What is Testosterone?

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Testosterone is the principle sex hormone responsible for the development of reproductive function in male vertebrates. Testosterone is one of the hormones referred to as androgens, which are also known of as anabolic steroids. As a steroid hormone, testosterone is derived from cholesterol and the structure of this hormone is similar across all mammals, reptiles, birds and fish.


In males, testosterone is required for the development of male sex organs such as increased penis and testes size. The hormone also promotes the development of sexual male characteristics during puberty such as voice deepening and the growth of armpit, chest and pubic hair. Testosterone plays an important role in maintaining sex drive, sperm production, muscle strength and bone mass. A healthy level of testosterone is also protective against bone disorders such as osteoporosis.

As testosterone is required for so many bodily functions, it is considered to be a general promoter of overall health and well being and has been described by The National Institutes of Health as the most important hormone in men.

Testosterone is also important for maintaining bone strength and lean muscle mass in women, as well as contributing to overall well-being and energy levels. This hormone plays a key role in a woman’s sex drive and is responsible for enhancing sexual pleasure during intercourse. However, the levels of testosterone produced by females is still between ten and times less than the amount produced by men.


In males, the majority of testosterone is secreted from the testes, hence the term “testosterone”. The hormone is also produced in small amounts by the adrenal gland. The production of this hormone is controlled by the hypothalamus and pituitary gland in the brain. The pituitary gland receives instructions from the hypothalamus on how much testosterone needs producing and passes this information onto the testicles via chemicals and hormones circulating in the bloodstream.

In females, half of the testosterone is produced by the ovaries and adrenal glands. The remainder is produced through conversion of adrenal androgens in other parts of the body.

Declining testosterone

In males, testosterone levels peak during adolescence and early adulthood and start to decline after the age of 30. This can lead to a progressive decline in physical energy and libido as a man ages. The testosterone level is thought to decline by approximately 1% every year after a man has reached 30 years of age.

However, in older men, it is important to establish whether any decline in well-being or function is due to a normal age-related decline in testosterone or whether symptoms are being caused by an abnormally low testosterone level, a condition referred to as hypogonadism.

Low testosterone

In men with hypogonadism, a low level of testosterone is produced due to a problem in the testicles or the pituitary gland. According to Harvard Medical School, determining exactly what constitutes a low testosterone level is a controversial matter. Levels of this hormone fluctuate wildly and even vary according to the time of day. However, generally physicians only decide to treat a patient for hypogonadism if the blood testosterone level is below 300 ng/dL and the following symptoms outlined by The National Institutes of Health are present.    

  • Reduced libido
  • Reduced sperm count
  • Hot flashes
  • Increased breast size
  • Impotence or erectile dysfunction
  • Shrunken testes
  • Reduced muscle mass
  • Hair loss
  • Increased propensity to bone fracture

Testosterone replacement therapy

Testosterone replacement therapy can help to improve some of the symptoms of hypogonadism and may be prescribed in the form of injections, patches, gels or pellets.

Non-medical use

For some men who are aging, the idea of testosterone replacement therapy seems like an enticing option. Effects such as increased vigour, increased muscle strength, enhanced memory, sharpened concentration, a boost in libido and increased energy levels can make this drug seem like the miracle anti-aging therapy. However, it is unclear whether or not this therapy can offer any health benefits to men who simply have a normal age-related decline in testosterone. Few large studies have examined the effects of this therapy in men who have a healthy testosterone level and the few smaller studies that have been conducted reveal conflicting results.

Furthermore, the use of testosterone replacement therapy is associated with several risks and men are advised to carefully consider the risks of taking this medication before making a decision.  Some of the risks associated with this therapy are describe below.

  • Heart attack
  • Stimulation of benign prostate hyperplasia
  • Growth of any existing prostate cancer
  • Acne and other skin reactions
  • Sleep apnea
  • Enlarged breast size
  • Reduced sperm production
  • Testicle shrinkage
  • Thrombosis

High testosterone

In females, a high testosterone level can lead to irregular periods, acne, voice deepening and an excess growth of body hair. A high testosterone level is also found in women with polycystic ovarian syndrome, a condition that can cause infertility.

Further Reading

Last Updated: Jun 19, 2023

Sally Robertson

Written by

Sally Robertson

Sally first developed an interest in medical communications when she took on the role of Journal Development Editor for BioMed Central (BMC), after having graduated with a degree in biomedical science from Greenwich University.


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  1. Mukombe Saidi Mukombe Saidi Uganda says:

    (Male Sexual Function) Analysis Report Card
    Name: Mukombe Saidi  Sex: Male  Age: 22
    Figure: Standard body weight(165cm,65kg)  Testing Time: 2015-01-09 04:14
    Actual Testing Results
    Testing Item  Normal Range  Actual Measurement Value  Testing Result
    Testosterone  3.342 - 9.461  2.993  
    Gonadotropin  4.111 - 18.741  9.142  
    Erection Transmitter  3.241 - 9.814  4.285  
    Reference Standard:  
      Normal(-)     Mildly Abnormal(+)
      Moderately Abnormal(++)     Severely Abnormal(+++)
    Testosterone:  3.342-9.461(-)  2.790-3.342(+)
      1.394-2.790(++)  <1.394(+++)
    Gonadotropin:  4.111-18.741(-)  2.790-4.111(+)
      1.737-2.790(++)  <1.737(+++)
    Erection Transmitter:  3.241-9.814(-)  2.617-3.241(+)
      1.821-2.617(++) please help me and reply Thanks

  2. Mukombe Saidi Mukombe Saidi Uganda says:

    But what can i use?

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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