Ferritin

Ferritin is the blood test that estimates how much iron your body has stored. It is the best single check for iron deficiency, but it also rises with inflammation, and the "normal" range most labs print is wide enough to miss both mild deficiency and a slow build-up of iron.

Have ferritin checked together with transferrin saturation once a year, with your routine bloods. Aim for above 30 µg/L and below 150 µg/L (women who menstruate) or 200 µg/L (men and other women), and read a high value against an inflammation marker before calling it iron overload.

What the test measures

Inside cells, almost all iron is locked inside ferritin, a hollow protein shell that holds thousands of iron atoms in a chemically inert form and releases them only on demand. A small amount of ferritin circulates in the blood, and its level tracks the size of those stores. The body has no regulated way to get rid of iron, so in adults who aren't losing blood, stores tend to creep upward with age — see Iron and aging.

Two weaknesses matter. Ferritin is also an inflammation marker, so infection, illness and hard training all push it up. And standard lab ranges, roughly 10 to over 300 µg/L, are built to catch overt anaemia or full iron-overload disease, not to judge where you sit within the healthy range.

Where deficiency starts

There is no single agreed cutoff:

UK gastroenterology guidance: under 30 µg/L usually means low iron stores.^[1]
World Health Organization (WHO): below 15 µg/L in healthy adults, or below 70 µg/L during infection or inflammation.^[2]
A proposed higher cutoff: some experts argue for about 45–50 µg/L.^[3] WHO has not adopted it, and critics note it would label nearly half of women deficient.
Restless legs: the US sleep-medicine guideline advises iron when ferritin is 75 µg/L or below. That is an expert-consensus figure, not a tested one.^[4]

Low iron can cause fatigue, hair loss, poor exercise recovery, restless legs and impaired thinking, sometimes before ferritin drops below the usual cutoffs. Premenopausal women, endurance athletes, frequent blood donors and long-term plant-based eaters are most at risk.

Where the upper limit sits

WHO treats ferritin above 150 µg/L in women who menstruate, or above 200 µg/L in men and other women, as a possible sign of iron overload in otherwise healthy people.^[5] In a Danish cohort, 200 µg/L or more went with about 10% higher mortality.^[6]

Nobody has tested a narrower target in a trial. In about 162,000 Copenhagen adults whose doctors had ordered blood tests, the lowest mortality sat near 60 µg/L in women and 125 µg/L in men without marked inflammation. That is an association, not a tested target.^[7]

Lower is not simply better. A large Catalan records study compared about 7,400 people who reached 100 with people over 92 who died before 100. After adjustment, those with ferritin in the middle of the range were about 20% more likely to reach 100 than those with the lowest levels, while the highest levels showed no clear advantage. It is observational, and the authors warn of possible residual confounding.^[8]

Reading a high result

Because ferritin rises with inflammation, read a high value alongside high-sensitivity C-reactive protein and transferrin saturation (the share of the blood's iron-carrying protein that is loaded with iron) before calling it overload. If ferritin is high with normal inflammation and transferrin saturation above about 45%, a work-up for hereditary haemochromatosis is appropriate. Don't act on a single value taken during an illness or a block of hard training; both inflate it.

Reading a low result in an older adult

In older adults, a low ferritin is a reason for a clinical work-up, not a reason to self-treat: it can flag hidden bleeding or cancer. Low blood iron can also come from inflammation rather than true depletion (anaemia of chronic inflammation, driven by the hormone hepcidin), and in that case supplementing does nothing useful and may cause harm. Even a real deficiency, once corrected, is not a longevity lever: intravenous-iron trials in heart failure showed no clear gain in survival.

What to do

Don't take iron, including multivitamins with iron, unless a test shows you are deficient.
Test ferritin with transferrin saturation yearly. The full panel and how to read it are on Midlife labs.
Aim above 30 µg/L and below 150 µg/L (women who menstruate) or 200 µg/L (men and other women). The goal is low-normal, not empty.
If it is high, check an inflammation marker and transferrin saturation, and repeat the test when you are well before drawing conclusions.
If it is low, find the cause before taking iron; if you are older, that means a clinical work-up.
If you give blood regularly, get ferritin checked; see Blood donation.