I'll describe a problem I'm going through now as we speak Rogue, my Rottweiler, was recently diagnosed with Addison's Disease :( this means his adrenal glands don't function at all and the only treatment available is hormone replacement therapy monthly and prednisone.
With hormone imbalance and daily use of steroid, I'm thinking irritability. Well, as soon as Rogue returned home after crashing (system shutting down) due to lack of electrolytes that are regulated by adrenal produced hormones, he began guarding his food. With otherwise a sweet, family-oriented dog, Rogue would growl. After corrections he got better but continues to guard. I moved the food outside on the deck and I'm having him eat there. So far, re-evaluating Rogue's obedience has helped keep the situation manageable. Sitting before fed. Stay and down commands and more adherence to the "first time I tell you" doctrine, has improved the situation. Guarding is difficult. The point is, if you're having behavior problems, don't forget to first look at obedience.