Iron Health Guide

Ferritin vs Iron: What's the Difference and Which Test Do You Need?

An independent UK guide to two of the most commonly confused blood tests. Understand what each result actually means so you can choose the right test for fatigue, iron deficiency or ongoing monitoring — without paying for markers you don't need.

Last updated 2026-07-19

The short answer

Ferritin measures stored iron — the reserves your body draws on. Serum iron measures circulating iron in your bloodstream at that moment. For most people investigating fatigue, low energy, hair thinning or heavy periods, ferritin is the more informative starting test because stores drop before your circulating iron or haemoglobin does. Serum iron is more useful as part of a full iron studies panel when investigating iron overload, monitoring supplementation or interpreting borderline ferritin results.

Ferritin vs iron at a glance

Trait	Ferritin	Serum iron
What it measures	Stored iron inside cells (mainly liver, spleen, bone marrow).	Iron currently circulating in blood, bound to transferrin.
Best for detecting	Early iron deficiency, depleted stores, cause of fatigue.	Acute changes, iron overload, response to supplementation.
Typical UK reference range	30 – 300 ng/mL (men), 15 – 200 ng/mL (women)	10 – 30 µmol/L (varies by lab and time of day)
Affected by inflammation	Yes — can appear falsely normal or high during infection.	Less so, but drops during acute illness.
Time-of-day sensitivity	Stable — any time of day is fine.	Highest in the morning; fast for accurate reading.
Sample type	Finger-prick or venous.	Usually venous (part of an iron studies panel).

When each test is the right choice

Persistent tiredness or low energy

Ferritin is the more useful first test — stores drop before circulating iron does, so low ferritin flags deficiency months earlier.

Heavy periods, pregnancy or plant-based diet

Groups at higher risk of depletion should track ferritin annually. A serum iron alone can miss early depletion.

Restless legs, hair thinning, brittle nails

Classic low-ferritin symptoms even when haemoglobin is still in range. Test ferritin plus a full blood count.

Suspected iron overload or haemochromatosis

Order iron studies (serum iron, transferrin saturation, ferritin together). High ferritin with high transferrin saturation warrants GP review.

Monitoring supplementation

Recheck serum iron and ferritin after 8 – 12 weeks. Ferritin recovers slower than serum iron and is the truer marker of replenished stores.

Typical UK prices (2026)

Provider	Ferritin only	Full iron studies panel
Medichecks	£29 (finger-prick)	£49 (Iron Status)
Randox Health	£45 (in-clinic)	£95 (Iron Profile)
Goodbody Clinic	£45 (in-clinic)	£89 (Iron Studies)
London Medical Lab	£39 (finger-prick)	£69 (Iron Profile)

Typical published prices from CQC-regulated UK providers. Phlebotomy or clinician review fees may apply — check each provider's listing before booking.

How to interpret your results

A single number rarely tells the whole story. Ferritin can be pushed up by inflammation, recent infection, alcohol or liver disease, so a "normal" ferritin during a flare of illness may still hide true deficiency. Serum iron swings across the day and rises after supplementation, so a one-off reading can mislead. Where possible, review ferritin alongside transferrin saturation, a full blood count and inflammatory markers (CRP) — most accredited providers include these in a full iron studies panel.

Symptomatic patients with ferritin under 30 ng/mL are widely accepted as iron-deficient in the UK, and many clinicians will treat borderline results (30 – 70 ng/mL) when symptoms persist. Ferritin above the upper reference range warrants review with a GP, especially alongside raised transferrin saturation.

This guide is educational and does not replace advice from your GP or a qualified healthcare professional. Always discuss abnormal results with a clinician before starting or stopping treatment.

Frequently asked questions

What is the difference between ferritin and iron?

Ferritin is a protein that stores iron inside your cells; the ferritin blood test estimates how much iron your body has in reserve. Serum iron measures the iron currently circulating in your bloodstream bound to transferrin. Ferritin falls first when you become deficient, which is why it is the preferred early-warning marker.

Which test should I ask for if I feel tired?

For unexplained fatigue in the UK, ferritin is typically the more informative starting test. Pairing it with a full blood count and, ideally, transferrin saturation gives a complete picture without over-testing.

Can ferritin be normal but iron still low?

Yes. Inflammation, infection or recent illness can push ferritin artificially high while iron availability remains poor. This is why clinicians often review ferritin alongside CRP and transferrin saturation, not in isolation.

What is a healthy ferritin level in the UK?

Most UK laboratories flag ferritin below 30 ng/mL as low in men and below 15 ng/mL in women, though many clinicians treat symptoms of deficiency when levels fall under 50 – 70 ng/mL. Ranges vary between accredited laboratories, so always interpret results against your provider's reference range.

Do I need to fast for a ferritin or iron test?

Ferritin does not require fasting. Serum iron is best drawn fasting in the morning because levels naturally vary throughout the day. Follow the specific instructions from your chosen UKAS-accredited provider.

Compare ferritin and iron tests side by side

See prices, biomarkers, sample method and turnaround from every CQC-regulated provider we track — with no pay-to-rank ordering.

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