Dr. Baistuff's easy approach to anemia which can be very frustrating.....
Despite all the causes of anemia, there are really only 3 ways to become anemic.
1) You don't make red blood cells (or enough of them)
2) You destroy red blood cells (hemolysis)
3) you lose red blood cells (bleeding)
Any and every cause of anemia will fall into 1 of these 3 categories. 3) is the simple one. Either someone is bleeding (GI, GU- including periods, retroperitoneal etc..) or they aren't.
2) Hemolysis- destruction- can occur via the immune system (spherocytosis, TTP, etc..) infection, meds, other. Usually detectable on labs ( check for LDH, haptoglobin, fibrinogen, and often the destruction will increase bilirubin as well having nothing to do with the liver. )
3) Inabilty to make (enough) red blood cells. VERY complicated. If there is not enough iron, b12, erythropoetin ( hormone made in the kidney) poor bone marrow function- due to many conditions ranging from benign to deadly, underactive thyroid, medications that stun or slow marrow production (chemotherapies, even some antibiotics,) then anemia is likely.
The other thing to keep in mind is what we call Anemia of Chronic Disease or inflammatory anemia. Often in chronic conditions- autoimmune, kidney or liver disease, cancers, and many many others, the ability for the body to keep up the process (and its a constant one since red cells live only 120 days in healthy folks) just isn't there, and people are chronically anemic.
A proper workup- check the CBC. If hgb/hct are low, then look at MCV- MCV can give a lot of information. If low, usually indicates low iron, or if very low an underlying thallasemia trait which impairs hemoglobin sysnthesis. If MCV is high, it can mean b12 or folate deficiency, chronic alcohol (poisons the marrow) liver disease, pregnancy. If the MCV is normal, then consider things like medication induced, thyroid disease, and the catch all "Chronic Disease."
So after looking at the MCV, check iron, TIBC, ferritin (not always helpful,) TSH, b12, folate, LDH, haptoglobin. Then look at meds, exposures (alcohol, lead etc...) Can do a hemoglobin electropharesis to rule out a thallasemia or other hemoglobinopthy, and finally, and ONLY if suspicious of a primary bone marrow problem, a bone marrow biopsy.
I would say on this board, this vast majority of folks who are anemic and NOT bleeding and with NORMAL irons, probably are anemic due to just 'anemia of chronic disease/ inflammatory anemia" or due to medications.
Can be frustrating. Good luck.