Androgens Sources of androgens males testes are a major source of androgens adrenal cortex is a minor source of androgens females adrenal cortex is a major source of androgens Androgen biosynthesis adrenal cortex in adrenal cortex, zona reticularis synthesizes and secretes adrenal androgens via a biosynthetic pathway that begins with cholesterol androstenedione and dehydroepiandrosterone (DHEA) testes Leydig cells of testes synthesize and secrete testosterone via a biosynthetic pathway that begins with cholesterol Leydig cells express 17β-hydroxysteroid dehydrogenase catalyzes conversion of androstenedione to testosterone Leydig cells lack 21β-hydroxylase and 11β-hydroxylase testes cannot synthesize and secrete glucocorticoids and mineralocorticoids Leydig cells of testis may also synthesize and secrete dihydrotestosterone (DHT) Leydig cells express 5α-reductase catalyzes conversion of testosterone to DHT androgenic target tissues in some androgenic target tissues, DHT is an active androgen androgenic target tissues express 5α-reductase catalyzes conversion of testosterone to DHT Androgen potency DHT > testosterone > androstenedione Androgen function actions mediated by testosterone fetal differentiation of internal male genitalia epididymis, vas deferens, and seminal vesicles pubertal growth spurt growth of penis and seminal vesicles deepening of voice sex drive/libido in pre-pubescent males increased muscle mass cessation of pubertal growth spurt epiphyseal plate closure mediated by estrogen testosterone → estrogen via aromatase in testes, Sertoli cells express aromatase enzyme catalyzes conversion of testosterone to estrogen additionally, adipose tissue expresses aromatase enzyme actions mediated by DHT fetal differentiation of external male genitalia penis, scrotum, and prostate male hair pattern and male pattern baldness sebaceous gland activity growth of prostate 5α-reductase inhibitors e.g., finasteride inhibit conversion of testosterone to DHT DHT mediates male hair pattern and growth of prostate 5α-reductase inhibitors may be used in treatment of benign prostatic hyperplasia (BPH) and hair loss in males Exogenous testosterone exogenous testosterone inhibits hypothalamic-pituitary-gonadal axis decreases testosterone levels in testes decreases size of testes causes azoospermia