Testosterone is the main steroid hormone (androgen) in men responsible for sex drive, enlargement of the penis, sperm production & development, increased muscle mass, fat distribution, red blood cell production and lowering of the voice. Most of the testosterone in men is produced in the testes, while a smaller amount is made by the adrenal glands.
Luteinising hormone (LH) and follicle-stimulating hormone (FSH) are two hormones produced in the brain which stimulate the production of testosterone and sperm respectively.
Once produced, testosterone is bound and transported in the blood by the protein, sex hormone binding globulin (SHBG) and, to a lesser extent, by albumin. To become active, testosterone must be released from SHBG and bind to androgen receptors (ARs) in target tissues.
Testosterone is also converted into two other important hormones: 5a- dihydrotestosterone (DHT) and estrogen.
DHT is about 3-5 times more potent than testosterone because it binds more strongly to the ARs. While DHT is important for developing and driving normal male characteristics, its strong androgenic action is also responsible for increased hair growth and eventually loss, acne and enlargement of the prostate gland in later life.