What is testosterone?

Testosterone is mainly a male sex hormone, belonging to a group of androgens (steroids) that are central to sexual and reproductive development, but females also produce testosterone, although in smaller quantities.

In males, testosterone is produced primarily in the testes, by the Leydig cells. In females, testosterone is mainly produced in the ovaries, which also produce estrogen. In both sexes, a small amount of testosterone is also produced in the adrenal glands.

Testosterone levels can fluctuate greatly, even over the course of a single day. The overall level of testosterone in the body tends to decrease as a result of aging. A deficiency of the hormone can also occur prenatally and during puberty. Some individuals may experience unusually high levels of testosterone. This is most common in children and young adults, and also occurs in some females as a result of a variety of underlying medical conditions.

Testosterone therapy, a treatment method involving an artificial form of the hormone, may be necessary for those experiencing low levels of testosterone. There are, however, possible side-effects, such as a lowered sperm count and acne, as well as cardiovascular problems.[1]

What does testosterone do?

Given its role in several bodily processes, testosterone is considered highly important to the overall health of males and females.

During the gestation of a fetus, testosterone is critical to the development of the internal and external male sex organs. The hormone also plays an important role during puberty, helping to enact many of the bodily changes that take place as the body matures.

Changes that occur during puberty include:

  • Growing taller
  • Developing pubic and body hair
  • In males, the enlargement of the penis and testes
  • Behavioral changes

During and after puberty – in males and females – testosterone also helps to enhance the libido and produce sperm (in males).[2]

By encouraging the production of red blood cells, testosterone also helps to maintain healthy bones and muscle.

How is testosterone controlled?

The human male and female reproductive cycles are controlled by the interaction of hormones from the brain’s hypothalamus and anterior pituitary[3] with hormones from reproductive tissues and organs. In both sexes, the hypothalamus monitors and causes the release of hormones from the pituitary gland.

Low levels of testosterone

Low testosterone, or a testosterone deficiency, can occur prenatally, during puberty and develop in adult males and sometimes females too.

If the deficiency occurs prenatally (during pregnancy), the sexual development and process of masculinization may be affected as a male child matures. A male child with low levels of testosterone may experience delayed puberty, which will typically involve slow or no growth and a failure to develop the characteristics associated with puberty, including penis and testes enlargement, voice deepening, as well as pubic and body hair growth. Those with a testosterone deficiency may also struggle to develop normal muscle mass and have lesser endurance capabilities.[2]

A testosterone deficiency can also develop as a result of aging, as well as being associated with conditions including obesity, type 2 diabetes and high blood pressure. The symptoms of a testosterone deficiency may be physical and psychological in nature.[4]

Testosterone deficiency in males may include the following symptoms:


  • Change in sleep patterns - insomnia or sleep disturbances
  • Increased body fat and reduced muscle mass
  • Reduced sexual desire, sexual dysfunction or infertility


  • Decreased motivation
  • Depression

A decreased libido and general fatigue are the most common symptoms of a testosterone deficiency. However, some males with low levels of testosterone may experience no symptoms at all, or only some of the symptoms listed above.

Females can also experience testosterone deficiency. Symptoms of testosterone deficiency in females may include:


  • Loss of muscle mass and strength
  • Loss of libido


  • Lethargy
  • Lack of motivation
  • Low levels of wellbeing
  • Low mood

  1. Live Science. “What is Testosterone?” June 22, 2017. Accessed September 14, 2017.

  2. You and Your Hormones. “Testosterone.” January, 2015. Accessed September 14, 2017.

  3. The hypothalamus and anterior pituitary are small glands that are part of the brain.

  4. The American Journal of Medicine. “Testosterone Deficiency.” July, 2011. Accessed September 15, 2017.