Causes of Low Ferritin (Early Iron Deficiency) in Adults
The most common causes of low ferritin in adults with otherwise normal labs are menstrual blood loss in premenopausal women, gastrointestinal blood loss (often occult), impaired iron absorption (celiac disease, atrophic gastritis, post-bariatric surgery), and inadequate dietary iron intake 1.
Primary Etiologic Categories
Blood Loss (Most Common)
- Menstrual bleeding: In premenopausal women, this is the predominant cause—approximately 38% have non-anemic iron deficiency 1. In the absence of GI symptoms or family history of GI pathology, GI investigation is generally not warranted in premenopausal women 2.
- Gastrointestinal bleeding: This is the leading concern in men and postmenopausal women. Even occult bleeding can deplete iron stores before anemia develops 2, 1.
- NSAID use: Increases risk of GI blood loss 1.
Malabsorption
- Celiac disease: Found in 3-5% of iron deficiency cases and should be routinely screened serologically 2.
- Atrophic gastritis: Reduces gastric acid needed for iron absorption 1.
- Post-bariatric surgery: Particularly gastric bypass procedures 1.
Increased Demand
- Pregnancy: Up to 84% of pregnant women in the third trimester have iron deficiency 1.
- Rapid growth phases: Though less relevant in adults.
Inadequate Intake
- Dietary insufficiency: Particularly in vegetarians/vegans or those with restricted diets 1.
Risk Stratification for Investigation
The threshold for investigation should be LOW in men, postmenopausal women, and those with GI symptoms or family history of GI pathology 2. This is critical because:
- Age, sex, hemoglobin, and MCV are independent predictors of GI cancer risk 2
- The overall prevalence of significant GI pathology is low in non-anemic iron deficiency, but cancer risk increases with male sex, older age, and presence of anemia 2
When to Investigate Aggressively:
- Men of any age with ferritin <30-45 μg/L
- Postmenopausal women with low ferritin
- Any patient with GI symptoms (change in bowel habits, abdominal pain, weight loss)
- Family history of GI malignancy or inflammatory bowel disease
When Investigation Can Be Deferred:
- Premenopausal women with heavy menstrual bleeding and no other risk factors
- Recent pregnancy or postpartum period
- Known dietary insufficiency in appropriate clinical context
Chronic Inflammatory Conditions
While you specified "otherwise normal laboratory values," be aware that inflammatory conditions can mask iron deficiency because ferritin is an acute phase reactant 2, 3. In inflammation, ferritin may appear falsely normal despite true iron deficiency. Consider:
- Inflammatory bowel disease (13-90% have iron deficiency) 1
- Chronic kidney disease (24-85% affected) 1
- Heart failure (37-61% affected) 1
- Cancer (18-82% affected) 1
A ferritin <45 μg/L warrants consideration of investigation even with mild inflammation, as this cutoff provides optimal sensitivity-specificity trade-off 2.
Diagnostic Approach
Initial workup should include 2:
- Urinalysis (to exclude urinary blood loss)
- Celiac serology (tissue transglutaminase antibody)
- Consider upper and lower endoscopy in high-risk groups (men, postmenopausal women, those with GI symptoms)
Common pitfall: Assuming menstrual bleeding is the cause in premenopausal women without first excluding celiac disease and ensuring no concerning GI symptoms are present 2.