Dysthymia, also known as persistent depressive disorder (PDD), is a mental and behavioral disorder, specifically a disorder primarily of mood, consisting of similar cognitive and physical problems as major depressive disorder, but with longer-lasting symptoms.

As dysthymia is a chronic disorder, those with the condition may experience symptoms for many years before it is diagnosed, if diagnosis occurs at all. As a result, they may believe that depression is a part of their character, so they may not even discuss their symptoms with doctors, family members or friends.

Dysthymia characteristics include an extended period of depressed mood combined with at least two other symptoms which may include insomnia or hypersomnia, fatigue or low energy, eating changes (more or less), low self-esteem, or feelings of hopelessness. Poor concentration or difficulty making decisions are treated as another possible symptom. Irritability is one of the more common symptoms in children and adolescents.

Read more about it here. [en.wikipedia.org]