Testosterone plays a key role in metabolism, physical performance and mental well-being. A deficiency can lead to fatigue, sexual problems and other health issues. Doctors explain the path from blood tests to diagnosis - and why obesity, lack of exercise and stress can lower hormone levels. Alongside lifestyle changes, short-term testosterone therapy may help break a harmful cycle. Also on our men’s health episode: The little-known functions, and diseases, of the prostate gland, new approaches