Fatigue is a symptom with many possible explanations. Need to define what patient means by fatigue: e.g. tired, lack of energy, muscle weakness, depressed, etc→ use further questions to identify etiology. Risk factors (e.g. age, sex), past history, meds and other historical clues should help to further categorize. Occasionally, no root cause(s) for fatigue is/are identified, in which case it's called a Medically Unexplained Symptom.