The hypothalamus is an area at the base of the brain that regulates much of the body's hormonal activity. It does this by sending chemical signals to the pituitary gland, a cherry-sized organ that produces a wide variety of hormones involved in the regulation of growth, thyroid function, blood pressure, pregnancy, birth and other critical body functions.
To stimulate testosterone production, the hypothalamus releases a substance to the pituitary gland called gonadotropin-releasing hormone (GnRH). This hormone, in turn, causes the gland to produce two other hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), collectively known as gonadotropins. LH is released into the bloodstream where it travels to the male testes and triggers the production of testosterone from cholesterol. If this process continues until the testosterone level becomes too high, the pituitary slows the release of LH so production slows down. FSH is similarly involved in the increase and decrease in sperm production.
When LH reaches the testes, it influences activity in the Leydig cells, which are where cholesterol is gradually changed into a series of compounds until it becomes testosterone. When the small but vital amount of testosterone produced is released into the bloodstream, it is mostly bound to a special "carrier" compound called sex hormone binding globulin or SHBG. SHBG, which is produced by the liver, plays an important role in regulating the amount of "free" testosterone circulating in the body at any one time. The more SHBG there is the less unbound, active testosterone is able to move from the blood stream into cells where it is needed. As SHBG levels rise and fall, so do free testosterone levels, except in reverse.
With such a complex chain of events leading to a normal testosterone level, many problems or interruptions along the process can lead to sub-normal or low testosterone levels in men at any age. If there are diseases or negative conditions involving the male testes, hypothalamus, pituitary gland or genetic material, the resulting state is called hypogonadism.
