Testosterone is the most important male sex hormone. The biological actions of testosterone differ in the different phases of life:

In the uterus the hormone is responsible for the development of a male fetus. During puberty it is testosterone that causes the development of the secondary sexual characteristics. In the adult male it controls all reproductive functions such as sperm production, erections and sex drive. In addition to these actions, testosterone also has a number of other important effects which play a crucial role in maintaining male health and wellbeing.

Testosterone is produced mainly in the testes.

Disease and aging can lead to the development of a testosterone deficiency which can cause a wide range of complaints and serious illnesses.