Good thought.
My low hemoglobin turned up after a routine physical c. November 2006. My hemoglobin was 119. I went back for more bloodwork, to confirm it wasn't bad labwork. This time it was worse: 114.
Given my near-vegetarian diet and my regular blood donations, the doctor thought iron pills would be a good first step.
With iron supplementation, my hemoglobin bounced back quickly.
I had run a reasonably respectable leg of a marathon in mid-June of that year, and had done some intense hiking in the Rockies in July, but felt I had underperformed in both situations. I attributed it to getting older, but later figured I would have done much better with higher hemoglobin.
No problems since then. Cycled c. 110 km today and felt pretty good.