Hemoglobin is iron in circulation. Ferritin is iron in storage. The body protects hemoglobin at all costs - cannibalising storage reserves for months before the circulating level drops. By the time your CBC flags low hemoglobin, iron deficiency has typically been present for the better part of a year.
This is why 'your blood work is normal' and 'you are not iron deficient' are not the same statement. A complete blood count with normal hemoglobin tells you that your body has not yet reached the stage of frank anaemia. It says nothing about your ferritin, which is where the story starts.
The storage-vs-circulating gap
Ferritin is the protein that stores iron inside cells - primarily in the liver, spleen, and bone marrow. When dietary iron is insufficient or losses exceed intake, ferritin depletes first. Only once stores are substantially exhausted does the body begin compromising red blood cell production, which is what eventually shows up as low hemoglobin.
The standard lab lower reference for ferritin is typically 10–15 ng/mL - the threshold at which you are clinically iron deficient by the narrowest definition. The functional floor for symptom relief is a different number entirely.
What low ferritin actually looks like
- Fatigue that doesn't improve with adequate sleep - iron is essential for mitochondrial energy production.
- Hair shedding (telogen effluvium) - follicles are iron-hungry; deficiency triggers the resting phase.
- Restless legs syndrome - dopamine synthesis in the brain requires iron; restless legs is often the first neurological sign.
- Reduced exercise tolerance - ferritin below 30 ng/mL impairs VO₂ max even in the absence of anaemia.
- Brain fog and difficulty concentrating - especially pronounced in the luteal phase when progesterone demands more oxygen.
Who is most at risk
Menstruating women
A typical menstrual cycle involves a loss of 20–80 mL of blood - roughly 10–40 mg of iron. Heavy periods (menorrhagia) can easily double or triple this. The WHO estimates that 30% of non-pregnant women of reproductive age are iron deficient worldwide; in the US, estimates range from 10–20%, with significantly higher rates in women with heavy flow.
Plant-forward eaters
Non-haem iron (from plant sources) is absorbed at 1–10% efficiency. Haem iron (from meat) is absorbed at 15–35%. Vitamin C at the same meal substantially improves non-haem absorption; coffee, tea, and calcium inhibit it. Vegetarian and vegan women need roughly 1.8× the iron intake of omnivores to achieve equivalent absorption.
Endurance athletes
Foot-strike haemolysis (red blood cell destruction from repetitive impact), sweat losses, and GI microbleeding during intense training all increase iron turnover. Female runners have some of the highest rates of iron depletion of any population group.
How to replete effectively
Ferrous bisglycinate is better tolerated than ferrous sulfate with equivalent or superior absorption - fewer GI side effects means better adherence. Take on an empty stomach with vitamin C where possible; avoid within two hours of calcium, antacids, or thyroid medication. Retest ferritin at 90 days - not hemoglobin, which will look normal before stores are rebuilt.