Controlling secondary sexual characteristics in men (including muscle mass, deep voice, and body hair) and the sex drive, testosterone is produced in the testicles. With age the concentration of testosterone decreases, and when it reaches less than 300ng/dL it is considered low. There are also possible other causes for low testosterone including (but not restricted to) injury, infection, HIV, diabetes, alcohol abuse, cancer, thyroid problems or as a side effect of other medication.
A low concentration of testosterone can cause several symptoms such as a decreased sex drive, erectile dysfunction, decreased sperm count, hair loss, fatigue and insomnia.
To get an accurate diagnosis of low testosterone you should visit your GP for a blood test.