Testosterone is a 19-carbon steroid hormone synthesized from cholesterol in the testes (men) and ovaries/adrenals (women).
Mechanism: Binds the androgen receptor to drive anabolic gene expression: muscle protein synthesis, erythropoiesis, bone mineralization. Converts to estradiol via aromatase and to DHT via 5α-reductase, both of which have distinct downstream activities.
Lab range: Men, 300–1000 ng/dL total (optimal often 600–900). Women, 15–70 ng/dL. Free testosterone is more biologically meaningful but harder to measure accurately.
Caveats: TRT in men suppresses HPTA function (testicular atrophy, fertility loss) unless paired with HCG or FSH analogs. Women's TRT is microdosed. Typically one-tenth the male dose. Estradiol conversion needs monitoring.