Fatigue

Hypothyroidism
Anemia
Depression
Sleep apnea or insufficient sleep
Infection, eg, tuberculosis, hepatitis, endocarditis, HIV, Lyme disease
Diabetes mellitus
Congestive heart failure, chronic obstructive pulmonary disease, or chronic renal failure
Cancer
Alcoholism
Hypercalcemia
Drugs, eg, sedatives, beta blockers
Chronic fatigue syndrome
Somatoform disorder (eg, somatization)
Fibromyalgia
Mononucleosis
Autoimmune disease
Adrenal insufficiency
Iron deficiency even in absence of anemia