Summarize the following:
One of the main ways in which low testosterone can manifest is as a low sexual desire and/or reduced spontaneous erections, or trouble with erections overall. Testosterone naturally declines in men as they age (testosterone levels decrease by about 1% per year after the age of 30 or 40). However, if you are noticing a significant decline in your sexual function, it is advisable to speak to your doctor about the possibility that you may have low testosterone. Sexual function is measured by the frequency of your orgasms and sexual satisfaction. Low testosterone can also lead to problems sleeping and even insomnia. It may cause heightened daytime fatigue and an overall reduced energy level. If you notice these things happening to you, book an appointment with your family doctor, as they may be correlated with reduced or low testosterone. Low testosterone may contribute to depression, irritability, and/or difficulty concentrating. Testosterone plays a significant role in controlling mood and emotional states. Therefore, if you feel "off" emotionally and like your moods have declined, there is the possibility that this is related to low testosterone. One study showed that testosterone can function as an antidepressant in men with depression and low testosterone. If you have unexplained hair loss or an unusual decline in your body's strength and muscle mass along with an increase in fat mass, this may be a sign that your testosterone levels are low. It is not a guarantee that the two are correlated, but it is worth exploring with your family doctor.

Summary:
Observe changes to your sexual function. Make note of changes to your sleep and energy levels. Be aware of changes to your mood. Notice changes to your physical body.