The cardinal rule is that appearance of iron deficiency in an adult male is most often due to ...... until proven otherwise:
Nutritional deficiency
Gastrointestinal blood loss
Renal disease
Malabsorption of iron due to worm infestation
The cardinal rule is that the appearance of iron deficiency in an adult male means gastrointestinal blood loss. Reference: Harrisons Principles of Internal Medicine, 18th Edition, Page 846