Ferritin

Ferritin is the protein that stores iron in the body, and the blood level is used as a proxy for how much iron you have in reserve. Iron is essential for carrying oxygen in the blood and for producing energy in cells.

It matters because low ferritin is one of the most common and most missed causes of fatigue and poor training performance — particularly in menstruating women and endurance athletes — often before full anaemia appears. Very high ferritin warrants attention too, as it can reflect iron overload or inflammation.

In practice, ferritin is read with the full blood count and, because it also rises with inflammation, is interpreted in context; low iron is corrected through diet or supplements, ideally guided by testing rather than guesswork.