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Updated: Jun 9 2021

Androgens

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  • 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
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