Testosterone is a hormone that plays a vital role in aspects of male health including the production of sperm and red blood cells, the accumulation and maintenance of muscle strength and muscle mass, and the typical distribution of body fat. Testosterone also appears to play a role in decreasing male risks for developing depression. In line with this viewpoint, several studies indicate that men who develop depression sometimes have unusually low testosterone levels. Depressed men with low testosterone often experience a decline in their depression symptoms when they receive a treatment known as testosterone therapy. Testosterone Basics Although testosterone is frequently viewed as an exclusively male hormone, women also have relatively small amounts of the hormone in their bloodstreams. While teenage boys and men produce most of their testosterone supplies in their testicles, women produce most of their supplies in their ovaries. According to the US National Library of Medicine, testosterone levels in postpubescent males normally range from 300 to 1,000 nanograms per deciliter of blood (ng\/dL). Testosterone levels in postpubescent females normally range from 15 to 70 ng\/dL. Testosterone-related problems in men typically appear when levels of this hormone fall too low; when men only produce very small amounts of testosterone, or no testosterone at all, they have a condition called male hypogonadism. Potential underlying causes of hypogonadism in men include immune system dysfunction, kidney disease, liver disease, infections, steroid use, certain hereditary disorders (including a condition called Klinefelter syndrome), lack of proper nutrient intake, radiation exposure, surgical complications, certain traumatic injuries and quick, substantial increases in body weight. Testosterone and Depression In the United States, men develop depression roughly half as often as women. Current scientific evidence indicates that maintenance of an adequate testosterone level is a vital factor in keeping male depression levels relatively low. For example, in a study published in 2005 in the Journal of Clinical Psychiatry, a team of researchers from the US Department of Veterans Affairs examined the connection between low testosterone levels and future depression risks in more than 700 men who were at least 50 years old. The authors of the study concluded that men in this age group with low testosterone develop depression at an earlier average age than men with normal testosterone levels. They also concluded that men with low testosterone develop depression more often than their age counterparts with normal testosterone levels. In another study, published in 2003 in the American Journal of Psychiatry, researchers from Harvard University examined the connection between low testosterone levels and depression symptoms in a group of 56 men with major depression who failed to respond to standard antidepressant treatment. The authors of the study found that over 40 percent of the men who failed to respond to antidepressants had unusually low testosterone levels. Effects of Testosterone Therapy Testosterone therapy, also known as testosterone replacement therapy, is a medical treatment normally used to artificially boost testosterone levels in people affected by hypogonadism. Depending on individual circumstances, methods used to administer this treatment may include the application of testosterone-containing gel on the midsection, shoulders or upper arms; attachment of testosterone-containing patches to parts of the body such as the scrotum, midsection, back or buttocks; or injection of testosterone directly into muscle tissue. In a study review published in 2009 in the Journal of Psychiatric Practice, a team of researchers from the West Virginia University School of Medicine examined the available scientific evidence regarding the effectiveness of testosterone therapy as a treatment for depression. After reviewing the results of seven different well-designed studies, the authors of the review concluded that testosterone treatment does indeed play a beneficial role in easing depression symptoms, particularly in men diagnosed with hypogonadism, older adults, and people diagnosed with HIV\/AIDS. In a study published in 2012 in the journal Biological Psychiatry, a team of Florida State University researchers used animal testing to investigate the reasons testosterone therapy helps relieve depression symptoms. The authors of this study concluded that testosterone probably produces its depression-related benefits by altering activity in a part of the brain called the hippocampus. This brain region plays a vital role in normal memory function, as well as the human ability to control the harmful effects of stress exposure. The study\u2019s authors believe that identification of testosterone\u2019s effects on the hippocampus may help future researchers design better depression treatments.