A synthetic form of the endogenous androgenic steroid testosterone. In vivo, testosterone is irreversibly converted to dihydrotestosterone (DHT) in target tissues by the enzyme 5-alpha reductase. Testosterone or DHT ligand-androgen receptor complexes act as transcription factor complexes, stimulating the expression of various responsive genes. DHT binds with higher affinity to androgen receptors than testosterone, activating gene expression more efficiently. In addition, testosterone is irreversibly converted to estradiol by the enzyme complex aromatase, particularly in the liver and adipose tissue. Testosterone and DHT promote the development and maintenance of male sex characteristics related to the internal and external genitalia, skeletal muscle, and hair follicles; estradiol promotes epiphyseal maturation and bone mineralization. Due to rapid metabolism by the liver, therapeutic testosterone is generally administered as an ester derivative.