Testosterone

Testosterone is the primary male hormone secreted by the testicles. It is responsible for inducing puberty and maintaining secondary male sexual characteristics. Testosterone production begins to taper off at about age 40 and continues to fall with age.

Testosterone blood testing is often done to aid in the diagnosis of sexual dysfunction or infertility.
 

It is less frequently studied to aid in the diagnosis of a variety of complicated hormonally influenced diseases.

While low testosterone is not usually a cause of impotence, a depressed testosterone may be an important factor.