An additional 40% of testosterone is metabolized in equal proportions into the 17-ketosteroids androsterone and etiocholanolone via the combined actions of 5α- and 5β-reductases, 3α-hydroxysteroid dehydrogenase, and 17β-HSD, in that order. Approximately 50% of testosterone is metabolized via conjugation into testosterone glucuronide and to a lesser extent testosterone sulfate by glucuronosyltransferases and sulfotransferases, respectively. The plasma protein binding of testosterone is 98.0 to 98.5%, with 1.5 to 2.0% free or unbound. The amount of testosterone synthesized is regulated by the hypothalamic–pituitary–testicular axis (Figure 2). In addition, the amount of testosterone produced by existing Leydig cells is under the control of LH, which regulates the expression of 17β-hydroxysteroid dehydrogenase. The number of Leydig cells in turn is regulated by luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Testosterone is also synthesized in far smaller total quantities in women by the adrenal glands, thecal cells of the ovaries, and, during pregnancy, by the placenta.
While a small, annual decline in testosterone levels is a normal part of the aging process, you can take action to optimize your levels and avoid symptoms of low testosterone. In addition to these, having too-high testosterone levels put you at risk for health conditions including cancer, cardiac complications, and irritability (9). A combination of age, genetics, and pre-existing medical conditions determines your testosterone levels. But determining normal testosterone levels by age?
Total levels of testosterone in the body have been reported as 264 to 916 ng/dL (nanograms per deciliter) in non-obese European and American men age 19 to 39 years, while mean testosterone levels in adult men have been reported as 630 ng/dL. Like most hormones, testosterone is supplied to target tissues in the blood where much of it is transported bound to a specific plasma protein, sex hormone-binding globulin (SHBG). Lipophilic hormones (soluble in lipids but not in water), such as steroid hormones, including testosterone, are transported in water-based blood plasma through specific and non-specific proteins. There is no FDA-approved androgen preparation for the treatment of androgen insufficiency; however, it has been used as an off-label use to treat low libido and sexual dysfunction in older women. A link has also been found between relaxation following sexual arousal and testosterone levels.
Providers call it male hypogonadism when you have symptoms along with these low levels. Healthcare providers consider testosterone levels below 300 nanograms per deciliter (ng/dL) as low in adults. Low testosterone (male hypogonadism) is a condition in which your testicles don’t produce enough testosterone. Swerdloff RS, Wang C. The testis and male hypogonadism, infertility, and sexual dysfunction.
Men who produce less testosterone are more likely to be in a relationship or married, and men who produce more testosterone are more likely to divorce. However, the testosterone changes observed do not seem to be maintained as relationships develop over time. Women's level of testosterone is higher when measured pre-intercourse vs. pre-cuddling, as well as post-intercourse vs. post-cuddling.