Testosterone serves as a circulating precursor or prohormone for two types of metabolites, which in turn mediate many androgen actions (Fig. 11.8). It can be converted to 5a-reduced steroids, principally dihydrotestosterone, which mediate many differentiative, growth-promoting, and functional actions. Alternatively, androgens can be aromatised in the extraglandular tissues to oestrogens that act independently or in concert with androgens to influence physiological processes. Thus, the actions of testosterone are the result of the combined effects of testosterone, dihydrotestosterone, and oestradiol. Dihydrotestosterone formation is mediated by two widely distributed membrane-bound 5a-reductase enzymes; the majority of dihydrotestosterone is produced in androgen-target tissues. Aromatisation also occurs in many tissues, perhaps the most important being the adipocyte.
