Ferritin

You are exhausted. Your hair is thinning. You can barely finish a workout that used to feel easy. You ask your doctor to check your iron. They run a CBC. Your hemoglobin comes back normal. You are told your iron is fine.

But nobody checked your ferritin.

Hemoglobin is the last domino to fall in iron deficiency. It stays propped up long after your iron stores have emptied, your energy has cratered, and your body has started rationing iron to only the most critical functions. By the time hemoglobin drops, the deficiency has been building for months. Ferritin measures what hemoglobin cannot: how much iron your body has in reserve. And for over a billion people worldwide, the answer is "not enough" ^[1].

The second problem runs in the opposite direction. Ferritin also rises when you are inflamed, when your liver is under stress, or when your body is fighting infection. In those cases, a high ferritin might look like plenty of iron while your tissues are starving unnoticed. This dual identity makes ferritin one of the most important and most misunderstood numbers on any lab panel.

There is a third story that's hard to hear until damage is already done. For roughly 1 in 200 people of Northern European descent, the problem is not too little iron but too much. Their bodies absorb more iron than they can use, and because humans have no regulated mechanism for excreting excess iron, it accumulates slowly and silently. Year after year, iron builds up in the liver, pancreas, heart, and joints. By the time symptoms appear (chronic fatigue, joint pain, unexplained blood sugar problems, liver abnormalities), the overload has been underway for decades. This is hemochromatosis, and a high transferrin saturation, sometimes alongside a ferritin that still reads "normal," is often the first clue ^[2]. It is also why this site exists. Read the lab result that started everything →

Iron is not a "more is better" nutrient. Too little, and your cells starve for oxygen and energy. Too much, and it generates free radicals that corrode your tissues from the inside. Ferritin is how you know which side of that line you are standing on.

The vault in this picture is ferritin: a hollow protein shell made of 24 subunits that can hold up to 4,500 iron atoms safely inside its core ^[3]. Iron enters as soluble Fe2+, is oxidized to Fe3+, and is laid down as a tiny solid mineral inside the shell, inert and harmless until the cell needs it back. These vaults sit inside cells throughout the body, concentrated in the liver, spleen, and bone marrow, where most of the body's iron is banked. The distribution network is the body's iron absorption and recycling system, centered on the small intestine (where dietary iron enters) and the spleen and liver (where macrophages break down aging red blood cells and release their iron back into circulation).

That recycling pathway is the dominant supply source. About 90% of the body's daily iron needs come not from new dietary absorption but from macrophages recovering iron from spent red blood cells. Dietary absorption through the gut accounts for only 1–2 mg per day, just enough to replace obligate losses from skin shedding and minor GI turnover.

Iron deficiency does not happen all at once. The vault empties long before the operation halts. Ferritin drops first, as the reserves drain. The transport system empties next. Only last does hemoglobin fall, by which point the shortage has been building for months. The manager has been rationing all along, well before anyone notices the lights dimming.

But inflammation can flip this picture. When the body is fighting an infection, it deliberately pulls iron out of circulation and locks it inside cells, partly to starve the invaders that feed on it, and it makes extra ferritin to hold the hidden iron. So your ferritin climbs, not because your stores grew, but because inflammation changed how your body handles iron. This is the security alert from the analogy playing out for real: the manager locking iron in the vault, so the readout looks fuller than the operation truly is ^[4].

A low ferritin is the earliest, most reliable sign of deficiency, but most labs set the bar too low. They flag it only below 12 to 15 ng/mL, while the WHO uses 30 ng/mL, the level matched against the gold-standard test, iron staining of the bone marrow ^[5]. The gap between 15 and 30 is where millions of people sit: normal on paper, depleted in practice.

A high ferritin is harder to read, because two very different things raise it. The first is inflammation. Ferritin spills out of inflamed or damaged cells, so infection, liver disease, and heavy alcohol can drive it up with no change in actual iron, and a value above 1,000 ng/mL usually signals serious illness rather than too much iron. This is why a high ferritin is never read without an hs-CRP for context.

The second cause is true iron overload, the one that does lasting harm. Once the vaults are full, the surplus packs into hemosiderin and begins spilling loose inside cells, where it throws off the free radicals that corrode the organs it settles into: the liver, pancreas, heart, and joints. The damage builds over years, and across large populations higher ferritin tracks with earlier death, with roughly a 50% higher risk of dying once it climbs above 600 ng/mL ^[6]. Some of that link is confounding, since a very high ferritin often flags serious illness rather than iron alone, but the dose-response trend is consistent. The aim is not simply to avoid running empty, but to stay where iron is plentiful without the excess that accelerates aging.