Gastrointestinal malignancy: Despite a normal colonoscopy 8 months ago, the possibility of a new or missed lesion, especially in the proximal colon or small intestine, cannot be ruled out. Iron deficiency anemia in men, particularly at this age, often points towards a gastrointestinal source of bleeding.

Gastritis or peptic ulcer disease: These conditions can cause chronic blood loss leading to iron deficiency anemia. The recent colonoscopy does not rule out upper GI pathology.
Angiodysplasia: These are vascular malformations in the GI tract that can cause chronic, occult bleeding. They are more common in the elderly and can be a cause of iron deficiency anemia.
Inflammatory bowel disease (IBD): Although less common in new diagnoses at this age, IBD (Crohn's disease or ulcerative colitis) can cause chronic blood loss and iron deficiency anemia.

Gastric cancer: Even though the patient had a recent normal colonoscopy, gastric cancer can present with iron deficiency anemia due to chronic bleeding. It's crucial to investigate the upper GI tract.
Small intestine tumors: These can be difficult to diagnose and may present with iron deficiency anemia due to chronic blood loss.
Dieulafoy's lesion: A rare cause of severe GI bleeding, but it can present with iron deficiency anemia if the bleeding is chronic and intermittent.

Hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu syndrome): A genetic disorder that leads to the formation of abnormal blood vessels, which can cause chronic bleeding and iron deficiency anemia.
Blue rubber bleb nevus syndrome: A rare condition characterized by venous malformations that can cause GI bleeding and iron deficiency anemia.
Celiac disease: Although more commonly associated with malabsorption, celiac disease can also lead to iron deficiency anemia due to mucosal damage and impaired iron absorption.