Iron storage protein and the usual first blood test for iron stores: highly relevant to fatigue and athletic performance.
What it is
Ferritin is a protein that stores iron safely inside cells, especially in the liver, spleen, and bone marrow. The small amount circulating in blood is a practical proxy for body iron stores. UK labs report serum ferritin in micrograms per litre (µg/L).
It is often the first iron-status test when anaemia, unexplained fatigue, or restless legs are explored. Because ferritin also rises as an “acute-phase” protein during inflammation, infection, or tissue injury, a normal or high ferritin does not always rule out iron deficiency if CRP or clinical inflammation is present: clinicians may then use transferrin saturation or other iron studies.
Ferritin’s hollow protein shell stores thousands of iron atoms in a safe Fe³⁺ form. Small amounts are secreted into plasma in proportion to cellular stores: hence its value as a blood test: until inflammation independently raises secretion.
Why it matters
Low ferritin is the clearest routine signal of depleted iron stores. People may still have a normal haemoglobin early on (“iron deficiency without anaemia”) yet feel tired, cold, or less sharp mentally. Causes include low dietary iron, menstrual blood loss, gut blood loss or malabsorption, frequent blood donation, and increased demand in pregnancy.
High ferritin has a long list of causes: inflammation, liver disease, metabolic syndrome, alcohol, infection, and iron overload conditions. Context (CRP, liver enzymes, family history, alcohol intake) matters more than the number alone.
Sex-specific laboratory ranges differ, and many sports clinicians use higher practical cut-offs for symptomatic athletes than the lab’s lower reference limit. Treatment decisions (diet, oral iron, further tests) belong with a clinician: self-supplementing high-dose iron without guidance risks constipation, gut upset, and masking other diagnoses.
This guide is educational only and cannot diagnose you. Discuss results, especially anything outside your laboratory’s reference interval, or that does not match how you feel, with a GP or other appropriately qualified clinician.
How it affects the body
Iron from ferritin stores is drawn into haemoglobin synthesis and into iron-dependent enzymes in muscle and brain. When stores run low, the body rationing iron can leave you short of oxygen-carrying capacity and of enzymes involved in energy metabolism, which is why fatigue can precede a frankly low haemoglobin.
Inflammation raises ferritin while potentially locking iron away from red-cell production (anaemia of chronic disease). That is why “high ferritin” is not automatically “too much iron”, and why the story always includes symptoms, CRP, and the wider blood count.
For athletes
Ferritin is a staple in athlete blood panels. Endurance sports, high training volumes, plant-based diets, and heavy menstrual bleeding raise the odds of low stores. Foot-strike haemolysis and gastrointestinal micro-losses in long events can add drain over a season.
Many coaches watch trends across a block rather than one off-season blood test. A fall in ferritin with rising training load, heavier legs, or declining session quality is a prompt to review fueling, recovery, and medical assessment, not to diagnose yourself. Avoid testing immediately after an ultra or iron race without noting recent inflammation; pair with haemoglobin, MCV, and CRP when possible.
