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Anabolic and metabolic effects of testosterone and other androgens: Direct effects and role of testosterone metabolic products : Chapter 11 : Part I Physiology of the Hypothalamus-Pituitary-Gonadal Axis, the Testis, and Testicular Function
Los Angles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, CA, USA | ||
Wang, Christina | Los Angles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, CA, USA | David Geffen School of Medicine at UCLA, Harbor-UCLA Medical Center, Torrance, CA, USA |
Swerdloff, Ronald S. | Los Angles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, CA, USA | David Geffen School of Medicine at UCLA, Harbor-UCLA Medical Center, Torrance, CA, USA |
Liu, Peter Y. | Los Angles Biomedical Research Institute at Harbor-UCLA Medical Center, Torrance, CA, USA | David Geffen School of Medicine at UCLA, Harbor-UCLA Medical Center, Torrance, CA, USA |
ISBN 978-3-319-44440-6
ISBN 978-3-319-44441-3 (eBook)
ISBN 978-3-319-44442-0 (print and electronic bundle)
Bibliogr.: p. 389-394
Sex hormones play a major role throughout the body starting from gender differentiation in utero to senescence. While they are undoubtedly important for development and conservation of reproductive function, their importance has been shown in most systems by experimental and clinical models. While testosterone (T) is considered to be the most important androgen in circulation, it may either directly exert effects on target tissues or be metabolized by 5α-reductase into dihydrotestosterone (DHT) or aromatized to estradiol (E2). Differential effects of T and its metabolites are not well known and are a topic of scientific debate. These pleiotropic effects lead to variable phenotypes, clinical manifestations, and significance throughout life and may be more pronounced in some systems (such as bone and reproductive organs) or stages (such as puberty) than in others. This chapter focuses on the available clinical evidence of the androgen effects on several important systems.Introduction The most important circulating androgen – testosterone (T) – is mainly produced by the testis with minimal amounts being secreted by zona reticularis in the adrenals. The androgens that are predominantly secreted by the adrenal gland are dehydroepiandrosterone (DHEA) and androstenedione. Approximately 98% of circulating T is bound to protein – 60% loosely bound to albumin, while 40% is tightly bound to sex hormone-binding globulin (SHBG). The remaining 2–3% is free or unbound testosterone. The classic understanding is that only free T is biologically active, while albumin-bound T is easily dissociated; thus both of these forms are available to tissues and are usually referred to as bioavailable T. Unlike T, most of the adrenal androgens (about 90%) are bound to albumin with only 3% bound to SHBG.[...].