When you think of mood disorders, depression and bipolar disorder likely come to mind first. That's because these are common, severe illnesses and leading causes of disability. Depression and bipolar disorder can be emotionally crippling, making it difficult to live life to its fullest. What you may not know is that two milder versions of these mood disorders can also take a toll, and can go undiagnosed. These are called dysthymic disorder and cyclothymic disorder.