Male Reproductive Endocrinology
The hypothalamus produces gonadotropin-releasing hormone which is released in a pulsatile fashion every 60 to 120 minutes. Its target organ, the anterior pituitary gland, responds to each pulse of gonadotropin-releasing hormone by producing a corresponding pulse of luteinizing hormone (LH) and, to a lesser degree, follicle-stimulating hormone (FSH). If the gonadotropin-releasing hormone pulses do not occur with the proper amplitude, frequency, and diurnal variation, hypogonadism may result of idiopathic hypogonadotropic hypogonadism. Continuous stimulation by gonadotropin-releasing hormone agonists actually suppresses pituitary release of LH and FSH and thus testosterone production. Testosterone is synthesized from cholesterol through several intermediate compounds, including dehydroepiandrosterone (DHEA) and androstenedione. Circulating testosterone is mostly protein bound, about 40% avidly bound to sex hormone binding globulin and 58% loosely bound to albumin. Thus, only about 2% of circulating testosterone is bioavailable as free testosterone. This bioactive component of total testosterone is responsible for male characteristics, libido, bone and muscle mass.