Low ferritin, and feeling flat when your blood test was "normal"

A specific thing happens to a lot of people. They feel exhausted, foggy, low and short of breath on stairs. They see a doctor. Bloods come back "normal". They leave with a suggestion about stress and sleep, and nothing changes.

Often, the test that would have found the problem was not done.

Haemoglobin is the last thing to fall

Iron deficiency happens in stages, and anaemia is the final one.

First, stored iron is depleted. Ferritin — the storage protein — falls. Nothing else changes yet, and a full blood count looks entirely normal.

Second, iron available for making red cells runs short. Transferrin saturation falls. The full blood count may still look normal.

Only third does haemoglobin drop and anaemia appear.

Symptoms begin in stage one. Iron is not only used for haemoglobin — it is a cofactor for enzymes involved in dopamine and serotonin synthesis, for thyroid hormone production, and for mitochondrial function. So the fatigue, the brain fog, the low mood, the hair shedding and the restless legs can all be present with a completely normal haemoglobin.

If your doctor ran a full blood count and told you your iron was fine, they may not have measured your iron at all.

What the numbers mean

Ferritin below 15 µg/L indicates depleted stores in essentially everyone. But the more useful clinical point is that the threshold at which symptoms appear is higher than the threshold at which laboratories flag a result.

Many labs report anything above 12 or 15 as normal. A good deal of clinical practice, and a reasonable body of evidence on fatigue and restless legs, treats ferritin below about 30 µg/L as deficient and below about 50 as worth treating in someone who is symptomatic. If your ferritin came back at 18 and was reported as normal, that is a number worth revisiting rather than accepting.

One complication: ferritin is an acute-phase reactant. It rises with inflammation, infection and liver disease, so a normal-looking ferritin can hide deficiency in someone with an inflammatory condition. A CRP alongside it helps interpret this, and transferrin saturation is more robust in that situation.

Why it is so common in women

Menstrual blood loss is the dominant cause in people of reproductive age, and it does not take a dramatic period — a moderately heavy period every month for years is enough to outpace absorption. People with genuinely heavy periods frequently run chronically depleted. How heavy is too heavy covers when bleeding itself needs addressing, and it usually does, because supplementing while the losses continue is bailing without patching.

Pregnancy is a large draw on stores, and postpartum depletion is common and under-checked — worth knowing alongside the fourth trimester.

Beyond that: coeliac disease, which impairs absorption and is under-diagnosed; gastrointestinal bleeding, which must be considered in anyone not menstruating and in older adults; diets low in bioavailable iron; and endurance training, which increases losses.

The overlap that causes the confusion

Iron deficiency symptoms overlap almost completely with hypothyroidism, depression, sleep deprivation and perimenopause. Fatigue, low mood, poor concentration, hair loss, feeling cold, low exercise tolerance.

That overlap cuts both ways. Iron deficiency gets mistaken for depression — and depression gets mistaken for iron deficiency. The only way through it is to test, treat properly, and see what remains. Fatigue that sleep doesn't fix walks through the wider differential.

Fixing it properly takes months

Two things about replacement are worth knowing, because most people get them wrong.

Alternate-day dosing absorbs better than daily. Taking iron raises hepcidin, which then blocks absorption for the next day or so. Trials have found that alternate-day single doses produce better total absorption than daily or twice-daily dosing, with fewer side effects. If daily iron gave you constipation and no improvement, this is worth raising.

Vitamin C helps; tea, coffee, calcium and antacids hinder. Take it away from those.

Replenishing stores, not just correcting haemoglobin, takes around three to six months of consistent supplementation. Stopping when you feel better is how people end up back where they started within a year. Recheck ferritin after three months.

Intravenous iron exists and is appropriate for people who cannot absorb or tolerate oral iron, or who need correction quickly. It is under-offered.

The thing to ask for

At your next blood test, ask specifically for ferritin, and ask for the actual number rather than "normal". Reading your own test results is relevant here — a value inside the reference range is not the same as a value that is good for you.

And if you are treating it, the useful measure of whether it worked is not the next blood test. It is whether the fatigue, the stairs, the fog and the mood actually changed — over months, against a baseline you recorded rather than remember.

Naked exists to hold that baseline: energy, mood and sleep logged over time, so when you start a treatment in February you can see in June whether it did anything. That is a surprisingly hard question to answer from memory, and it is the one that decides whether you keep taking it.