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A mood swing describes a notable change in immediate behavior that is not the result of an external stimulus and that appears contrary to the person's normal personality.

Mood swings can go in either direction. A calm person can become angry with no warning or stimulus and then, just as quickly, can go from anger to being calm again.

Moodiness and anger are typical personality traits that respond to stimuli. For example, House gets angry when his Vicodin is taken away from him. Such behavior is normal.

However, with a mood swing, an individual goes from one behavior to another without an intervening event. For example, in the episode Heavy, the patient goes from polite to combative during a test.

Mood swings can indicate that a person has a serious underlying illness, often a mental illness, but often an organic illness. For example, porphyria can cause mood swings. However, one of the difficulties in establishing a diagnosis is that mood swings rarely occur in clinical settings. Individuals who know the patient will pick up on their sudden behavioral changes, but doctors and other strangers will either see the behavior as the patient's normal behavior, will not observe the behavioral change (as mood swings are often transitory in nature) or will write off the mood change as a consequence of being examined by the physician. For example, in Honeymoon, Stacy told House about her husband Mark's mood swings, but Mark put them down to being around House.

Mood swing at Wikipedia