What are the mechanisms of iron deficiency?

Mechanisms of Iron Deficiency

Iron deficiency develops through three primary mechanisms: inadequate dietary intake/absorption, increased physiological demands exceeding supply, and pathological blood loss—with hepcidin-mediated iron sequestration representing a distinct fourth mechanism in inflammatory states. 1, 2

Primary Mechanisms

1. Inadequate Iron Supply (Absolute Iron Deficiency)

Insufficient dietary intake is the most common cause globally, particularly affecting:

• Infants aged 4-18 months when maternal iron stores are depleted and dietary iron is inadequate 1
• Premenopausal women due to menstrual losses (the leading cause worldwide) 1, 3
• Populations with limited access to iron-rich foods 4

Malabsorption occurs through:

• Gastrointestinal disorders reducing iron uptake at the duodenal enterocyte level 5, 6
• Reduced expression of iron transport proteins (DMT1, ferroportin) on enterocyte surfaces 2

2. Increased Physiological Demands

Rapid growth periods create iron deficiency when stores cannot meet erythropoietic needs:

• Infancy (ages 9-18 months): Full-term infants deplete maternal iron stores by 4-6 months; preterm/low-birthweight infants deplete stores by 2-3 months due to lower initial stores and faster growth 1
• Adolescence: Rapid body growth increases total iron requirements 7
• Pregnancy: Increased maternal blood volume and fetal iron demands (30 mg/day requirement vs. 10 mg/day baseline) 3

3. Pathological Blood Loss

Chronic blood loss depletes iron stores progressively:

• Gastrointestinal bleeding (most common in men and postmenopausal women, including occult colon cancer) 6
• Menstrual blood loss exceeding dietary replacement 3
• Hemodialysis-related losses: 165 mL/year (conventional 3x/week) to 385 mL/year (daily dialysis) from residual blood in tubing and dialyzer 3
• Uremic enteropathy causing occult intestinal bleeding 3

4. Functional Iron Deficiency (Inflammation-Mediated)

Hepcidin overproduction blocks iron mobilization despite adequate stores 2, 8, 9:

• Inflammatory cytokines (IL-6, IL-1, TNF) stimulate hepatic hepcidin synthesis 8
• Hepcidin binds ferroportin on enterocytes and reticuloendothelial macrophages, causing internalization and degradation of this iron export protein 2, 8
• Result: Iron becomes sequestered in macrophages and hepatocytes, unavailable for erythropoiesis despite normal/elevated ferritin levels 8, 9

Clinical contexts where this occurs:

• Cancer-related anemia (cytokine-driven hepcidin elevation) 8
• Chronic kidney disease (decreased hepcidin excretion plus inflammation) 2
• Chronic heart failure (affects 40-70% of patients due to systemic inflammation) 9
• ESA therapy creating "kinetic iron deficiency" where erythropoietic demand exceeds iron mobilization capacity 9, 10

Pathophysiological Spectrum

Iron deficiency progresses through three stages 1:

1. Iron depletion: Reduced storage iron (low ferritin) without functional impairment
2. Iron-deficient erythropoiesis: Depleted stores plus reduced transport iron (low transferrin saturation), with increased erythrocyte protoporphyrin as iron becomes rate-limiting for heme synthesis 1
3. Iron-deficiency anemia: Severe shortage causing underproduction of hemoglobin, resulting in microcytic, hypochromic red blood cells 1

Regulatory Failure Mechanisms

Hepcidin dysregulation underlies several iron disorders 2:

• Genetic hemochromatosis: Deficient hepcidin synthesis causes iron overload
• IRIDA (iron refractory iron deficiency anemia): TMPRSS6 gene mutation causes unregulated hepcidin synthesis, creating refractory iron deficiency 3

The body cannot excrete iron in a regulated manner, making prevention of deficiency entirely dependent on adequate absorption matching losses 2. This physiological constraint makes iron balance particularly vulnerable during high-demand states or when absorption is impaired.

Critical Clinical Pitfall

Ferritin is an acute-phase reactant, making it unreliable in inflammatory conditions 8, 9. A patient with ferritin 60 ng/mL and active inflammation may actually have absolute iron deficiency, as the inflammatory state artificially elevates ferritin levels 8. In obesity-related inflammation, ferritin up to 100 μg/L may still represent true iron deficiency 11.