Iron, decoded.

The editorial TLDR.

What serum iron actually is.

Iron is a mineral the body uses for a small number of very important jobs — most prominently, building haemoglobin, the molecule inside red blood cells that carries oxygen from your lungs to your tissues. Iron is also part of myoglobin (the version of haemoglobin that lives in muscles), several enzymes involved in energy production, and processes ranging from thyroid hormone synthesis to immune function.

The body handles iron with unusual care because it's both essential and potentially dangerous in excess. Iron isn't actively excreted — there's no major mechanism to get rid of it once it's in the body — so the system regulates it tightly at the absorption stage in the gut and stores any surplus inside cells, principally in the liver, bone marrow and spleen. The protein that does the storage work is called ferritin, which is why a ferritin blood test is the best single proxy for total body iron reserves.

"Ferritin is the savings account. Serum iron is the cash in your wallet. You can have a thin wallet and a fat savings account, or a healthy wallet and an empty savings account — they tell you different things."

Most of the iron in your bloodstream isn't free. It travels bound to a transport protein called transferrin. The iron panel reports both how much iron is currently riding on transferrin (the serum iron number) and how much extra capacity transferrin has to carry more (the TIBC, total iron binding capacity). Divide one by the other and you get transferrin saturation — the percentage of transferrin currently carrying iron. A low saturation suggests not enough iron in circulation; a very high saturation can suggest the opposite problem.

The four numbers — serum iron, ferritin, TIBC and transferrin saturation — work together. Looking at any one in isolation can mislead. A normal serum iron with a low ferritin is a classic pattern of early iron deficiency: you've stopped saving but you're still spending. A low serum iron with a normal ferritin can be inflammation hiding the picture. Each combination tells a slightly different story. This is why "the iron panel" rather than "an iron test" is what most clinicians actually want.

Why iron matters for women.

The numbers here are arresting. U.S. surveys consistently find that low iron and iron-deficient anaemia are several times more common in pre-menopausal women than in men of the same age. A meaningful share of U.S. women carry depleted iron stores at any given time, and many of them are walking around with fatigue, hair shedding, low exercise tolerance and brain fog without realising iron is the driver. There are good biological reasons for the pattern.

Heavy or long periods. Menstruation is iron loss, and the amount of loss varies dramatically between women and across a single woman's life. A "normal" period of 40 mL of blood per cycle loses around 16 mg of iron each month. A heavier period — defined clinically as over 80 mL, but most women's reference is "I'm changing a super tampon every hour" or "I bleed through clothing" — can lose 30 mg or more. The body absorbs only 1 to 2 mg of iron per day from food on average, which means heavy periods can outpace replacement and slowly drain the savings account over years. This is the single most common driver of low iron in pre-menopausal U.S. women, and it's the one most often missed in standard annual physicals.

Pregnancy and postpartum. Pregnancy raises iron demand sharply — the mother's blood volume expands, the placenta needs iron, the fetus is building its own iron stores from the maternal supply. Many women come out of pregnancy with depleted iron reserves, and the bleeding around birth itself adds to the cost. Many U.S. obstetricians screen ferritin in addition to haemoglobin during pregnancy for this reason. Postpartum fatigue overlaps heavily with low iron, and it's worth checking even when the obvious explanation is the new baby.

Vegetarian and vegan eating. Plant-based iron (non-heme iron) is real iron, but the body absorbs it less efficiently than the heme iron found in animal foods. A well-planned plant-based diet can absolutely meet iron needs, but it requires more attention — vitamin C alongside iron-rich meals, awareness of coffee and tea timing (they reduce absorption), and sometimes supplementation. Vegetarian and vegan women in the U.S. are statistically more likely to test with lower iron stores, particularly during the menstruating years.

The midlife shift. The pattern flips at menopause. Once monthly bleeding stops, iron loss drops dramatically, and iron stores generally rise. Many post-menopausal women find their ferritin trending upward in their fifties — this is normal. Higher iron in this window is rarely a problem on its own; persistently very high iron in a post-menopausal woman is worth investigating for hereditary haemochromatosis, which is more common than commonly recognised but often missed.

The hair, skin and energy story. Low iron — well before formal anaemia — is associated in research with hair shedding (telogen effluvium), poor exercise recovery, restless legs at night, cold extremities, and brain fog. Women often present to dermatologists or primary care with these symptoms without anyone running a ferritin. The diagnostic miss happens at the test-ordering stage, not at the interpretation stage.

What the ranges generally mean.

Most U.S. labs report serum iron in micrograms per deciliter (µg/dL). The reference range below is widely used; your lab's exact figures may vary.

