Testosterone is an important hormone for both men and women. It has a far larger affect in men than women however.

In men testosterone is produced within the testes by specialised cells called Leydig cells. A small amount of testosterone is also produced by the adrenal glands located above the kidneys.

Testosterone is produced from cholesterol under the influence of luteinising hormone (LH) produced by the pituitary gland.

Testosterone is sometimes called a pro-hormone as it can be broken down into two separate compounds. Each of these compounds has their own unique function.

Sometimes doctors will test for levels of these two hormones as well as testosterone itself. There are conditions which can cause symptoms like those seen in Kallmann syndrome that are caused by the failure to produce one of these two compounds.

Testosterone is not vital for survival but it is crucial for developing “maleness”. Its primary function is in the sexual development differentiation and development of the male, i.e. the development of the testes & penis and subsequent sperm production and male sex drive.

It does have a number of other functions around the body as the above table illustrates.

Testosterone and in particular DHT are crucial for the development of the male foetus. At around the 8th week of growth the sexual organs are differentiated into male and female. If testosterone or DHT is absent or unable to be used there is likely to be a disruption in the growth of the male sex organs.

Just how much of an effect this will be will depend on the amount of testosterone and DHT available at that time. In some form of Kallmann’s syndrome or HH the amount of testosterone available is lower than would be expected resulting in abnormal penis growth.

During puberty testosterone is the driving force behind all the physical and psychological changes seen. Normally luteinising hormone produced by the pituitary gland will stimulate testosterone production in the testes. This will allow the testes to grow and produce even more testosterone and eventually sperm. The testosterone will also cause the penis to lengthen and thicken.