Depression is a mood disorder wherein feelings of sadness, loss, anger, or frustration interfere with everyday life for a long period of time. The exact cause of depression is not known. Many researchers believe it is caused by a chemical imbalance in the brain, which may be hereditary or caused by events in a person's life. Other risk factors include alcohol or drug abuse, social isolation, medical conditions such as hypothyroidism, cancer, major illness, or prolonged pain.