Western medicine treats Depression as a chemical imbalance in the brain. TCM sees this as a meridian imbalance involving heart and kidneys. Chi is blocked, chi is unhealthy. Balance the chi, balance the meridians, and the problem resolves itself.
More info on Depression on Wikipedia:
Depression (also called major depressive disorder, unipolar depression or clinical depression) is a diagnosis listed in the DSM 5 as a mood disorder describing people who feel sad and pessimistic. Most people feel sad, discouraged, or “down” sometimes, but that is not depression. In clinical depression, these feelings last for a long time and are combined with other symptoms. Someone with depression might have these symptoms: strong sad feelings or no strong feelings at all, feeling guilty, feeling helpless or hopeless, anxiety, fear, low self-esteem or a loss of interest in life. These are symptoms of depression when they last for a long time (usually for more than two weeks). Most people who have not had depression do not completely understand its effects. Instead, they see it as simply being sad. Since it is not understood, many people with depression are criticized by others for not helping themselves. Some people with depression commit suicide; today, it is thought that over half the people who committed suicide had suffered from depression at least once.