The panel / The causes nobody else checks

Ferritin, iron, TIBC

You do not have to be anaemic to be short of iron, and you do not have to be anaemic to feel it. Iron deficiency without anaemia produces fatigue, poor exercise tolerance, brain fog and low mood, and a standard blood count will look entirely normal while it is happening.

What it measures

Ferritin reflects stored iron and is the most useful single marker. Serum iron is what is circulating right now, which moves with what you ate. TIBC reflects the capacity to carry it. Together they produce transferrin saturation, which is the more stable read on whether supply is meeting demand.

What each marker shows

FerritinReflects iron stores, and falls first

Transferrin saturationReflects whether supply is meeting demand

TIBCTypically rises in true deficiency

Iron does far more than carry oxygen. It is required for myoglobin in muscle, for the cytochromes that run mitochondrial energy production, and for neurotransmitter synthesis. That is why symptoms appear before the haemoglobin ever moves.

Serum iron on its own is close to useless because it swings with the last meal and with time of day. It earns its place as an input into transferrin saturation. We draw CRP alongside because without it a ferritin result cannot be interpreted safely, for the reason below.

Why it is on the panel

Because it is a genuinely common, genuinely treatable cause of the presentation, and because it is routinely missed. The usual failure is checking a full blood count, seeing a normal haemoglobin, and concluding iron is fine. Ferritin is a separate test and it has to be asked for.

How it is measured

SpecimenSerum

TimingMorning, with the panel

FastingPreferred, serum iron moves with recent intake

Read asTransferrin saturation, not serum iron alone

Drawn alongsideCRP, to detect inflammation distorting ferritin

TurnaroundTypically 1 to 3 days

Interpretation

Reading the result

There is no single validated ferritin number at which deficiency starts producing fatigue. Different thresholds appear in the evidence depending on the question being asked, and honest practice means saying so.

Bands used in practice

Ferritin below 30 ng/mLAbsolute iron deficiency in the absence of inflammation.

Ferritin below 50 ng/mLThe threshold used in the fatigue treatment trials, where iron helped.

Ferritin below 100 ng/mL with inflammationDeficiency is still likely. Inflammation falsely raises the number.

Transferrin saturation below 20 percentSupports deficiency regardless of where the ferritin sits.

The most important trap on this page: ferritin is an acute phase reactant. It rises with infection, chronic inflammation, liver injury and malignancy. A normal ferritin does not exclude iron deficiency in an inflamed man, which is why CRP is drawn with it and why the threshold moves upward when CRP is raised.

What distorts the result

Mostly things that push ferritin up and make a deficient man look replete.

7 things that move it

Any acute or chronic inflammation, which raises ferritin independently of iron stores.

Infection, recent or ongoing.

Liver disease and liver injury, which release stored ferritin.

Malignancy.

An overactive thyroid, which raises ferritin in an inflammation-like pattern that resolves once treated.

Recent food intake and time of day, which move serum iron specifically.

Inflammation or protein loss, which can falsely lower TIBC and mask deficiency that TIBC would otherwise reveal.

What happens next

Confirmed deficiency is treated, and the honest expectation is set: pooled trial data show a real but modest improvement in fatigue, independent of any change in haemoglobin.

In a man with a raised CRP, the ferritin threshold is raised and transferrin saturation carries more weight.

Iron deficiency in an adult man is not a nutritional finding by default. It prompts a look for a source of blood loss, which usually means the gut.

Levels are rechecked after repletion rather than assumed, because absorption varies widely.

Straight answers

Not necessarily, and this is the single most common misunderstanding on this page. Haemoglobin is the last thing to fall. You can run your stores down far enough to feel exhausted while your blood count stays entirely within range. Ferritin is a separate test that has to be ordered specifically.