What does low ferritin mean?

Ferritin is reported in ng/mL or µg/L, which are the same number. The thresholds:

• Below 15 ng/mL: absent iron stores, with a specificity of 0.99 for iron deficiency [1]. This is the WHO cutoff for healthy people aged 5 and older [5].
• Below 30 ng/mL: low body iron stores [1]. ASH 2026 uses 30 ng/mL or lower as the threshold for most adults, including menstruating and pregnant people [2].
• Below 45 ng/mL with anemia: the AGA cutoff for diagnosing iron deficiency when anemia is present [6].
• Up to 50 ng/mL: ASH suggests this higher threshold may be considered in some higher-risk groups, such as heavy menstrual bleeding [2].

Some lab reports still print a lower limit near 15 ng/mL, so a result of 20 may not be flagged. The lower limit for most labs lies anywhere from 15 to 30 µg/L [1]. A clinician reads the number against your symptoms, hemoglobin and history.

Iron deficiency causes problems before anemia, because many enzymes and tissues besides red cells need iron [7].

• Fatigue. In 198 non-anemic menstruating women with unexplained fatigue and ferritin below 50 ng/mL, oral iron cut fatigue scores by 47.7% versus 28.8% with placebo over 12 weeks [3]. Pooled trials show a modest benefit for self-reported fatigue [8].
• Exercise. The same pooled trials found no improvement in measured physical capacity such as VO2max in non-anemic adults [8].
• Restless legs. Iron is recommended for restless legs when ferritin is 75 ng/mL or lower [9].
• With anemia. Breathlessness on effort, a fast heartbeat, pallor and headaches become more common as hemoglobin falls.

Fatigue is common and has many causes, from sleep and thyroid disease (TSH) to depression. A low ferritin makes iron one likely contributor; it does not rule out the others.

• Menstruation and pregnancy. These are the most common causes of iron deficiency anemia in premenopausal women [4]. Ferritin thresholds for deficiency during pregnancy sit around 25 µg/L in the first trimester [10].
• Diet. Vegetarian diets provide iron with about 5% to 12% bioavailability, against 14% to 18% for mixed diets [11]. Tea and coffee at meals cut absorption further [12].
• Blood donation. One whole blood donation removes a large amount of iron. Without supplements, 67% of donors had not recovered their iron stores 168 days later [13].
• Endurance training. Exercise raises hepcidin, which lowers absorption; about 15% to 35% of female and 5% to 11% of male athlete cohorts are iron deficient [14].
• Gut blood loss and malabsorption. These are the most common causes in men and postmenopausal women [4]. Coeliac disease is a classic cause of poor absorption, and screening for it is often worthwhile [7].

A low ferritin can have more than one cause at once, such as heavy periods plus a vegetarian diet plus running.

Approximately a third of men and postmenopausal women presenting with iron deficiency anaemia have an underlying pathological abnormality, most commonly in the gut [1]. In the UK, unexplained iron deficiency anaemia in at-risk people is a reason for fast-track referral, because gut cancers can present this way without other symptoms [1]. The AGA guideline sets out when to perform upper and lower endoscopy [6].

Signs that make investigation more urgent include:

• being a man or postmenopausal
• low ferritin that comes back after treatment, or does not respond to it
• black or bloody stools, weight loss or a change in bowel habit
• symptoms or a family history of coeliac disease

For menstruating women with an obvious cause, a clinician may treat and recheck before investigating further. That is a clinical decision, not something to settle with a supplement on your own.

A cross-sectional study found mean ferritin of 37.3 ng/mL in women with androgenetic alopecia and 24.9 ng/mL in alopecia areata, versus 59.5 ng/mL in women without hair loss. Women with telogen effluvium, the diffuse shedding most often blamed on iron, did not differ significantly [15].

A review of the evidence found studies split on the link, and concluded there was not enough evidence to recommend iron screening for everyone with hair loss, or iron supplements for hair loss without anemia [4]. A later review reached a similar view and called for large placebo-controlled trials [16].

Online claims that ferritin must reach 40, 70 or 100 ng/mL for hair growth are not backed by trials. If you have hair loss and low ferritin, it is reasonable to ask a clinician to treat the iron deficiency for its own sake, and to look at other causes of hair loss as well.

Typical next steps:

1. Confirm and complete the picture. Hemoglobin, transferrin saturation and sometimes CRP, so inflammation is not masking a lower true level [2].
2. Find the cause. History, and gut investigation where needed [1].
3. Replace iron. BSG recommends one tablet a day of ferrous sulphate, fumarate or gluconate, with every other day dosing or another preparation if it is not tolerated [1]. Alternate-day dosing gave equal ferritin with fewer gut side effects in a 2023 trial [17].
4. Recheck. The fatigue trial suggests checking blood markers after about six weeks of treatment [3].

Details and the evidence for each lever are on how to increase ferritin. Taking iron without a test can hide the warning sign of a bleed and harms people who actually have iron overload.