in the same man on the same day. One low result is not a diagnosis — guidelines call for two confirmed morning measurements before labeling a man hypogonadal.
Healthy aging causes total T to drop roughly 1-2% per year after 30, with steeper declines when metabolic disease, obesity, poor sleep, chronic stress, or opioid use stack on top. SHBG climbs with age, so free T drops faster than total T. By 60-70, around 20% of men meet clinical hypogonadism criteria; the proportion rises further with comorbidities.
Free testosterone (Vermeulen)
Calculated via Vermeulen equation. Ranges vary by lab.
Use that as a rough intuition tool, not a diagnostic instrument. Real labs measure free testosterone by equilibrium dialysis or calculate it from total T, SHBG, and albumin using validated equations. Immunoassays for free T directly are notoriously inaccurate — if your free T number comes from an immunoassay rather than a calculated or dialysis value, treat it skeptically.
When it goes wrong
Male hypogonadism splits into primary (the testes themselves fail, LH and FSH climb) and secondary (the pituitary under-signals, LH and FSH are low or inappropriately normal). Primary causes include Klinefelter syndrome (47,XXY), testicular trauma, mumps orchitis, chemotherapy, radiation, and cryptorchidism. Secondary hypogonadism is far more common in adults and is driven by obesity, opioid use, anabolic steroid abuse, chronic illness, sleep apnea, pituitary tumors, and idiopathic hypogonadotropic hypogonadism.
Symptoms of low testosterone in men overlap heavily with depression, poor sleep, and general metabolic misery — low libido, weak or unreliable erections, fatigue, irritability, muscle loss, increased body fat, reduced bone density, and reduced cognitive drive. This overlap is why you should not diagnose yourself from a symptom list. A proper workup requires two morning total T draws, SHBG, LH, FSH, prolactin, and usually a hematocrit and PSA before starting therapy.
Excess testosterone is almost always exogenous — anabolic steroid use, overdosed TRT, or rarely androgen-secreting tumors. Supraphysiologic androgen raises hematocrit (stroke risk), suppresses endogenous production (collapsing fertility), raises estradiol via aromatization (gynecomastia), accelerates male pattern hair loss in the predisposed, and can aggravate sleep apnea. The cardiovascular picture for physiologic-range TRT in hypogonadal men looks neutral to mildly positive in the recent TRAVERSE trial; the picture for recreational supraphysiologic dosing is a different and worse story.
In women, androgen excess shows up as PCOS (the most common endocrine disorder in reproductive-age women, with hyperandrogenism, anovulation, and polycystic ovaries), congenital adrenal hyperplasia, or rarer androgen-secreting tumors. Clinical signs include hirsutism, acne, scalp hair loss, menstrual irregularity, and in severe cases virilization.
Interactions
Obesity raises aromatase activity in adipose tissue, which converts testosterone to estradiol and drags total T down. Losing significant weight in obese men reliably raises testosterone, often more than any intervention short of TRT itself. Poor sleep crashes testosterone — one week of 5-hour nights drops total T roughly 10-15% in healthy young men. Alcohol is directly testicularly toxic and raises aromatase; regular heavy drinking is one of the most underappreciated causes of low-T in otherwise healthy guys.
Opioids crash testosterone through central suppression of GnRH — chronic opioid use produces hypogonadism rates above 50%. Glucocorticoids (both endogenous in Cushing's and exogenous in chronic prednisone) do the same. Exogenous testosterone, anabolic steroids, and even high-dose DHEA suppress the HPG axis via negative feedback, which is why coming off these without proper restart protocols can leave men in prolonged suppression. Common meds that raise SHBG and therefore drop free T include certain anticonvulsants and thyroid hormone at replacement doses.
Zinc deficiency mildly impairs testosterone production; supplementing zinc in someone already replete does nothing. Vitamin D status correlates with T in observational studies but supplementation trials are mixed — correcting a real deficiency may nudge T, but it is not a lever the way sleep and body composition are.
Honest take
Most men under 50 complaining of low-T should fix sleep, body composition, and alcohol intake before considering pharmacologic treatment. The effect size of those levers in unoptimized men often matches what TRT would provide, without the lifelong dependency. That said, TRT in men with confirmed, symptomatic hypogonadism (two morning total T draws well below 300 ng/dL with consistent symptoms) is one of the most satisfying interventions in endocrinology — well-run TRT changes lives. The gray zone is men in the 300-450 range with vague symptoms. The evidence for treating them is thin, the placebo effect is large, and the fertility cost of starting TRT is real. Anyone not done having kids should see a reproductive urologist first — HCG or enclomiphene preserves testicular function in ways straight testosterone does not.
Sources
- Bhasin et al., Endocrine Society Clinical Practice Guideline (2018) — the reference standard for diagnosing and treating male hypogonadism.
- Lincoff et al., TRAVERSE trial, NEJM (2023) — the largest randomized trial on cardiovascular safety of TRT in hypogonadal men at elevated cardiac risk.
- Handelsman et al., Endocrine Reviews (2016) — a careful review of assay limitations that explains why free T measurement is so often mishandled.
- Traish et al., Journal of Andrology — on the full clinical picture of testosterone deficiency and the limits of the "just a number" approach.