Illustrative ranges, not diagnostic. Reference ranges vary by laboratory, assay, time of day, supplement use, menstrual cycle phase and individual context. Always discuss your specific result with a qualified healthcare provider.

What may drive iron low or high.

A single iron reading is a snapshot. The interesting question is what's shaping it, and what the rest of the panel says alongside it.

What may drive iron low.

• Heavy or long periods. The most common driver in U.S. women under 50. Menstrual blood loss outpaces dietary replacement.
• Pregnancy and postpartum. Increased iron demand, blood loss at birth, and continued demand during lactation.
• Vegetarian and vegan eating without attention to iron. Plant iron is real but less absorbable. Often manageable with planning and sometimes supplementation.
• GI blood loss. Ulcers, polyps, certain gut conditions and (rarely) gastrointestinal cancers can drive chronic, slow blood loss. Persistent low iron in a woman with no obvious menstrual or dietary explanation generally warrants a conversation about GI evaluation, particularly after menopause.
• Celiac disease and other malabsorption conditions. Iron absorption happens in the upper small intestine — damaged or inflamed tissue reduces uptake.
• Frequent blood donation. Each whole-blood donation removes around 250 mg of iron. Worth tracking for women who donate regularly.
• Athletic training load. Endurance sports, particularly running, are associated with lower iron stores in women through several mechanisms — foot-strike haemolysis, GI losses, and competition with other demands.
• Chronic inflammation. Inflammation can reduce iron availability by trapping it inside cells. Ferritin may look paradoxically normal or even high in this pattern, because ferritin is also an inflammatory marker.

What may drive iron high.

• Iron supplementation. By a wide margin the most common reason for a high reading. A serum iron drawn within hours of an iron tablet will read elevated.
• Recent iron-rich meal. Red meat, organ meats, fortified cereals — all can lift serum iron temporarily.
• Hereditary haemochromatosis. An inherited condition causing the body to absorb too much iron. More common in people of Northern European descent. Typically picked up through persistently high transferrin saturation. Worth discussing with a healthcare provider, particularly if there's family history.
• Recent blood transfusion. Will raise iron readings.
• Some liver conditions. Damaged liver tissue can release stored iron into circulation.
• Cessation of menstruation. After menopause, iron stores generally rise as monthly losses stop. A trend upward in this window is usually normal — but persistently very high warrants the same conversation as any other unexplained high.

Your provider will guide the conversation based on your full medical context. The point of these questions isn't to second-guess the workup — it's to make sure the right markers actually get measured before any number is read.

When to test, and how it's measured.

The iron panel is a standard venous blood draw. Most clinicians prefer a morning sample, ideally fasting, because serum iron is meaningfully higher after meals — particularly meals containing iron-rich foods or coffee/tea. A non-fasting sample isn't useless; it just adds noise to a number that's already variable.

If you take an iron supplement, most labs and clinicians recommend pausing iron for at least 24 hours, often 48, before the draw — otherwise the serum iron reflects the supplement, not your baseline. Ferritin is much more stable and less affected by recent intake. Never stop a prescribed iron supplement without asking your provider.

Cycle timing matters more than most people expect. Drawing iron in the few days after a heavy period can produce a lower-looking serum iron and a different picture than the same panel drawn mid-cycle. If you're tracking iron over time, it's worth running the panel at a consistent point in the cycle.

Inflammation matters too. Ferritin rises in response to inflammation, infection or recent illness — a process called the acute-phase response. Iron stores can be genuinely low while ferritin reads paradoxically normal because inflammation is masking the picture. If you've been recently ill, it's worth waiting a couple of weeks before testing, or interpreting the panel alongside an inflammation marker like C-reactive protein (CRP).

Direct-to-consumer at-home options exist — services like Function Health, LetsGetChecked and Quest Direct include iron and ferritin in most of their broader panels. These can be useful for a baseline, particularly for women whose periods or fatigue have been on the conversation list for a while. They don't replace a clinical evaluation when iron is consistently low or the panel is genuinely unclear.

Related biomarkers worth understanding.

Serum iron is one number in a small ecosystem. The markers below come up most often alongside it.

• Ferritin — your iron savings account. The single most important number alongside serum iron, and the one most often missed in U.S. physicals.
• TIBC and transferrin saturation — round out the iron panel and tell you whether the body is upregulating capacity to grab more iron.
• Complete blood count (CBC) — picks up the anaemia stage. Best read alongside the iron panel, not in place of it.
• Vitamin B12 and folate — the other nutrients essential for red blood cell production. Worth checking alongside iron in any fatigue workup.
• TSH — thyroid function. Iron and thyroid interact; one can hide a problem with the other.
• C-reactive protein (CRP) — inflammation marker. Helps interpret ferritin readings that don't match the rest of the picture.