You have anemia testing mostly for "loss" of red blood cells.
What should also be investigated is:
- low red blood cell production
-destruction of red blood cells
-low iron intake or absorption
There are many conditions and imbalances which may contribute to or cause anemia, which are not associated with loss of blood.
The iron supplementation was an effective trial and because it worked, it indicates that the cause is likely related to low iron.
I would bet on this as a cause, however, the question for you remains unanswered, since you mentioned the iron pills masked the anemia!
What is/was causing the suspected iron deficiency?
-Is it low intake of iron-rich foods?
-Is it low Vitamin C and/or Vitamin A and/or Beta carotene, casing low iron absorption?
-Is it too much phytic acid interfering with iron absorption?
Also your doctor could have had your thyroid function tested (not just an Ultrasound) by checking Free T3, FreeT4 and Reverse T3.
But if you have no related symptoms, you can forgo further thyroid testing.
Food for thought.
I hope this helps.