Low Iron: Test Before You Supplement
Low ferritin (iron deficiency) is a real, common, and frequently missed cause of fatigue, brain fog and poor exercise recovery, most often in menstruating women, vegetarians and endurance athletes. Correcting a confirmed deficiency has solid evidence behind it. But iron is not a supplement to take on a hunch: too much iron is genuinely harmful, men and postmenopausal women rarely need it and are the group most likely to carry the gene for iron overload. Get a ferritin blood test first. Never start iron because you are tired, start it because a test says you are low.
Get a ferritin blood test, not just hemoglobin Ask specifically for serum ferritin; a standard hemoglobin/CBC alone can miss early iron deficiency
Ferritin reflects your iron stores and drops well before hemoglobin does, which is why Sims argues the standard hemoglobin-only check catches deficiency too late.
Prioritize testing if you fall into a higher-risk group Menstruating women (especially with heavy periods), endurance athletes, vegetarians and vegans, pregnant people, and anyone with unexplained fatigue, brain fog, restless legs, hair shedding or poor recovery
These are the groups with meaningfully higher rates of iron deficiency in the research Sims cites (15 to 35 percent in female athletes versus 5 to 11 percent in male athletes).
Compare your ferritin number to more than one bar Below 15 micrograms per liter is the WHO's clinical deficiency threshold; some researchers argue real symptoms often start closer to 25 to 30, and Sims specifically flags a minimum of 50 (preferably 100) as a performance target for active people, a higher, expert-opinion bar, not a diagnostic one
A ferritin of 20 can be called normal on a standard lab report and still be low enough to cause fatigue in an active person, a genuine, unresolved gap between the diagnostic cutoff and the performance-optimization number some experts use.
Supplement with a high-bioavailability iron, dosed correctly Reference point only, confirm with your doctor: Sims describes supplementing every other day (not daily) with a highly bioavailable form such as iron bisglycinate or carbonyl iron, since every-other-day dosing appears to improve absorption and reduce gut side effects versus daily dosing
Iron competes with itself for absorption when taken daily; every-other-day dosing is a real, published strategy for improving uptake, not just a personal preference.
Take iron away from coffee, tea, calcium and dairy Space iron at least 1 to 2 hours from coffee, tea, calcium supplements or dairy; pairing with vitamin C (like a glass of orange juice) can help absorption
Tannins in coffee and tea and calcium both meaningfully block iron absorption; timing it away from them is free and makes the dose you take actually count.
Recheck ferritin after about 8 to 12 weeks of correcting a confirmed deficiency One repeat blood test before deciding to continue, adjust or stop
This is what prevents both under-treating (stopping too early) and over-treating (sliding into overload); do not supplement indefinitely without checking.
Understand that men and postmenopausal women rarely need iron, and can be harmed by it About 1 in 200 people of Northern European descent carry two copies of the most common hemochromatosis gene mutation; affected men have roughly 24 times the rate of iron-overload disease that women do, because women lose iron monthly and men do not. If you are a man or a postmenopausal woman, do not supplement iron without a confirmed low ferritin result
This is the single most important safety fact on this page: iron overload causes real organ damage over time (liver, heart, joints, pancreas) and is far more preventable than it is treatable once advanced.
This protocol works from your actual levels. Testing partner coming soon.
- Ferritin blood test
We may earn a referral fee if you book testing through a future partner link; it will never affect which tests are listed here. This is not medical advice or a diagnosis.
Ferritin, not hemoglobin alone, is the marker that catches iron deficiency before it becomes anemia; the WHO's working threshold is below 15 micrograms per liter in non-pregnant adults, though Sims and others argue meaningful symptoms show up well before that, closer to 25 to 30.
View sources
Huberman Lab: Dr. Stacy Sims, Iron, Fatigue & Blood Testing segment
The Diary Of A CEO: Dr. Stacy Sims, Recommended Supplements for Women
Is Iron Supplementation Necessary for Women? (Dr. Stacy Sims on the Huberman Podcast)
Are You Iron Deficient? Are You Sure? (Dr. Stacy Sims)
Hereditary Hemochromatosis (American Academy of Family Physicians, 2013)
Iron: Fact Sheet for Health Professionals (NIH Office of Dietary Supplements)
WHO Guideline on Use of Ferritin Concentrations to Assess Iron Status in Individuals and Populations (WHO, 2020)
Iron deficiency, supplementation, and sports performance in female athletes: A systematic review (2024)
Not medical advice. This page is for education only and is not a substitute for professional medical care. Consult a qualified clinician before changing your health routine.
Independent curation. YourProtocol is an independent platform. This protocol is based on the publicly available work of Stacy Sims and is not created, reviewed, endorsed by, or affiliated with Stacy Sims or Author, ROAR & Next Level.
Is this for you
- Anyone with unexplained fatigue, brain fog or poor exercise recovery who has not actually checked ferritin
- Menstruating women, endurance athletes, and vegetarians or vegans, the groups with the highest real rates of deficiency
- Men and postmenopausal women who assume 'more iron' is automatically safe (it is not for this group)
- Anyone already taking an iron supplement without ever having tested
Cautions
- The single biggest risk on this page is the opposite of deficiency: iron overload. Men and postmenopausal women rarely need iron supplements and are the group most likely to carry the hereditary hemochromatosis gene; unnecessary iron supplementation in this group can cause real harm over time. Do not supplement iron without a blood test confirming low ferritin
- High-dose iron supplements can cause nausea, constipation and stomach upset, and accidental overdose (especially in children who get into a bottle of iron pills) is a genuine medical emergency; store iron supplements like a medication, out of reach of kids
- Iron can interact with levothyroxine and some antibiotics (tetracyclines, fluoroquinolones); if you take prescription medication, ask your doctor or pharmacist about timing before starting iron.
- If you are pregnant or breastfeeding, have a GI condition (like IBD or celiac disease), have unexplained heavy bleeding, or your ferritin does not improve after a few months of correct supplementation, see a doctor rather than continuing to self-treat
- Educational only, not medical advice, and not a diagnosis
Common questions
Some links earn us a commission. It never affects our picks or grades. See our affiliate disclosure.