The causes of depression are not fully understood, but some studies suggest that an imbalance in the brain's signaling chemicals may be responsible for the condition in many patients. Moreover, a variety of distressing life situations such as substance abuse, early childhood trauma, a job loss, the death of a loved one, financial troubles or a divorce can trigger depression. Certain medical conditions like underactive thyroid gland, cancer, heart disease or prolonged pain may also trigger depression. Most likely, depression is caused by a combination of genetic, biological, environmental, and psychological factors.