Testosterone is the primary and most talked about male sex hormone. When in balance, testosterone is associated with energy, sex drive, and an overall sense of well-being. It is mainly secreted from the gonads or “testes” in males with a small amount being secreted from the adrenal glands. Testosterone is involved in development of the male reproductive tissues and in secondary sexual characteristics such as the testicles, prostate, muscle and bone strength, and body hair. Men are not the only ones who produce testosterone, women also produce some of the hormone in the ovaries. In women testosterone has an impact on the reproductive system, strength, and overall health. Too much or too little testosterone can cause issues in both men and women.
Testosterone production is regulated by the hypothalamus and the pituitary which are located in the brain and send signals to the testes. Together this hormone regulating system is known as the hypothalamus-pituitary-gonadal (HPG) axis. The axis allows for the body to monitor the amount of testosterone in the body and adjust the amount produced according to development age and physical factors.
Gonadotropin-releasing hormone (GNRH) is the hormone that regulates the sex hormones. GNRH is low in childhood and increases in puberty when sex hormones are needed for maturation and sexual urges. GNRH is released from the hypothalamus in the brain into the bloodstream and then absorbed into the pituitary gland. In response the pituitary gland releases follicle stimulating hormone (FSH) and luteinizing hormone (LH), which belonging to a group of glycoprotein hormones called gonadotrophins.
FSH targets cells that regulate sperm production, while LH targets Leydig cells in the testicles that produce testosterone. Leydig cells release testosterone into the blood stream where nearly 30% of it will be bound by another protein called sex hormone binding globulin (SHBG). Binding of SHBG leaves the testosterone inactive, meaning it cannot have any effect on the body. Approximately 68% of testosterone is loosely bound by blood protein albumin. Only 2% of circulating testosterone is unbound and active.
Outside of the testicles, testosterone can be converted to highly potent sex steroid dihydotestosterone (DHT) by the 5α-reductase enzyme or other androgens by the aromatase enzyme. Because testosterone is essential to sperm production, testosterone concentrations within the testicles are nearly 100 fold higher than in any other tissue. This high level is maintained by androgen-binding proteins released by other testicular cells involved in sperm production. Serum levels of testosterone are maintained by a negative feedback loop at the hypothalamus and pituitary glands.