Does inflammation cause functional iron deficiency by increasing hepcidin and trapping iron?

The physiological mechanism linking inflammation to iron metabolism is well-documented and forms the pathological basis for Anemia of Inflammation (AI), also known as anemia of chronic disease.

Mechanistic explanations

The process is driven by the hepcidin-ferroportin axis, which acts as the master regulator of systemic iron homeostasis:

• Hepcidin Induction: During inflammatory states, cytokines—most notably Interleukin-6 (IL-6)—activate the JAK/STAT3 signaling pathway in hepatocytes. This leads to the upregulation of the HAMP gene, significantly increasing the systemic production of hepcidin.
• Ferroportin Degradation: Hepcidin acts as a negative regulator by binding to ferroportin (FPN1), the only known cellular iron exporter. Upon binding, hepcidin triggers the ubiquitination, internalization, and lysosomal degradation of ferroportin.
• Cellular Sequestration:
  • Intestinal Enterocytes: Degradation of ferroportin on the basolateral membrane of duodenal cells prevents dietary iron from entering the bloodstream, effectively halting absorption.
  • Macrophages: In the reticuloendothelial system, macrophages are responsible for recycling iron from senescent red blood cells. Without functional ferroportin, this recycled iron remains trapped within the macrophage cytoplasm.

Clinical and biochemical evidence

This sequestration creates a distinct biochemical profile that differentiates functional iron deficiency (FID) from absolute iron deficiency:

• Low Transferrin Saturation (TSAT): Because iron is trapped in cells and absorption is blocked, serum iron levels drop significantly (hypoferremia). This results in a TSAT typically below 20%, indicating that there is insufficient iron available to saturate the transport protein transferrin for delivery to bone marrow.
• Normal or Elevated Ferritin: Ferritin serves a dual role; it is an intracellular iron storage protein and a positive acute-phase reactant. In inflammatory states, ferritin increases due to both the physical sequestration of iron (increasing storage) and direct induction by inflammatory signals. In patients with chronic disease, FID is often diagnosed when ferritin is ≥100 μg/L (and sometimes up to 500 μg/L) despite a low TSAT.

Bottom line

Inflammation increases hepcidin via IL-6, which degrades the iron exporter ferroportin. This prevents iron from leaving macrophages and enterocytes, leading to a state of functional iron deficiency characterized by low serum iron availability (low TSAT) despite abundant internal stores (normal/elevated ferritin).