Testosterone is the primary sex hormone in males and is responsible for the development of male reproductive tissues (e.g., the testes and the prostate), as well as secondary sexual characteristics such as increased muscle mass and the growth of body hair. It is found in men and women, and is integral in regulating health and well-being, including maintaining bone mass.
Testosterone is a steroid, a large class of organic compounds that includes hormones, vitamins, and more. It is secreted by the testicles of males and the ovaries of females, though the levels are significantly higher in men (around 7-8 times as much). It can be used as a treatment for breast cancer in women, and older men can be prescribed testosterone treatments as levels in men decrease with age.
For adult men, testosterone is vital for sperm development as it activates Sertoli cells. These are a specialized cell type found in the testicles, specifically in the seminiferous tubules of the testes, and they facilitate the production of new sperm. Testosterone levels increase during sexual arousal in males and females, though to a greater degree in men.
Interestingly, both the act of falling in love and becoming a father correlate to decreased levels of testosterone in men, with the latter believed to be related to promoting paternal care.