The biggest problem with adding synthetic testosterone to the system is that the body stops being able to

Synthetic testosterone was applied to men with significant medical condition who have low levels of testosterone. Moreover, it is also used by millions to enhance sexual desire, energy and mood. Also treat weak erections and osteoporosis for it help increase bone density.  

However, adding synthetic testosterone to the body system produced a tendency that the body will stop being able to make natural testosterone and these can be felt when men stop testosterone therapy.