What ferritin level warrants iron supplementation?

How it works

Ferritin is an acute-phase reactant: it rises in inflammation, infection, malignancy, liver disease, and obesity, independent of iron status. A normal or elevated ferritin in an inflamed patient can mask genuine iron deficiency. CRP measured alongside ferritin is the standard adjunct. In confirmed inflammation, some references use ferritin below 70 µg/L as the iron deficiency threshold; transferrin saturation (TSAT) below 16-20% is more reliable in inflammatory contexts because TSAT is not an acute-phase reactant. Soluble transferrin receptor (sTfR) is unaffected by inflammation but availability is variable in UK NHS labs.

Effective dose

Alternate-day or reduced-frequency dosing is BSG 2021 supported for tolerability. Moretti 2015 (Blood 126(17):1981-1989, PMID 26289639) showed 60 mg elemental iron on consecutive days produced hepcidin elevations reducing absorption from the next dose by 35-45%. Stoffel 2017 (Lancet Haematol 4(11):e524-e533, PMID 29032957), Stoffel 2020 (Haematologica 105(5):1232-1239) confirm alternate-day achieves higher cumulative fractional absorption than consecutive-day. Iron bisglycinate has reported tolerability advantages but limited cost-benefit evidence vs ferrous sulphate.

Forms compared

Switching between traditional iron salts for tolerability is NOT evidence-supported per BSG 2021; all three have similar side-effect profiles per Tolkien 2015 systematic review. Modified-release ferrous sulphate is BNF-listed as less suitable for prescribing because it reduces GI exposure at the cost of absorption. Newer chelated forms (bisglycinate, ferric maltol, sucrosomial iron) have varying tolerability profiles but do not change the repletion timeline materially.

Timing

If iron is given without anaemia (low ferritin only), duration is more individualised; typically 3-6 months with ferritin retest before stopping. No specific ferritin target per BSG 2021. Iron absorbs best on empty stomach (1 hour before or 2 hours after food); GI tolerance often requires taking with food, with modest absorption penalty (acceptable trade-off).

Safety profile

Failure of Hb response at 4 weeks suggests ongoing blood loss (GI, gynaecological with heavy menstrual bleeding very common per NICE NG88), malabsorption (coeliac most commonly missed; H. pylori; atrophic gastritis), poor adherence, or wrong diagnosis. Rare causes include hereditary haemochromatosis with selective bowel iron absorption defect, refractory iron deficiency anaemia (genetic transferrin or transcobalamin disorders), and iron-refractory iron deficiency anaemia (IRIDA, TMPRSS6 mutations, paediatric or young adult presentation).

Special populations

Inflammatory bowel disease, coeliac disease, post-bariatric anatomy: malabsorption magnifies all supplement-related concerns. Per BSG 2021, parenteral iron should be considered. Chronic kidney disease with EPO therapy: separate ferritin and TSAT thresholds apply. Pregnancy iron requirements follow specific antenatal guidance. Paediatric RLS: separate ferritin threshold of below 50 µg/L per AASM 2024.

Interactions

Vitamin C may modestly enhance non-haem iron absorption (mechanism: keeps iron in reduced ferrous form). Clinical importance in iron-deficient supplementation context is debated; Hurrell and Egli 2010 found minimal added benefit. BSG 2021 does not include routine vitamin C co-administration as a recommended intervention. Tea, coffee polyphenols, and calcium reduce non-haem iron absorption acutely; separate by 1-2 hours where practical.

Guideline positions

The Stoffel/Moretti programme (Moretti 2015 Blood 126(17):1981-1989, PMID 26289639; Stoffel 2017 Lancet Haematol 4(11):e524-e533, PMID 29032957; Stoffel 2020 Haematologica 105(5):1232-1239) underpins alternate-day dosing. Tolkien 2015 (PLoS One 10(2):e0117383, PMID 25700159) covers GI tolerability. Camaschella 2015 (NEJM 372(19):1832-1843, PMID 25946282) and Ganz 2019 (NEJM 381(12):1148-1157, PMID 31532961) cover the canonical iron deficiency review and inflammation-confounded ferritin biology.

Practical framework

Parenteral iron (iron sucrose, ferric carboxymaltose, ferric derisomaltose) per BSG 2021 / NICE CKS where (a) oral iron is contraindicated, (b) oral iron not tolerated despite alternate-day or reduced-dose attempts, (c) oral iron ineffective despite adequate trial, (d) clinical urgency (preoperative correction), (e) chronic kidney disease with EPO therapy, (f) malabsorption (severe IBD, post-bariatric). Monitor for hypophosphataemia (especially ferric carboxymaltose) and rare hypersensitivity reactions. This is a summary of published research, not personal health advice. Discuss any health or supplement decisions with a qualified healthcare professional, particularly during ongoing care, pregnancy, or with chronic conditions.

Common misconceptions

Outside RLS-specific clinical context, the higher general targets are not UK-clinical-evidence-based. Patients with ferritin 30-100 µg/L without anaemia and without specific clinical features should not be assumed to need supplementation under UK general guidance.

Claim: a normal ferritin during inflammation reflects adequate iron stores. Ferritin is an acute-phase reactant; CRP-paired interpretation is essential. Switching between ferrous sulphate, fumarate, and gluconate for tolerability is not evidence-supported (Tolkien 2015).