The steroid hormone testosterone is the most important male sex hormone (androgens). It is synthesised from cholesterol and pregnelonon and androstenedion, in men in the Leydig cells of the testes and in women in the ovaries. A low proportion of testosterone is produced in the adrenal cortex. The hormone is responsible for protein biosynthesis, muscle growth, preservation of bone density and determines male sexual behaviour.

In the serum, the majority of the testosterone is bound with a high affinity to the sex hormone binding globulin (SHbG) and with a low affinity to albumin. Maximally 2 to 3% of the total testosterone are freely available in the serum.  Free testosterone enters the saliva through the salivary glands. Free and albumin-bound testosterone are considered as biologically available. At an older age, concentration of free testosterone is significantly lowered due to the increased synthesis of SHbH, since more testosterone is bound.