Though not entirely understood, the link between testosterone and a wide variety of health problems is one of the most interesting discoveries in men\u2019s health. Testosterone is a steroid hormone that humans produce naturally. It is generally thought of as a male hormone, although it is actually produced by both men and women. Men do, however, produce an average of 20 times more testosterone than do women and use it up at a much higher rate. Testosterone is the most important hormone for male sexual development. It plays a big role in the growth of primary reproductive organs, as well as the development of so-called secondary sexual characteristics, such as body hair and muscle mass. In addition, testosterone seems to have a strong connection to overall male health, although the exact nature of this connection is still something of a mystery.\r\nCause or Effect\r\nResearch has begun to identify a number of serious medical conditions that have some connection to a man\u2019s testosterone levels. Low testosterone is a condition that affects about 30 percent of men 55 years of age and older. Men suffering from diabetes, high blood pressure or obesity were all more likely to have abnormally low testosterone levels. The reverse is also true: men with low testosterone levels are more likely to have or develop diabetes or obesity. However, the way in which these conditions are connected to low testosterone is something researchers are still working on. One possibility is that low testosterone levels somehow increase a man\u2019s likelihood of developing one of these or several other major health problems. An equally likely possibility is that conditions such as diabetes and obesity, or just poor health in general, may result in the lowering of an individual\u2019s testosterone levels. There are other possibilities as well, including the chance that both low testosterone and the conditions that have been linked to it are all caused by some other factor or factors.\r\nDepression and Low Testosterone\r\nDepression is another of the major health problems that has been strongly linked to low testosterone levels. Like conditions such as diabetes and obesity, men with low testosterone have been found to be more likely to have or develop depression, and men with depression are more likely to have low testosterone levels. As with other conditions, the exact cause and effect relationship between these health problems\u2014if there is one\u2014has yet to be pinned down. One confusing factor is that some of the known effects of low testosterone are also symptoms, or can appear to be symptoms, of depression. When men have low levels of this important hormone they will often experience fatigue, increased irritability and a decreased sex drive. These symptoms are also common symptoms of depression. A study published in 2004 attempted to shed more light on this relationship and to help medical professionals get a better idea about whether low testosterone is really a risk factor for depression. The study, led by Dr. Molly M. Shores, followed 278 men over the age of 45 for a period of four years. At the conclusion of the study, Dr. Shores and her research team found that the men who had low testosterone levels were four times as likely to be diagnosed with clinical depression as the men who had normal levels of testosterone. There are several possible explanations for the high rate at which depression affects men with low testosterone. One possibility is that some of the known symptoms of low testosterone, such as irritability and decreased sex drive, may develop into true depression because they cause a person\u2019s mood to suffer. A study of low testosterone in mice suggests another possibility: that testosterone levels may actually directly affect serotonin levels in the brain. Serotonin is another natural hormone and one of its many roles is to help regulate our moods. As people get older, their ability to absorb the serotonin that their bodies produce decreases. If low testosterone lowers serotonin levels, it may help to explain why low testosterone in older men seems to put them at particular risk for depression.