In this section we will learn regulation of Testosterone by Pitutary Gland which is also called Hypothalamic–pituitary–testicular axis.
In males, testosterone is synthesized primarily in Leydig cells. The number of Leydig cells in turn is regulated by luteinizing hormone (LH) and follicle-stimulating hormone (FSH). In addition, the amount of testosterone produced by existing Leydig cells is under the control of LH, which regulates the expression of 17-β hydroxysteroid dehydrogenase.
The amount of testosterone synthesized is regulated by the hypothalamic–pituitary–testicular axis. When testosterone levels are low, gonadotropin-releasing hormone (GnRH) is released by the hypothalamus, which in turn stimulates the pituitary gland to release FSH and LH. These latter two hormones stimulate the testis to synthesize testosterone. Finally, increasing levels of testosterone through a negative feedback loop act on the hypothalamus and pituitary to inhibit the release of GnRH and FSH/LH, respectively.
This important hormone is produced mainly in the testes in males (more than 95 percent) and in the ovaries in females; however, small amounts are made in the outer layer of the adrenal glands in both sexes. The process that carefully regulates the amount and timing of testosterone production is complex and begins in the brain. When a man feels aroused or successful, the cerebral cortex, the most sophisticated area of the brain, sends a signal to another part of the brain called the hypothalamus to stimulate the production of testosterone. 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.
A blood test is an easy way to assess blood levels of testosterone. An analysis of testosterone levels is typically needed to assess early or late puberty in boys, infertility in men or excess hair growth and irregular menstrual cycles in women. According to the National Institutes of Health, for a male, a normal testosterone level is between 300 to 1,200 ng/dL, nanograms per deciliter. For a female, testosterone levels should fall between 30 to 95 ng/dL.