Testosterone

Testosterone is the primary male hormone responsible for regulating sex differentiation, producing male sex characteristics, spermatogenesis, and fertility. Testosterone's effects are first seen in the fetus. During the first 6 weeks of development, the reproductive tissues of males and females are identical.
Testosterone is a hormone found in humans, as well as in other animals. In men, the testicles primarily make testosterone. Women’s ovaries also make testosterone, though in much smaller amounts.

The production of testosterone starts to increase significantly during puberty and begins to dip after age 30 or so.

Testosterone is most often associated with sex drive and plays a vital role in sperm production. It also affects bone and muscle mass, the way men store fat in the body, and even red blood cell production.

A man’s testosterone levels can also affect his mood.