Testosterone is a hormone produced mainly in men, but also present in women and children. It plays an important role in the development of male characteristics such as facial hair growth, deepening of voice, muscle mass, bone density, and sex drive. Testosterone levels can be measured through blood tests to determine if a person has low or high levels of this hormone. Low testosterone levels may indicate various health issues while high levels could also have negative effects on the body such as aggressive behavior, acne, and early puberty in children.