Produced primarily by the testicles, testosterone is the hormone responsible for developing male sexual traits and maintaining muscle mass, bone density and red blood cell levels. Testosterone levels peak in adolescence and early adulthood then begin to decline with age, typically at a rate of 1 to 2 percent per year after age 30. Testosterone levels influence physical, emotional and sexual well being, with higher testosterone generally having a favorable effect on attitude and performance. Though increasing testosterone can have benefits, changes to testosterone levels can affect hormonal production elsewhere in the endocrine system, so consult a doctor prior to attempting to raise your testosterone.
Increased testosterone can have an impact on body composition. Possible benefits include gains in lean muscle mass, reduced body fat and increased bone density. Testosterone inhibits uptake of triglycerides and increases lipid mobilization from adipose tissue, and the increase or decrease of testosterone will usually have an inverse effect on fat stores, with higher testosterone generally causing a decrease in body fat. "The Journal of Clinical Endocrinology and Metabolism" published a study in 2007 that showed decreases in body fat and increases in lean mass in HIV-positive obese men given testosterone therapy. In 1989, a study of the effects of testosterone on muscle mass at the University of Rochester School of Medicine and Dentistry suggests that increasing testosterone increases protein synthesis in muscles. Body composition changes from increased testosterone were also demonstrated in a 1999 study at the School of Exercise Science and Sports Management, Southern Cross University in Australia performed on male weight-training subjects, which showed increases in arm girth and body weight and decreased body fat following a 12-week cycle of testosterone enanthate.
Testosterone replacement therapy can successfully treat erectile dysfunction and loss of libido in men with low testosterone from either advancing age or hypogonadism. Although the effects of increased testosterone are more dramatic in hypogonadal men there are also benefits to the libido of men with normal gonadal, also called eugonadal, function. In a 2004 study published in the "Journal of Endocrinology and Metabolism," researchers found that increasing peak testosterone levels to between 400 and 500 percent above baseline in subjects resulted in a significant increase in sexual arousability over placebo subjects.
Low testosterone levels have a dramatic effect on emotional state. The American Academy of Family Physicians lists depression, impaired cognition and increased fatigue as symptoms of low testosterone production, or hypogonadism. Testosterone replacement therapy for hypogonadal men has successfully reduced negative moods relating to fatigue and depression while increasing feelings of self-esteem. Increasing testosterone in eugonadal men has also shown positive emotional benefits such as increased feelings of self-esteem and reduction of fatigue. The intensity of these benefits is dependent on dosage; a wide body of literature on testosterone increase has shown that at higher dosage aggression, and aggressive response, can become pronounced.