According to MedicineNet, most of men's testosterone is produced in their testicles. Much lower amounts are also made in women's ovaries. Testosterone is needed in men for the development of secondary sex characteristics, such as chest hair and voice deepening.
The hormone also plays an important factor in men's sex drive, energy level and muscle growth. Testosterone levels in men are greatest at about 20 years of age. After that, production typically decreases. Medline Plus indicates that testosterone levels are sometimes measured in males due to early onset or delay of puberty, impotence, low sex drive or infertility. In women, tests are often given if there are physical signs that too much testosterone is being produced, such as unusual hair growth, missing menstrual periods or thinning hair.