Endogenous depression is a type of major depressive disorder (MDD). Although it used to be seen as a distinct disorder, endogenous depression is now rarely diagnosed. Instead, it’s currently diagnosed as MDD. MDD, also known as clinical depression, is a mood disorder characterized by persistent and intense feelings of sadness for extended periods of time. These feelings have a negative impact on mood and behavior as well as various physical functions, including sleep and appetite. Nearly 7 percent trusted Source of adults in the United States experience MDD each year. Researchers don’t know the exact cause of depression.