Ferritin: why this one number tells you more than you think.

What ferritin actually is

Ferritin is a protein that stores iron inside cells, primarily in the liver, spleen, and bone marrow. Each ferritin molecule can hold up to 4,500 iron atoms. A small amount of ferritin leaks into the bloodstream, and this serum ferritin correlates with total body iron stores — under normal conditions, 1 ng/mL of serum ferritin corresponds to approximately 8-10 mg of stored iron[1].

But ferritin is also an acute phase reactant. It rises during inflammation, infection, liver damage, and malignancy — independently of iron status. This dual nature is both ferritin's diagnostic power and its interpretive challenge. A ferritin of 80 ng/mL in a healthy person means well-stocked iron. The same ferritin in someone with active inflammation might mask iron deficiency, because inflammation can triple or quadruple ferritin while actual iron stores are depleted[2].

The reference range problem

Most labs report a "normal" ferritin range of approximately 12-150 ng/mL for women and 12-300 ng/mL for men. These ranges are population-based — they describe what is statistically common, not what is optimal. The lower end is particularly problematic: a ferritin of 15 ng/mL is classified as "normal" but represents nearly empty iron stores[3].

Research consistently shows that symptoms of iron depletion — fatigue, reduced exercise capacity, impaired cognitive function, hair loss, restless legs — begin at ferritin levels well above the lab's lower limit. Multiple studies have demonstrated that iron supplementation improves fatigue in non-anemic women with ferritin levels below 50 ng/mL[4].

Too low: iron deficiency without anemia

Iron deficiency is the most common nutritional deficiency worldwide, affecting approximately 1-2 billion people[1]. But the traditional diagnostic model focuses on the end stage — iron deficiency anemia — and misses the far more common earlier stages.

Iron depletion progresses through three stages[3]:

1. Storage depletion: Ferritin drops below 30 ng/mL. No change in hemoglobin or serum iron yet. Symptoms may already be present.
2. Iron-deficient erythropoiesis: Transferrin saturation falls below 20%. Red blood cell production begins to suffer. Hemoglobin may still be normal or borderline.
3. Iron deficiency anemia: Hemoglobin falls below normal. MCV decreases (microcytic anemia). This is what most doctors screen for — and by this point, you have been depleted for months or years.

The clinical impact of stage 1-2 iron deficiency (without anemia) is increasingly recognized. A randomized trial in non-anemic women with ferritin below 50 ng/mL and unexplained fatigue found that intravenous iron significantly improved fatigue scores compared to placebo[4]. Similar results have been found for exercise capacity and cognitive function.

Women vs men: the biology of iron loss

The gender disparity in iron status is enormous and physiologically straightforward. Menstruating women lose approximately 30-40 mL of blood per cycle, which translates to 15-30 mg of iron per month. Heavy menstrual bleeding (menorrhagia) can double or triple this loss[5].

Dietary iron absorption from a mixed diet is approximately 1-2 mg per day — barely enough to replace menstrual losses in women with normal periods, and insufficient in women with heavy periods. This is why the average ferritin in premenopausal women (30-40 ng/mL) is dramatically lower than in men (100-150 ng/mL), and why up to 30% of premenopausal women have ferritin below 30 ng/mL[5].

After menopause, iron loss stops and ferritin gradually rises to approximate male levels. Conversely, any new iron deficiency in a postmenopausal woman or a man of any age should prompt investigation for a source of blood loss — most commonly GI (ulcers, polyps, colon cancer, celiac disease)[3].

Athletes: the iron demand multiplier

Athletes have increased iron requirements through multiple mechanisms[6]:

• Increased erythropoiesis: Training stimulates red blood cell production, consuming iron for hemoglobin synthesis
• Foot-strike hemolysis: Impact forces during running mechanically destroy red blood cells in capillaries of the feet
• Sweat iron losses: Iron is lost in sweat, which can be significant in hot environments or prolonged exercise
• Exercise-induced hepcidin: Intense exercise triggers an inflammatory response that raises hepcidin — a hormone that blocks iron absorption from the gut. Hepcidin peaks 3-6 hours post-exercise, which means iron supplements taken in this window are poorly absorbed[6]
• GI blood loss: Endurance exercise can cause subclinical GI bleeding, particularly during long runs

Sports medicine consensus suggests athletes maintain ferritin above 30-50 ng/mL, with many experts recommending above 50-70 ng/mL for optimal oxygen-carrying capacity and performance. Female endurance athletes are at the highest risk of iron depletion and should be screened routinely[6].

