Does your partner seem like the proverbial grumpy old man these days? There may be a physiological reason for that crankiness. If you feel like your man is exhibiting that perma-scowl, wagging finger and "Get off my lawn!" proclamation, then he may suffer from low testosterone, experts say. The hormone testosterone is responsible in both men and women for supplying energy, reducing fat, building muscle and increasing libido. When there are inadequate levels of testosterone, then the most apparent outward symptoms can be irritability and mood swings. Experts say concentration can be affected as well. Metabolic conditions such as kidney disease and diabetes can exacerbate the problem, as can certain drugs like opiates. If your guy is exhibiting the symptoms of low testosterone, then it's a good idea for him to get a complete physical workup.