Major depressive disorder (MDD) has been ranked as the third cause of the burden of disease worldwide in 2008 by WHO, which has projected that this disease will rank first by 2030. It is diagnosed when an individual has a persistently low or depressed mood, anhedonia or decreased interest in pleasurable activities, feelings of guilt or worthlessness, lack of energy, poor concentration, appetite changes, psychomotor retardation or agitation, sleep disturbances, or suicidal thoughts. Per the Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5), an individual must have five of the above-mentioned symptoms, of which one must be a depressed mood or anhedonia causing social or occupational impairment, to be diagnosed with MDD. History of a manic or hypomanic episode must be ruled out to make a diagnosis of MDD. Children and adolescents with MDD may present with irritable mood. Per DSM-5, other types of depression falling under the category of depressive disorders are: Persistent depressive disorder, formerly known as dysthymia. Disruptive mood dysregulation disorder . Premenstrual dysphoric disorder. Substance/medication-induced depressive disorder. Depressive disorder due to another medical condition. Unspecified depressive disorder.
重度抑郁症（MDD）在2008年已被世界卫生组织列为全球疾病负担的第三大原因，预计到2030年该疾病将升至首位。当个体持续情绪低落或沮丧、快感缺失或对愉悦活动的兴趣降低、有内疚感或无价值感、缺乏精力、注意力不集中、食欲改变、精神运动性迟缓或激越、睡眠障碍或有自杀念头时，即可诊断为重度抑郁症。根据《精神疾病诊断与统计手册》第5版（DSM-5），个体必须具备上述症状中的五条，其中一条必须是导致社交或职业功能受损的情绪低落或快感缺失，才能被诊断为重度抑郁症。诊断重度抑郁症时必须排除躁狂或轻躁狂发作史。患有重度抑郁症的儿童和青少年可能表现为易激惹情绪。根据DSM-5，属于抑郁障碍类别的其他类型抑郁症包括：持续性抑郁障碍，以前称为心境恶劣障碍。破坏性心境失调障碍。经前烦躁障碍。物质/药物所致抑郁障碍。由另一种躯体疾病所致的抑郁障碍。未特定的抑郁障碍。
