TY - CHAP M1 - Book, Section TI - Diagnosis and Treatment of Androgen Deficiency Syndromes in Men A1 - Hayes, Frances J. A2 - Bhasin, Shalender A2 - O’Leary, Michael P. A2 - Basaria, Shehzad S. PY - 2021 T2 - Essentials of Men’s Health AB - Regulation of testosterone secretion in men involves a coordinated series of signals between the hypothalamus, pituitary, and testes. Gonadotropin-releasing hormone (GnRH) is secreted in pulses from the hypothalamus approximately every 2 h and stimulates the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) from the anterior pituitary, which, in turn, act on the testes to stimulate testosterone production and spermatogenesis.1 From a functional standpoint, the testis can be viewed as a 2-compartment structure. LH acts on Leydig cells to stimulate steroidogenesis, causing an increase in both intratesticular and systemic testosterone levels. FSH acts on Sertoli cells and, in the presence of adequate amounts of intratesticular testosterone, stimulates sperm production. Gonadal steroids, in turn, exert a negative feedback effect on the hypothalamus and pituitary to maintain LH and FSH levels within a tight range. There is additional regulation of FSH by a nonsteroidal factor made by Sertoli cells called inhibin B.2 The majority of circulating testosterone is bound to sex hormone–binding globulin (SHBG), albumin, cortisol-binding globulin, and orosomucoid, with only approximately 2% to 4% being unbound or free. Epidemiologic studies suggest that it is the free fraction of testosterone that is biologically active based on the demonstration that it correlates better with symptoms of hypogonadism than total testosterone.3 SN - PB - McGraw Hill CY - New York, NY Y2 - 2024/03/28 UR - accessmedicine.mhmedical.com/content.aspx?aid=1174515468 ER -