Mặc dù nội tiết tố androgen được cho là đã quan hệ tình dục hormone steroid lớn ở nam giới, nghiên cứu gần đây trên động vật và con người mà không có một thụ thể estrogen (ERα) cho thấy rằng estradiol đóng một vai trò quan trọng | 278 Wierman Hypothalamus GnRH Pituitary a LHp FSHp Testosterone Testes Dihydrotestosterone Estradiol Inhibin B Fig. 1. Diagram of the hypothalamic-pituitary-testicular axis. GnRH from hypothalamic neurons activates the gonadotropin subunit genes a LHP FSHP to release LH and FSH from the pituitary. These in turn stimulate spermatogenesis and production of sex steroids testosterone estradiol and the gonadal peptide inhibin B. B together regulate spermatogenesis in the presence of high intratesticular levels of testosterone 4 . Testosterone is a prohormone and is converted by 5a-reductase to dihydrotestosterone a more potent androgen or by aromatase into estradiol. Although androgens were thought to be the major sex steroid hormone in men recent studies in animals and humans without an estrogen receptor ERa suggest that estradiol plays a critical role in normal spermatogenesis and hormone feedback in the male 5 6 . LH levels are controlled primarily by GnRH from the hypothalamus and negative feedback from testosterone and estradiol from the testes 7-9 . FSH levels in contrast are controlled by GnRH gonadal steroid hormone feedback and the actions of the gonadal peptides inhibin B activin A and follistatin derived from both the gonad and the pituitary 10 . A critical feature of this endocrine system is the negative feedback of steroid hormones on hypothalamic and pituitary hormone production 2 . Testosterone levels are secreted in a circadian rhythm with increases at night 3 . Another feature is the requirement for an episodic pattern of hormone secretion for normal reproductive function 1-3 . Continuous production of GnRH-induced LH secretion turns off the system and is the basis for GnRH analogues used as reversible medical castration in hormone-dependent malignancies such as prostate cancer. This episodic pattern of hormonal signaling is important to remember when obtaining samples for hormone levels. Chapter 14 Hypogonadism 279 NORMAL PHYSIOLOGY OF ERECTION There .
