By measuring testosterone levels in the blood, low testosterone can be diagnosed. Since testosterone levels tend to fluctuate throughout the day, it may take several measurements to determine whether a patient has low testosterone. Most testosterone levels are highest in the early morning, near 8 a.m. It is for this reason that doctors prefer to measure testosterone levels early in the morning. As men age, testosterone levels decrease gradually, a process known as andropause or male menopause. Most men don't experience any significant symptoms as a result. Some people may notice that their muscle mass is declining, that they are depressed, or that they are less interested in sex. Males with low testosterone can develop large breasts, smaller, softer testicles, thinner muscles (this happens gradually over time), and loss of body hair (also slow, usually over time). Osteoporosis is often caused by low testosterone levels. The bones may become thinner, weaker, and more prone to breaking as testosterone levels fall. In some cases, a drop in testosterone does not interfere with sex, but it can make it more difficult for your body and brain to become aroused. Some men may notice a decrease in libido, while others may lose interest in sex entirely. Erections can also be harder to get or maintain when testosterone levels are low. Men sometimes have subtle problems like mood changes, difficulty concentrating, and low energy. It is also possible for these symptoms to be caused by other health concerns, such as anemia, depression, sleep issues, or chronic illnesses.