Testosterone is a sex hormone produced by the testes in men. Women also produce testosterone in their ovaries and adrenal glands, but usually in smaller quantities. Testosterone governs many essential body functions, such as sex drive (libido), energy levels, healthy bones, muscle mass, strength, and mood. It also controls sperm production, so changes in this hormone can affect fertility. Stress levels, how much alcohol you drink, and the amount of exercise you do can all affect the amount of testosterone your body makes. Low testosterone levels can sometimes cause unwanted symptoms, such as low sex drive and low mood. Both men and women may experience a natural decrease in their testosterone levels with age.