Ferritin

What this marker measures

Ferritin is an intracellular protein that stores iron in a non-reactive form. The serum value is a small fraction that leaks out of cells and into the bloodstream. Under normal conditions it tracks total body iron stores reasonably well. Under inflammatory conditions it does not.

What the conventional reference range means

The lab's range was built on a working-age, ambulatory population without chronic inflammation. In that population a low ferritin means low iron stores and a high ferritin means high iron stores. The range is correct for the population it was built on. It is misleading in the chronic illness population, because that population almost always carries some background inflammation, and inflammation pushes ferritin up regardless of iron status.

What the Host Capacity Model reads in this marker

Two readings sit in tension. The first is the iron-status reading. The second is the acute-phase reading. The clinically useful question is which reading dominates in a given patient.

Ferritin in the low-normal range (say 15–50 ng/mL) in a symptomatic patient is almost always functional iron deficiency, regardless of where the lab range puts the lower bound. The patient feels worse than the number suggests because ferritin's lower bound was set for people who do not need iron-dependent processes running at full capacity. People with mast cell activation, mitochondrial strain, or post-viral states do.

Ferritin in the 200–500 ng/mL range, with no overt iron loading, is almost always inflammation. The iron status here is not the story. The story is that the body is keeping iron out of circulation, away from microbes and from Fenton chemistry. This is a defensive response, not a problem to correct with phlebotomy. The leverage point is whatever is driving the inflammation.

Ferritin above 1,000 ng/mL in a symptomatic patient is a different conversation and warrants ruling out hemochromatosis, hemophagocytic syndromes, and macrophage activation states.

Common patterns I look for

• Low ferritin (15–40) with high RDW, low MCV, low MCH: classic iron deficiency. Often comorbid with heavy menstruation, occult GI loss, or malabsorption from chronic gut inflammation.
• Low-normal ferritin (40–70) with normal CBC but exercise intolerance and hair loss: functional iron deficiency. Iron-dependent enzymes (including mitochondrial Fe-S cluster assembly) are running on insufficient substrate.
• Mid-high ferritin (150–400) with elevated hs-CRP: inflammation-driven. Ferritin is reflecting the inflammation, not iron loading.
• High ferritin (400–1000) with normal hs-CRP: rare but meaningful. Warrants iron studies and hereditary hemochromatosis workup.
• Ferritin that drops sharply after a viral illness: iron sequestration during acute infection has resolved. Now read iron status cleanly.

What I usually want to see alongside this marker

• hs-CRP, to anchor the inflammation reading
• Iron, TIBC, and transferrin saturation, to anchor the iron-status reading
• Soluble transferrin receptor, when ferritin and CRP both look ambiguous
• CBC indices (MCV, MCH, RDW), to see if the marrow is already responding

A ferritin number by itself is interpretable only at the extremes. In the middle, you need the other markers to disambiguate.