As we age, our body changes and our sex life may become less satisfying. Seemingly we may lose interest in sex. The barrier is not just psychological. The testosterone that once made your body virile, may be binding to globulins in your blood instead of stimulating the sex centers in your brain. When naturally abundant and free, testosterone can be a vital source of energy, stamina and sexual virility. When deficient, it can cause fatigue, loss of muscle mass, increased body fat and decreased sexual drive.