Ferritin becomes the number that matters when iron stores are in question: the World Health Organization's 2020 guideline sets a serum ferritin below 15 micrograms per liter as the cutoff for iron deficiency in adults, and below 70 when inflammation is present, because inflammation inflates the reading. The dosing news of the last decade is counterintuitive: a 2017 study in the Lancet Haematology by Stoffel and colleagues found that iron taken every other day produced higher fractional absorption than the same total taken daily, because daily dosing raises the hormone hepcidin, which blocks the gut's next dose from getting in.
The usual and here unusually firm caveat: this site publishes information, not medical advice. Iron is the supplement where self-diagnosis is most consequential — excess stored iron causes organ damage, and anemia has many causes that only a clinician can sort out. Diagnosis and treatment belong with a clinician.
Why is ferritin the number everyone orders?
Ferritin is the storage form of iron, and its blood level tracks the body's reserves — low when stores run down, long before hemoglobin falls into anemia. That is why the pattern called iron deficiency without anemia is common: the World Health Organization's estimates and United States surveillance data place iron deficiency, with or without anemia, at meaningful levels among women of reproductive age, roughly one in three in some surveys. Fatigue, hair shedding, and restless legs are the symptoms people bring to the test, and the honest evidence summary is that supplementation improves symptoms most reliably when ferritin is genuinely low — the cutoffs above, interpreted with a clinician.
The reference numbers, in one place
| Measure | Value | Source |
|---|---|---|
| Deficiency cutoff (no inflammation) | Ferritin < 15 µg/L | WHO guideline, 2020 |
| Deficiency cutoff (with inflammation) | Ferritin < 70 µg/L | WHO guideline, 2020 |
| RDA, women 19–50 | 18 mg/day | NIH Office of Dietary Supplements |
| RDA, adult men and women 51+ | 8 mg/day | NIH Office of Dietary Supplements |
| Elemental iron in 325 mg ferrous sulfate | 65 mg | Product monographs / NIH ODS |
These are published reference values, not personal targets — interpretation depends on the full blood count, inflammatory markers, and history.
Why did every-other-day dosing enter the conversation?
The daily-everything convention met its limit in pharmacology. Stoffel's 2017 trials in iron-deficient women showed that consecutive-day dosing raised hepcidin — a liver hormone that blocks the intestinal iron transporter ferroportin — and that a 24-hour gap allowed fractional absorption to recover; alternate-day regimens absorbed proportionally more iron from the same total. Follow-up work in 2020 examined morning-versus-consecutive-day schedules with similar logic. The practical translation is not a regimen to copy but a reason the old “twice a day with meals” instruction has been questioned in the literature: absorption, not just the milligram count, determines what a dose delivers.
The hepcidin finding inverted the folk rule: more frequent is not more available. The gut takes one door at a time, and the door closes between visits.
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What form and what food rules apply?
Ferrous sulfate remains the reference preparation — 325 mg of the salt carrying 65 mg of elemental iron — and it is the form in nearly all of the efficacy trials. Its downside is gastrointestinal: constipation and stomach upset at rates around 15 to 20 percent in trial reports, which drives more nonadherence than any other factor. The absorption rules are documented and simple. Vitamin C taken alongside increases uptake; the polyphenols in tea and coffee and the calcium in dairy or supplements decrease it, per the NIH fact sheet's summary of absorption studies. Taking iron on an empty stomach improves uptake but worsens tolerance — the trade every prescriber balances.
What are the limits and the dangers?
Two categories, both documented. The tolerable upper intake level for adults is 45 mg of elemental iron per day, set by the National Academies, and accidental overdose in children remains a leading cause of pediatric poisoning death in older toxicology registries — child-resistant storage is not decorative. At the other end, hemochromatosis and other iron-loading conditions turn supplementation from unnecessary into harmful, which is the single strongest reason ferritin is read before iron is bought. Anyone with a ferritin above range, an unexplained anemia, or a chronic inflammatory condition needs a clinician's workup, not a supplement.
Frequently asked questions
Can low ferritin explain my fatigue?
It can contribute when ferritin is below the deficiency cutoffs — trials show symptoms improve with repletion in that range — but fatigue has many causes, and studies in people with normal ferritin have not shown benefit. A clinician interpreting the full picture, including hemoglobin and inflammation markers, is the correct first step.
How long does repletion take?
Iron stores refill slowly: trial and clinical references commonly describe hemoglobin normalizing within weeks while ferritin recovery takes months of continued supplementation. Rechecking ferritin after a treatment interval is how clinicians confirm the dose worked.
Should I take iron with orange juice?
Vitamin C increases absorption, and citrus alongside a dose is a documented absorption aid. Keep tea, coffee, and calcium an hour or more away — their polyphenols and calcium measurably reduce uptake per the NIH fact sheet.
Is every-other-day dosing now the standard?
The 2017 Lancet Haematology findings changed the conversation and are reflected in current clinical reviews, but the right schedule depends on the formulation, the degree of deficiency, and tolerance. It is a prescribing decision to make with a clinician, not a rule to self-apply.