Testosterone

Testosterone is the primary male sex hormone, an androgen produced mainly in the testes that regulates libido, muscle mass, bone density, red blood cell production, and mood.

Testosterone is the primary androgen in men, produced predominantly in the testes under signalling from the pituitary's luteinising hormone (LH). It regulates libido, muscle and bone mass, erythropoiesis, mood, and a range of metabolic processes. Levels peak in early adulthood and decline by roughly 1-2% per year from age 30.

Production and regulation

Testosterone synthesis happens in the Leydig cells of the testes (~95%) with a small adrenal contribution. The hypothalamic-pituitary-gonadal (HPG) axis governs the cycle: hypothalamic GnRH stimulates pituitary LH and FSH, LH drives testosterone production, and circulating testosterone feeds back negatively on the hypothalamus and pituitary.

Roughly 98% of circulating testosterone is bound — mostly to sex hormone-binding globulin (SHBG), partly to albumin. The remaining 1-2% is unbound (free testosterone) and immediately bioavailable.

Functions

Anabolic: muscle protein synthesis, bone density, erythropoiesis (red blood cell production).

Androgenic: libido, sperm production, facial and body hair, voice deepening at puberty.

Metabolic: insulin sensitivity, lipid profile, body fat distribution.

Central: mood regulation, motivation, cognitive function.

Clinical reference ranges

Total testosterone in adult men typically ranges 12-30 nmol/L (350-1,000 ng/dL) on a morning fasted draw. Calculated free testosterone reference ranges vary by lab but ≥250 pmol/L is considered adequate.

Hypogonadism is generally diagnosed when total testosterone is consistently below the lab reference range (typically <8-10 nmol/L) alongside characteristic symptoms.

References

Related concepts

Sex Hormone-Binding Globulin (SHBG) — SHBG is a liver-produced glycoprotein that binds sex hormones (primarily testosterone and oestradiol) in the bloodstream, regulating their bioavailability.
Estradiol in Men — Estradiol (E2) is the most potent oestrogen, produced in men primarily through aromatisation of testosterone in adipose, brain, and bone tissue, and is essential for bone health, libido, cognition, and cardiovascular function.
Cortisol — Cortisol is the principal glucocorticoid stress hormone produced by the adrenal cortex, regulating glucose metabolism, immune function, and the body's response to physiological and psychological stress.