testosterone

Testosterone is a steroid androgen and the principal male sex hormone. It is synthesized mainly in the Leydig cells of the testes, with smaller amounts produced by the ovaries and adrenal glands in women. Production is regulated by the hypothalamic-pituitary-gonadal axis: GnRH from the hypothalamus stimulates LH and FSH from the pituitary, which drive testosterone synthesis. Most circulating testosterone is bound to sex hormone-binding globulin and albumin, with a small fraction free. Levels vary with age and time of day; typical reference ranges are about 300–1,000 ng/dL in adult men and 15–70 ng/dL in adult women, though ranges differ by assay.

Testosterone acts via the androgen receptor and is metabolized to dihydrotestosterone by 5-alpha reductase and to estradiol by aromatase. Prenatally, it supports male genital development; at puberty it drives secondary sexual characteristics, increases muscle mass and bone density, and promotes spermatogenesis and libido. In adults, it maintains sexual function, mood, energy, and red blood cell production; it also influences bone health.

Clinically, low testosterone (hypogonadism) can cause fatigue, reduced libido, and decreased muscle mass. Diagnosis combines symptoms with biochemical testing, typically by measuring total testosterone and sometimes free testosterone, with SHBG levels affecting interpretation. Treatments include testosterone replacement therapy via injections, gels, patches, or implants, supervised by clinicians because of potential risks such as erythrocytosis, prostate effects, sleep apnea, and cardiovascular concerns. Testosterone is also used in gender-affirming therapy for transgender men. Abuse for performance enhancement carries health risks and is illegal in many contexts; monitoring is essential.