The possibility that fatigue may be an early manifestation of a serious medical illness not elicited through blood testing has been investigated by monitoring fatigued patients prospectively over time for the development of new diagnoses. At 1 year of follow-up, one study 3 showed no significant differences in new diagnoses, the incidences of diagnosed cancers, number of physician visits, or number of hospital admissions or hospital days.
This gives more weight to the conclusion that patients with chronic fatigue who otherwise have normal histories and no abnormalities on physical examination do not require extensive workups for occult medical illnesses.
You can read the full article here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071607/http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071607/
Source: Stevens DL. Chronic fatigue. West J Med. 2001 Nov;175(5):315-9. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1071607/ (Full article)