By far, the most common cause of asymptomatic anemia is iron deficiency resulting from the inadequate dietary replacement of iron lost from the body. Daily iron requirements for men and postmenopausal women are between 0.5 and 1 mg. Because additional iron is needed by menstruating and pregnant women, their daily requirements are 2 and 2.5 mg, respectively. Because only 5% to 10% of the 10 to 20 mg of the iron contained in the average adult diet is absorbed, it is not surprising that iron deficiency is common in women of childbearing age. Population studies have found that 10% to 20% of menstruating women have abnormally low concentrations of hemoglobin (usually <12 g/100 mL). Between 20% and 60% of pregnant women have hemoglobin levels less than 11 g/100 mL. Anemia is less likely to occur in women taking birth control pills and more likely to occur in women with intrauterine devices. Iron deficiency is rare in male adults; if present, it is a clear indication for diligent investigation of the gastrointestinal tract. Absorption of iron may be decreased after gastrectomy or in the presence of achlorhydria. Sideroblastic and megaloblastic anemias are much less common. The prevalence of pernicious anemia, the most common form of vitamin B₁₂ deficiency, is 0.1% in persons of northern European extraction. Pernicious anemia is much less common among other ethnic and racial groups. Folate deficiency is common during pregnancy and in patients with alcoholic liver disease, in whom it is often accompanied by sideroblastic anemia. Anticonvulsant drugs, including phenytoin, primidone, and phenobarbital, may interfere with folate absorption, so megaloblastic anemia results. Thalassemia minor is a common cause of mild anemia in patients of Mediterranean or east Asian or southeast Asian extraction. Sickle cell disease and trait, by far the most common forms of hemoglobinopathy, are discussed separately (see Chapter 78).

Erythropoietin deficiency represents an important and treatable cause of anemia and debility in persons with chronic kidney disease (see Chapter 142). Poor responsiveness to erythropoietin in chronic kidney disease patients has in some instances been associated with low testosterone levels, leading to the suspicion that hypogonadism may be an important risk factor for anemia not only in chronic kidney disease but also in other settings where testosterone levels might be low (e.g., antiandrogen therapy for prostate disease, advanced age). In a population study of elderly persons without anemia, low levels of total and free testosterone were common and independently associated with increased risk of developing anemia over 3 years (relative risk 2.1). Whether this purported association represents a proxy for other anemia-producing illness(es) or is indicative of low testosterone being an important independent risk factor for anemia remains to be fully established.