Testosterone is the most important male sex hormone (androgen). In men more than ninety percent of testosterone production takes place in the testes. A considerably smaller amount is produced in the outer layer (cortex) of the adrenal glands.

An adult male produces about 6 - 7 mg testosterone per day. The normal blood level of testosterone fluctuates between 12 and 35 nmol/l. The amount of testosterone produced varies in the course of the day and blood levels can be up to 30 % higher in the early morning. This is the explanation for morning erections. Testosterone levels are usually at their lowest between 6 and 10 pm.

Testosterone production declines in the course of a man's life. On average, testosterone levels begin to decrease gradually after the age of about 35 to 40.

Testosterone has different functions in different phases of life.

In the unborn child it causes male sexual differentiation while during puberty it is responsible for virilization (masculinization).

In the adult male testosterone controls all sexual functions (libido, potency, fertility). In addition, it preserves the typical male build and appearance. It is important for physical health and performance and also has a decisive influence on mood and sensation of wellbeing.