Testosterone (T) (T)

Testosterone is the principal androgenic steroid hormone responsible for the development of male secondary sexual characteristics and the maintenance of muscle mass and bone density (StatPearls, 2023). In systemic circulation, it exists in a "free" state or bound to proteins like sex hormone-binding globulin (SHBG) and albumin; "Testosterone free levels" specifically refer to the unbound fraction that is biologically active and capable of diffusing into target cells (Mayo Clinic, 2023). Once inside the cell, testosterone or its metabolite dihydrotestosterone (DHT) binds to the Androgen Receptor (AR), a ligand-dependent transcription factor that regulates the expression of genes involved in male physiology (PubChem, 2024). Clinically, this pathway is targeted either through testosterone replacement therapy to treat hypogonadism or through androgen deprivation therapy—using GnRH analogs or AR antagonists—to treat androgen-dependent prostate cancer (NIH, 2023). Because factors like age and obesity can significantly alter protein binding, measuring free levels is often considered a more accurate biomarker of a patient's functional androgenic status than measuring total testosterone alone (PubMed, 2022).

Other names

Free testosteroneUnbound testosteroneBioavailable testosteronefT17beta-Hydroxyandrost-4-en-3-onetrans-Testosterone

Drugs targeting this pathway act through androgen receptor agonism, androgen receptor antagonism, inhibition of testosterone biosynthesis (CYP17A1), inhibition of 5-alpha reductase, or suppression of the hypothalamic-pituitary-gonadal axis (StatPearls, 2023).