The normal range of testosterone in men is between 300 and 1100 nanograms per deciliter of blood, which is a fairly wide range of “normal.” Levels anywhere in this range usually indicate a normal degree of sexual desire and support normal masculine physical and personality traits. Only when men have either very low or very high testosterone levels are physical or mental changes noticeable.
Men with levels below 300 nanograms, which is a condition called hypogonadism, tend to have little interest in sex and are often non-confrontational, socially inhibited and physically weak. Men with higher-than-normal testosterone levels tend to be just the opposite: obsessed with sex, competitive, aggressive, extroverted, physical and tending toward action-oriented activities or careers. But within the normal range, testosterone levels play only a background role and other aspects of personality dominate.
Whatever a man’s basic level, his testosterone fluctuates widely over the course of a day, with peak levels occurring in early morning, and often dropping by 30 to 40 percent by mid-afternoon.
Testosterone levels usually begin a slow downhill slide of about 1% a year starting around age 30. This adds up over the years. Men with lower but still normal testosterone levels such as 400 nanograms per deciliter might hit the threshold of clinically significant testosterone loss by age 55. The signs of below-normal testosterone levels include fatigue, depressed mood, low or absent sex drive, muscle weakness, and a general feeling of malaise.
Due to a man's low interest in sex, testosterone is a disorder related to Male Infertility. However, testosterone, in and of itself, does not directly affect fertility.