ABSTRACT
In addition to the sex steroids of gonadal origin that are synthesized in the testicles and ovaries before being released into the blood in order to reach all tissues to exert their action in the classical endocrine manner (Figure 12.1a), it is now well established that an additional source of sex steroids plays a major role in the humans of both sexes (Figure 12.1b). For example, in men who have their testicles surgically removed or who are treated with gonadotropin-releasing hormone (GnRH) agonists to completely block testicular androgen secretion (Labrie et al. 1980), it can be observed that while the blood levels of testosterone are reduced by 95%–97%, the concentrations of intraprostatic dihydrotestosterone (DHT) are only decreased by an average of 40%, as shown by the various data available in the literature (Labrie 2007; Labrie, Dupont, and Bélanger 1985). Such ‰ndings clearly indicate the existence of an important local biosynthesis of androgens in the prostate. Most peripheral tissues, including the skin, liver, adipose tissue, and brain also show local biosynthesis of sex steroids that exert their action locally in an intracrine manner (Labrie 1991; Labrie, Luu-The et al. 2005). In other words, the action of the sex steroids made from dehydroepiandrosterone (DHEA) is exerted in the same cells where the steroids are synthesized and inactivated (Labrie 1991).
