Testosterone is a sex hormone produced by males and females, although males produce approximately 10 times as much as females. It is responsible for promoting sperm production in the male testes but also travels in the blood to increase muscle and bone growth. Low testosterone levels can be an indication of reduced fertility in males, while high levels can be suggestive of certain types of cancer or precocious puberty.
Testosterone: A Hormone
Testosterone has many functions, most of which can be catagorized as anabolic, which means the growth of tissues such as your bones and muscles, and androgenic, which means the development of your sex hormones. Its main function is to bring about puberty in boys with the accompanying bodily changes such as hair growth, deepening of the voice, the building of muscle mass and the continuation of sexual function throughout life.
Testosterone Production
Around 95 percent of circulating testosterone is produced by the Leydig cells in the testes, although the adrenal glands also produce the hormone and its precursors. Testosterone is synthesized from the precursor molecule cholesterol, which goes through a complex reaction pathway producing several intermediate molecules on the way. Testosterone production occurs in the Leydig cells under the control of another hormone called luteinizing hormone. This causes an increase in the number of Leydig cells and an increase in the level of testosterone that they produce in puberty and throughout life.
Average Testosterone Levels
Testosterone can be measured in the blood in nanograms per deciliter. The first time testosterone is produced is in the first two months of life where levels can reach
300 ng/dL, followed by a steady rise until 6 months. This is followed by a sharp drop in levels until puberty is reached, when testosterone levels rise until adult levels of 300 to 1,000 ng/dL are reached around age 16. The National Institute of Health states that the average testosterone levels in adult males is 700 ng/dL, with the healthy range between 300 and 1,200 ng/dL. In adult females, it should be between 30 and 95 ng/dL.
Daily Testosterone Fluctuations
Within an individual, testosterone levels in the blood can vary 10 percent to 15 percent from one moment to the next. Levels are typically higher in the morning. Levels can also fluctuate during the day because of periodic testosterone secretions that occur about every seven hours. The increased levels of secretion are modest and so the rise and fall of blood hormone levels as a result of this are also minor.
Testosterone Binding Proteins
One major factor determining blood testosterone levels is the amount of free testosterone compared to bound testosterone. The hormone is normally bound to a sex hormone-binding globulin (SHBG), which is produced in the liver and prevents the testosterone from being metabolized as soon as it enters the blood. However, this means that bound testosterone travels around the blood in an inactive form. Only when testosterone is free in the blood can it perform its biological tasks, yet only 1 percent to 3 percent of testosterone is unbound in the blood. A decrease in the SHBG levels will result in higher blood testosterone levels. It is also known that variations in the testing methods can result in a variation of 33 percent in the levels of testosterone found in an individual. These factors should be taken into account when performing a testosterone measurement. If you have concerns about your testosterone levels, consult your physician.
