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This is Dr. David
you could be loosing microscopoic blood in your stool which you can't see
you need a stool occult blood test to check for occult blood in your stool you can't see.
that is good you are getting a colonoscopy.
the stool is the most common place to loose blood and iron from.
most likely the low testosterone is not causing your low iron levels
let me know if you have questions.