| Testosterone not only influences gonadotropin regulation and spermatogenesis, but is also a natural anabolic steroid
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The testes secrete about 7 mg of testosterone each day into the peripheral circulation. A further 500μg of testosterone originates from the adrenal gland. More than 97% of the circulating testosterone is bound to protein, with equal amounts bound to albumin and to the specific sex hormone-binding globulin (SHBG), which is similar in structure to ABP. Testosterone is further metabolized to dihydrotestosterone (DHT) via 5-α reduction; this modification more than doubles its affinity for the nuclear androgen receptor. In addition to effects on gonadotropin regulation and spermatogenesis, androgens induce the development of the reproductive tract from the Wolffian ducts during male sexual differentiation. They also have anabolic effects, stimulating protein synthesis and an increase in muscle mass (Fig. 37.11).
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| Figure 37.10 Variations in serum FSH and LH during life. Serum FSH and LH levels vary depending on age and sex. |
| Figure 37.11 Mechanism of action of testosterone. Testosterone from the testis enters a target cell and binds to the androgen receptor, either directly or after conversion to 5α-dihydrotestosterone (DHT). DHT binds more tightly than testosterone, and the DHT-receptor complex binds more efficiently to chromatin. Actions mediated by testosterone are shown by purple lines, those mediated by DHT are shown by blue lines. |
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