Timing tip: To maximize absorption, take iron supplements on rest days or at least 2-3 hours before exercise, not in the post-exercise window when hepcidin is elevated.

Too high: when to investigate

Elevated ferritin (above 200 ng/mL in women, above 300 ng/mL in men) always warrants investigation. The differential diagnosis falls into two main categories[2]:

Iron overload

Hereditary hemochromatosis (HH) is the most common genetic disease in people of Northern European descent, affecting approximately 1 in 200-250 people (homozygous C282Y mutation in the HFE gene). It causes excessive intestinal iron absorption, leading to iron deposition in the liver, heart, pancreas, joints, and endocrine glands[7].

The screening algorithm is straightforward: if ferritin is elevated, check transferrin saturation. If transferrin saturation is above 45%, order HFE gene testing. Hemochromatosis is treatable (by regular phlebotomy/blood donation) but can cause cirrhosis, diabetes, cardiomyopathy, and arthritis if undetected[7].

Non-iron causes of elevated ferritin

• Chronic inflammation: Autoimmune disease, chronic infection, obesity. CRP will be elevated. Transferrin saturation normal or low.
• Liver disease: Fatty liver (NAFLD), hepatitis, alcoholic liver disease. Liver enzymes (ALT, AST, GGT) typically elevated[8].
• Metabolic syndrome: Insulin resistance is independently associated with elevated ferritin. The combination of high ferritin + high triglycerides + high fasting glucose + normal transferrin saturation is a classic metabolic syndrome pattern[8].
• Heavy alcohol use: Alcohol directly increases ferritin synthesis in the liver.
• Malignancy: Rarely, very high ferritin (above 1,000 ng/mL) can indicate lymphoma, leukemia, or other malignancy.

The inflammatory confound: ferritin + CRP

This is the most clinically important nuance in ferritin interpretation. In the presence of inflammation, ferritin rises as an acute phase response — but actual iron stores may be normal or even depleted. This is called "anemia of chronic disease" or "anemia of inflammation" when it progresses far enough[2].

The solution is to always check CRP (or hs-CRP) alongside ferritin. If CRP is elevated:

• Ferritin below 100 ng/mL likely still indicates iron deficiency (the threshold for iron deficiency is raised in the setting of inflammation)[9]
• Transferrin saturation below 20% supports iron deficiency regardless of ferritin level
• Soluble transferrin receptor (sTfR) is an additional test that rises in true iron deficiency but not in inflammation, and can help distinguish the two when ferritin is ambiguous[9]

Supplementation: when and how

When to supplement

• Ferritin below 30 ng/mL in a premenopausal woman with menstrual blood loss: supplement
• Ferritin below 50 ng/mL with symptoms (fatigue, hair loss, exercise intolerance, restless legs): trial of supplementation is reasonable
• Ferritin below 30 ng/mL in a man or postmenopausal woman: investigate for GI blood loss before (or simultaneously with) supplementing[3]

How to supplement

• Form: Iron bisglycinate (chelated iron) is better tolerated than ferrous sulfate, with fewer GI side effects[10]
• Dose: 25-50 mg elemental iron every other day is as effective as daily dosing and better tolerated. Every-other-day dosing takes advantage of the hepcidin cycle — hepcidin rises after an iron dose and suppresses absorption for approximately 24 hours[11]
• Vitamin C: Take iron with 100-200 mg of vitamin C to enhance absorption (vitamin C reduces ferric to ferrous iron)
• Avoid taking with: Calcium, dairy, tea, coffee, and antacids all inhibit iron absorption. Separate by 2 hours.
• Retest: 8-12 weeks after starting supplementation. Ferritin responds slowly — do not retest at 4 weeks and conclude it is not working.

When to consider IV iron

Intravenous iron infusion (ferric carboxymaltose, iron sucrose) is indicated when oral iron is not tolerated, not absorbed (celiac, inflammatory bowel disease, post-bariatric surgery), or when rapid repletion is needed (severe deficiency, heart failure, heavy ongoing bleeding, pre-surgical optimization)[9]. IV iron bypasses the gut entirely and can raise ferritin by 100-200 ng/mL with a single infusion.

The complete iron panel

Ferritin alone tells an incomplete story. The full iron workup includes: