Testosterone Research: Physiology & Evidence

Summary

Testosterone is the principal androgen in men and an important hormone in women. Its age-related decline is real but modest — roughly 1% per year after 30 — and much smaller than the decline attributable to obesity and illness. Large trials have clarified both benefits and cardiovascular safety questions.

Last reviewed 2026-09-01

What it is

Testosterone is a 19-carbon steroid hormone synthesised primarily in testicular Leydig cells, and in smaller quantities by the ovaries and adrenal cortex. It circulates mostly bound to sex hormone-binding globulin and albumin, with 1–2% free.

Isolated and synthesised in 1935, work for which Butenandt and Ruzicka shared the Nobel Prize. Clinical use expanded rapidly from the 2000s, prompting regulatory concern about prescribing outside diagnosed hypogonadism.

Endogenous steroid hormone regulated by the hypothalamic-pituitary-gonadal axis.

Testosterone sits at the intersection of muscle, bone, mood, sexual function and metabolic health, which makes both genuine indications and overstated claims common.

How it works

In plain terms

The brain signals the testes to produce testosterone, and rising levels signal the brain to slow down — a thermostat. Testosterone acts on muscle, bone, brain and sexual tissue, and part of it converts to oestrogen, which matters for bone and libido.

Technical detail

GnRH released in pulses from the hypothalamus stimulates pituitary LH and FSH; LH drives Leydig cell steroidogenesis. Testosterone binds the androgen receptor, a nuclear receptor regulating transcription in muscle, bone, CNS and reproductive tissue. Peripheral conversion by 5-alpha-reductase yields dihydrotestosterone, a more potent AR agonist important in prostate and skin, while aromatase converts testosterone to oestradiol, essential for bone density, libido and adipose regulation in men. Negative feedback at hypothalamic and pituitary level means exogenous administration suppresses endogenous production and spermatogenesis.

Pathways involved

Hypothalamic-pituitary-gonadal axis and pulsatile GnRH
Androgen receptor-mediated transcription
5-alpha-reductase conversion to DHT
Aromatase conversion to oestradiol
SHBG binding and free hormone availability

Current research

Laboratory research

Androgen receptor biology and steroidogenesis pathways are fully characterised. Assay methodology matters: mass spectrometry is the reference standard and immunoassays perform poorly at low concentrations.

Animal research

Castration and replacement models established the muscle, bone and behavioural roles of androgens.

Human research

The Testosterone Trials (T-Trials, 2016) in men over 65 with low levels reported modest improvements in sexual function, mood and bone density, with no benefit for vitality or cognition. TRAVERSE (2023), a cardiovascular safety trial in over 5,000 men with hypogonadism and cardiovascular risk, found no increase in major adverse cardiovascular events but reported more atrial fibrillation, pulmonary embolism and acute kidney injury. Obesity, sleep apnoea and chronic illness lower testosterone substantially and are often reversible causes.

Ongoing research

Fertility-preserving alternatives and the long-term prostate safety question remain active areas.

What is being investigated

Sexual function and libido in men with low testosterone
Bone mineral density and fracture risk
Body composition, muscle mass and strength
Mood and depressive symptoms
Cardiovascular safety of replacement therapy

These are research directions reported in the literature, not established effects or recommendations.

Risks, limitations and unknowns

Read this section before the rest

Exogenous testosterone suppresses the HPG axis and impairs spermatogenesis, sometimes persistently
TRAVERSE reported increased atrial fibrillation, pulmonary embolism and acute kidney injury despite neutral MACE findings
Erythrocytosis is common and requires monitoring
Symptoms attributed to low testosterone frequently have other causes — obesity, poor sleep, depression, thyroid dysfunction
Diagnosis requires repeated morning measurements by a validated assay, not a single result or symptom questionnaire

Evidence ratings

Laboratory studies

Strong

Receptor and steroidogenesis biology fully characterised.

Animal studies

Strong

Classic castration/replacement models are definitive.

Human studies

Strong

T-Trials and TRAVERSE are large and well conducted.

Long-term safety

Moderate

TRAVERSE covered years, not decades; prostate questions remain.

References

[1]The Testosterone Trials — PubMed / NEJM, 2016
[2]TRAVERSE cardiovascular safety trial — PubMed / NEJM, 2023
[3]Endocrine Society testosterone guidelines — PubMed, 2018
Reference links open searches and records on PubMed and ClinicalTrials.gov so that every statement above can be traced to primary literature.

Frequently asked questions

What is testosterone?+

The principal androgen, a steroid hormone produced mainly in the testes and regulated by the hypothalamic-pituitary-gonadal axis.

Does testosterone decline with age?+

Yes, by roughly 1% per year after age 30 on average — a modest decline compared with the effect of obesity, illness or poor sleep.

What did the TRAVERSE trial find?+

No increase in major adverse cardiovascular events with testosterone replacement in men with hypogonadism, but higher rates of atrial fibrillation, pulmonary embolism and acute kidney injury.

Should testosterone be measured free or total?+

Total testosterone by mass spectrometry is the starting point; free or bioavailable testosterone matters when SHBG is abnormal, as in obesity or ageing.

Does testosterone therapy affect fertility?+

Yes. Exogenous testosterone suppresses LH and FSH, reducing sperm production, sometimes with slow or incomplete recovery.

Why does oestrogen matter in men?+

Aromatised oestradiol is essential for male bone density, libido and fat regulation, which is why aggressive aromatase suppression causes problems.

Can lifestyle raise testosterone?+

Weight loss, treating sleep apnoea and improving sleep duration have measurable effects in men where those factors are contributing.

Is testosterone the same as anabolic steroids?+

Testosterone is the reference androgen; anabolic steroids are synthetic derivatives designed to alter the anabolic-to-androgenic ratio, usually at supraphysiological doses.

What are symptoms of low testosterone?+

Reduced libido, erectile difficulty, fatigue, low mood and loss of muscle mass — all non-specific and commonly caused by other conditions.

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