Testosterone

Testosterone acts by binding to the androgen receptor (AR), a nuclear receptor that functions as a transcription factor to regulate gene expression across virtually every tissue in the body. Upon binding testosterone, AR translocates to the nucleus and modulates hundreds of genes involved in muscle protein synthesis, red blood cell production, bone mineralization, sexual function, and mood regulation. Testosterone is also converted to estradiol by aromatase in adipose tissue — estrogen contributes to bone density, cardiovascular protection, and libido in both sexes. In muscle, androgen receptor activation directly stimulates satellite cell proliferation and protein synthesis through mTOR pathway upregulation. The hypothalamic-pituitary-gonadal (HPG) axis tightly regulates endogenous testosterone production through negative feedback on LH and FSH secretion.