Giant Microbes, Testosterone Molecule, Small
Testosterone is a hormone that is the main driver in the development of increased musculature, body and facial hair, deeper voice, and other secondary sex characteristics typically seen in those assigned male at birth when they reach puberty. In utero it is involved in the development and growth of the penis and testes. Testosterone may also used by trans men and other trans-masculine people as part of their gender affirmation process.
The hormones commonly considered to be "sex hormones" in the body are testosterone, estrogen, and progesterone. Testosterone is often referred to as a "male" hormone, and estrogen and progesterone are often referred to as "female" hormones. However, it is interesting to note that no exclusively "male" or "female" hormones have been identified. All hormones characterized to date are present in all people regardless of sex, as are the receptor mechanisms that respond to those hormones.
In fact, the observation of the sexes we call "male" and "female" in nature is in part the result of differences in the amounts of individual hormones in the body and differences in their patterns of secretion (first in utero and then again during puberty) rather than their strict presence or absence. In other words, testosterone, estrogen, and progesterone are produced by all people, but in differing amounts and in different patterns.
Like all hormones, testosterone is a chemical messenger secreted by one body tissue that travels to another tissue to affect growth and behavior. Hormones are secreted in bursts and in amounts that change hourly and daily, as well as varying during stages of life or phases of fertility.
In people assigned male at birth, testosterone is mostly produced in the testicles. People assigned female at birth also produce low amounts of testosterone in their ovaries. All people produce a small amount of testosterone in the adrenal glands. The brain's hypothalamus and pituitary gland control testosterone production.
In addition to the functions listed above, testosterone also affects bone and muscle mass, the way fat is stored in the body, and even red blood cell production. For many people, low testosterone levels have been associated with feelings of depression or lowered libido.