Ferritin

Ferritin is the protein that stores iron inside cells, and the blood level of it is the standard proxy for how much iron your body has in reserve. It is the first marker to fall when iron stores are being drained, well before haemoglobin drops and anaemia appears.

That early warning is why it matters for anyone training seriously. Iron deficiency without anaemia degrades endurance performance and leaves people tired and flat, and it is common in menstruating women, endurance athletes, vegetarians and blood donors. Fatigue that does not respond to more sleep is worth a blood test rather than a guess.

Its weakness is that it is also an acute phase reactant, meaning it rises with inflammation, infection, liver disease and heavy alcohol use. A normal ferritin in the presence of inflammation can therefore hide genuine iron deficiency, which is why it is usually interpreted alongside CRP, transferrin saturation and a full blood count rather than on its own.

High ferritin is not automatically an iron problem either. It is more often a marker of inflammation or fatty liver than of iron overload, though genuine overload does exist, notably in haemochromatosis, where it needs proper investigation rather than reassurance.

Optimal ferritin is a contested number and a favourite of private testing. Laboratory reference ranges are wide, athletic recommendations are often narrower, and a value near the bottom of a normal range with symptoms is a different situation from the same value without them.

Self treating with iron is a genuinely bad idea. Iron supplements cause gastrointestinal problems, they interfere with absorption of other minerals, and taking them when you do not need them is harmful rather than neutral.

In practice: if you suspect iron deficiency, get ferritin measured with the markers that interpret it, find the cause rather than only correcting the number, and take supplementation as a medical decision rather than a purchase.