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PH-HE01

Iron-deficiency anaemia

How the body absorbs, recycles and stores iron, why stores empty before haemoglobin falls, why iron deficiency makes small, pale red cells, and how the symptoms and the search for a cause follow from the mechanism.

By the end, you can

  • Describe how iron is absorbed in the duodenum, carried, stored and recycled.
  • Explain why ferritin falls before haemoglobin as iron deficiency develops.
  • Explain why iron deficiency produces microcytic, hypochromic red cells.
  • Relate fatigue, breathlessness, pallor and nail and mouth changes to the mechanism.
  • Explain why unexplained iron deficiency in an adult needs a cause found.
  • Recognise the blood film, iron studies and other tests of iron deficiency, tell it from other microcytic anaemias, and explain how iron replacement and treatment of the cause act on the mechanism.

What it is

Iron-deficiency anaemia is a fall in haemoglobin because the bone marrow has too little iron to make haem. It is the commonest anaemia worldwide. It is usually caused by blood loss, too little dietary iron for the body’s needs, or poor absorption, and it is a sign of an underlying cause rather than a diagnosis on its own.

2 types to tell apart
Iron deficiency without anaemia
Stores are empty (low ferritin) but haemoglobin is still in range. Fatigue may already be present.
Functional iron deficiency
Stores are adequate but iron is locked away by inflammation (via hepcidin), as in the anaemia of chronic disease. It is a different mechanism with a different pattern of tests.

US spelling: anaemia → anemia, haemoglobin → hemoglobin, haem → hem.

Normal function

How the body handles iron

Most of the iron in the body is in haemoglobin. Each day the marrow needs about 20–25 mg to make new red cells, but only 1–2 mg is absorbed from food: almost all the rest is recycled. Macrophages, many of them in the , break down red cells at the end of their roughly 120-day life and return their iron to the blood.

Dietary iron is absorbed in the and upper small intestine. Haem iron from meat is absorbed easily; non-haem iron from plants is absorbed less well, better with vitamin C and worse with tea, calcium and stomach acid suppression. The body has no active way to excrete iron, so absorption is the control point.

In the blood, iron travels bound to transferrin; it is stored in the liver, spleen and marrow as ferritin. A liver hormone, hepcidin, adjusts absorption: it falls when stores are low, letting more iron in, and rises with inflammation, trapping iron in cells.

Point to it on the model
Iron is mostly recycledSchematic
NormalDuodenum1–2 mg absorbed a day; the control point
  • Absorbed iron (from Duodenum to Plasma transferrin)
Plasma transferrinCarries iron to the marrow
  • Delivered for haem (from Plasma transferrin to Marrow and red cells)
NormalMarrow and red cellsAbout 20–25 mg a day used for haem
  • Old red cells broken down (from Marrow and red cells to Macrophages and stores)
Macrophages and storesRecycle old red cells; store ferritin

Schematic. Recycled iron returns from macrophages to plasma transferrin; there is no active route for excreting iron.