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The Physiology of Androgens

by Robert S. Tan, from Opal Medical Clinic, on Dec 31
In a healthy man, approximately 95% of the androgens present are accounted for by testosterone produced in the testes.
The physiology of androgens

In a healthy man, approximately 95% of the androgens present are accounted for by testosterone produced in the testes.  Luteinizing hormone (LH) is secreted by the pituitary gland, which regulates the conversion of cholesterol to testosterone in Leydig's cells. Testosterone, in turn, is metabolized to dihydrotestosterone (DHT) by 5-alpha-reductase and undergoes aromatization to estradiol by aromatase. Dihydrotestosterone has been linked to prostate hypertrophy and male alopecia. The balance of the androgens is mainly in the form of dehydroepiandrosterone produced by the adrenal glands. Ninety-eight percent of circulating testosterone is bound to plasma proteins, with only 2% defined as free testosterone. About 40% of the bound testosterone is bound to sex hormone-binding globulin (SHBG). The remainder is weakly bound to albumin and is readily available to tissue when needed. Bioavailable testosterone includes free testosterone and testosterone that is loosely bound to albumin. Total testosterone levels decline in men between the ages of 40 and 70 years, falling at the rate of about 1-2% per year. Simultaneously, the level of sex hormone-binding globulin (SHBG) increases by 1-2% per year. The net result is that the amount of testosterone that is physiologically available decreases by about 3% per year. On the basis of total testosterone measurements alone, 20% of men older would be defined as hypogonadal. However, when the level of testosterone physiologically available is considered, the figure rises to 50%. Spermatogenesis is primarily regulated by the effect of LH on Leydig's cells in the testes. Testosterone has only a minor effect on sperm production. As such, andropause may not affect male fertility, although the number of sperm with normal motility and morphology may be altered. In contrast, the ultimate result of menopause is total termination of function of the ovaries and complete loss of reproductive capability. This difference aside, the long-term deprivation of sex hormones in men and women can have similar results, including effects on muscle, bone and cognition. We at the OPAL Medical Clinic pride ourselves as one of the pioneers in the delivery of androgens to both men and women. If you need further information, you can reach us at

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