Depression is a state of low mood and aversion to activity that can affect a person's thoughts, behavior, feelings and sense of well-being. Depressed people can feel sad, anxious, empty, hopeless, worried, helpless, worthless, guilty, irritable, hurt, or restless. They may lose interest in activities that once were pleasurable, experience loss of appetite or overeating, have problems concentrating, remembering details, or making decisions, and may contemplate, attempt, or commit suicide. Insomnia, excessive sleeping, fatigue, loss of energy, or aches, pains, or digestive problems may also be present.
Depressed mood is not always a psychiatric disorder. It may also be a normal reaction to certain life events, a symptom of some medical conditions, or a side effect of some drugs or medical treatments. Depressed mood is also a primary or associated feature of certain psychiatric syndromes such as clinical depression.
Lifestyle factors that may play a role in depressed moods include irregular sleep, poor diet, and lack of exercise.
Life events and changes that may precipitate depressed mood include childbirth, menopause, financial difficulties, job problems, a medical diagnosis (cancer, HIV, etc.), bullying, loss of a loved one, natural disasters, social isolation, relationship troubles, separation, and catastrophic injury.
Certain medications are known to cause depressed mood in a significant number of patients. These include hepatitis C drug therapy and some drugs used to treat high blood pressure, such as beta-blockers or reserpine.
Depressed mood can be the result of a number of infectious diseases, neurological conditions  and physiological problems including hypoandrogenism (in men), Addison's disease, Lyme disease, multiple sclerosis, chronic pain, stroke, diabetes, cancer, sleep apnea, and disturbed circadian rhythm. It is often one of the early symptoms of hypothyroidism (reduced activity of the thyroid gland). For a discussion of non-psychiatric conditions that can cause depressed mood, see Depression (differential diagnoses).
A number of psychiatric syndromes feature depressed mood as a main symptom. The mood disorders are a group of disorders considered to be primary disturbances of mood. These include major depressive disorder (MDD; commonly called major depression or clinical depression) where a person has at least two weeks of depressed mood or a loss of interest or pleasure in nearly all activities; and dysthymia, a state of chronic depressed mood, the symptoms of which do not meet the severity of a major depressive episode. Another mood disorder, bipolar disorder, features one or more episodes of abnormally elevated mood, cognition and energy levels, but may also involve one or more depressive episodes. When the course of depressive episodes follows a seasonal pattern, the disorder (major depressive disorder, bipolar disorder, etc.) may be described as a seasonal affective disorder.
Outside the mood disorders: borderline personality disorder commonly features depressed mood; adjustment disorder with depressed mood is a mood disturbance appearing as a psychological response to an identifiable event or stressor, in which the resulting emotional or behavioral symptoms are significant but do not meet the criteria for a major depressive episode;:355 and posttraumatic stress disorder, an anxiety disorder that sometimes follows trauma, is commonly accompanied by depressed mood.
Many psychological assessments related to testing for depression and/or for the severity of depressive symptoms exist, assessments such as the Beck Depression Inventory and Children's Depression Inventory test for depression and/or depressive symptoms.
Depressed mood may not require any professional treatment, and may be a normal reaction to certain life events, a symptom of some medical conditions, or a side effect of some drugs or medical treatments. A prolonged depressed mood, especially in combination with other symptoms, may lead to a diagnosis of a psychiatric or medical condition by a counselor or doctor, which may benefit from treatment. Different sub-divisions of depression have different treatment approaches.
Over-diagnosis of depression may be common, because while the prevalence of major depressive disorder has remained unchanged, its diagnosis has doubled in 20 years. Depression is more likely over-diagnosed than under-diagnosed in primary care, resulting in patients without evidence of major depressive disorder being prescribed medications. The elderly may be at more risk of over-diagnosis.
Given an accurate diagnosis of major depressive disorder, in general the type of treatment (psychotherapy and/or antidepressants, alternative therapies, or active intervention) is "less important than getting depressed patients involved in an active therapeutic program."
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