Lord knows, I don't have any expertise in any of this. I just pulled the Merck Medical Manual off the shelf and gave it a look.
This is what it said. If you're iron is slowly falling, it's because you're not ingesting enough, or you're not absorbing it, or you're losing it through occult blood loss.
Of these possibilities, the most catastrophic of them is slow blood loss from occult colon cancer.
You're 51. Have you ever had a colonoscopy?
Occasionally iron malabsorption is the first manifestation of celiac disease, not rare among white folk.
That's what the manual said. It's not meant to be medical advice.