Which organ system is most directly affected by testosterone levels?

Testosterone is a primary male sex hormone that plays a crucial role in the development and functioning of the reproductive system. It is produced mainly in the testes and is responsible for the development of male secondary sexual characteristics such as increased muscle mass, facial hair, and deeper voice. Moreover, testosterone regulates libido, sperm production, and overall reproductive health.

While testosterone may influence aspects of other systems, such as the nervous system in terms of mood and cognition or muscle growth associated with the muscular system, its most direct and primary effect is squarely within the reproductive system. This is evident not only in the physiological changes it drives during puberty but also in its ongoing role in maintaining reproductive functions throughout an individual's life.