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Low Iron: Test Before You Supplement

The test-first approach to iron: find out if you are actually deficient before you supplement, correct a confirmed low ferritin properly, and understand why 'just take iron' is genuinely bad advice for a lot of people.

Not endorsed · based on the published work of Stacy Sims
At a glance
Time
Quarterly
Steps
7
Difficulty
Beginner
Moderate 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.
The protocol

Get a ferritin blood test, not just hemoglobin

Ask specifically for serum ferritin; a standard hemoglobin/CBC alone can miss early iron deficiency
Why ↓

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.

Dr. Stacy Sims newsletter; WHO ferritin guideline
In your stack: Ferritin blood test

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
Why ↓

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).

Dr. Stacy Sims newsletter; iron deficiency systematic review, 2024

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
Why ↓

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.

WHO ferritin guideline; Dr. Stacy Sims, Diary of a CEO

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
Why ↓

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.

Dr. Stacy Sims, Huberman Lab; Diary of a CEO
In your stack: Iron supplement (bisglycinate or carbonyl form)

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
Why ↓

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.

NIH Office of Dietary Supplements

Recheck ferritin after about 8 to 12 weeks of correcting a confirmed deficiency

One repeat blood test before deciding to continue, adjust or stop
Why ↓

This is what prevents both under-treating (stopping too early) and over-treating (sliding into overload); do not supplement indefinitely without checking.

NIH Office of Dietary Supplements; WHO ferritin guideline

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
Why ↓

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.

NIH Office of Dietary Supplements; AAFP, Hereditary Hemochromatosis, 2013
The evidence 8
Moderate

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.

Huberman Lab: Dr. Stacy Sims, Iron, Fatigue & Blood Testing segment Listen open.spotify.com Plays here
The Diary Of A CEO: Dr. Stacy Sims, Recommended Supplements for Women Watch youtube.com
Is Iron Supplementation Necessary for Women? (Dr. Stacy Sims on the Huberman Podcast) Watch youtube.com
Are You Iron Deficient? Are You Sure? (Dr. Stacy Sims) Read drstacysims.com
Hereditary Hemochromatosis (American Academy of Family Physicians, 2013) Read aafp.org
Iron: Fact Sheet for Health Professionals (NIH Office of Dietary Supplements) Read ods.od.nih.gov
WHO Guideline on Use of Ferritin Concentrations to Assess Iron Status in Individuals and Populations (WHO, 2020) Read ncbi.nlm.nih.gov
Iron deficiency, supplementation, and sports performance in female athletes: A systematic review (2024) Read pubmed.ncbi.nlm.nih.gov

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
How do I know if I'm actually iron deficient, or just tired?
The only way to know is a blood test, specifically serum ferritin, not just a standard hemoglobin check. Fatigue, brain fog, restless legs, hair shedding and poor exercise recovery are common symptoms, but they overlap with a lot of other causes, so a test is the only way to confirm iron is the reason.
What ferritin level counts as low?
The WHO's clinical threshold for deficiency is below 15 micrograms per liter in non-pregnant adults. Some researchers, including Dr. Stacy Sims, argue real symptoms often start higher, around 25 to 30, and Sims specifically targets a minimum of 50 for active people. Treat the higher numbers as an expert-opinion performance target, not an official diagnostic cutoff.
Is it safe to just start taking an iron supplement if I feel tired?
No. Taking iron without a confirmed low ferritin result is a real risk, especially for men and postmenopausal women, who are the group most likely to carry the hereditary hemochromatosis gene and are far less likely to actually be iron deficient. Test first.
How much iron should I take if I am deficient?
This page gives reference points, not a personal prescription. Published approaches, including Sims' own guidance, describe every-other-day dosing of a high-bioavailability form (like bisglycinate or carbonyl iron) rather than daily dosing. The right dose and duration for you should be confirmed with your doctor based on your actual ferritin result.
Who is most likely to be iron deficient?
Menstruating women (especially with heavy periods), endurance athletes, vegetarians and vegans, and pregnant people. Research Sims cites found iron deficiency in 15 to 35 percent of female athletes versus 5 to 11 percent of male athletes.
Can too much iron actually hurt me?
Yes. Iron overload is a real condition, most often from hereditary hemochromatosis (about 1 in 200 people of Northern European descent carry two copies of the main gene mutation), and it can cause liver, heart and joint damage over time if it goes uncorrected. This is exactly why this protocol leads with testing, not dosing.
Ferritin blood test
Look for: Ask for serum ferritin specifically, at a clinic or a direct-to-consumer lab
Iron supplement (bisglycinate or carbonyl form)
Look for: Only after a confirmed low ferritin result; every-other-day dosing, confirm with your doctor
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