What is the most likely cause of a ferritin level of 18 ng/mL in a 48‑year‑old woman?

Root Cause of Low Ferritin (18 ng/mL) in a 48-Year-Old Woman

In a 48-year-old woman with a ferritin of 18 ng/mL, the most likely causes are menstrual blood loss (if premenopausal) or gastrointestinal blood loss/malabsorption (if perimenopausal/postmenopausal), and you must systematically evaluate both dietary intake and GI pathology to identify the source. 1

Understanding the Ferritin Level

A ferritin of 18 ng/mL definitively indicates iron deficiency. The AGA guidelines establish that ferritin <45 ng/mL has 85% sensitivity and 92% specificity for iron deficiency 2. Your patient's level of 18 ng/mL falls well below this threshold, confirming true iron depletion rather than borderline status.

Systematic Approach to Identifying the Root Cause

Step 1: Determine Menopausal Status and Menstrual History

If premenopausal:

• Heavy menstrual bleeding (menorrhagia) is the leading cause in this age group
• Quantify menstrual blood loss: duration >7 days, clots, flooding, or need for double protection suggests significant loss
• At age 48, consider perimenopausal transition with irregular heavy bleeding

If postmenopausal or minimal menses:

• Proceed directly to GI evaluation as the primary focus

Step 2: Evaluate Dietary Iron Intake

Specifically assess:

• Vegetarian or vegan diet (lack of heme iron from meat)
• Restrictive eating patterns or food insecurity
• Bariatric surgery history (impaired absorption)
• Heavy tea/coffee consumption with meals (inhibits iron absorption)

Step 3: Gastrointestinal Evaluation

The AGA strongly recommends bidirectional endoscopy (both upper endoscopy and colonoscopy) for postmenopausal women with iron deficiency anemia 2. While your patient may not yet have anemia, the same GI pathology applies to isolated iron deficiency.

Key GI causes to investigate:

• Upper GI: H. pylori gastritis, celiac disease, atrophic gastritis, gastric/duodenal ulcers, gastric antral vascular ectasia
• Lower GI: Colon polyps, colorectal cancer, inflammatory bowel disease, angiodysplasia
• Small bowel: Celiac disease (check tissue transglutaminase antibodies), small bowel angioectasias

Test for H. pylori infection specifically - this is a common, treatable cause of iron malabsorption 1

Screen for celiac disease with serologic testing (tissue transglutaminase IgA with total IgA level), as this causes malabsorption even without overt GI symptoms 1

Step 4: Consider Non-GI Blood Loss

• Frequent blood donation history
• Hematuria (check urinalysis)
• Chronic epistaxis or other bleeding sites

Critical Pitfalls to Avoid

Do not assume dietary deficiency alone in a 48-year-old woman without thorough investigation. Even with poor dietary intake, ferritin this low in a middle-aged adult warrants evaluation for pathologic blood loss, particularly GI malignancy 2.

Do not delay endoscopic evaluation in postmenopausal women. The AGA provides a strong recommendation for bidirectional endoscopy in this population because of the risk of missing GI malignancy 2.

If inflammatory conditions are present (autoimmune disease, chronic infections), ferritin may underestimate iron stores. However, at 18 ng/mL, true iron deficiency is still highly likely. Consider checking transferrin saturation or soluble transferrin receptor if inflammation is suspected 1.

Algorithmic Approach Summary

1. Menstrual history → If heavy menses present, address gynecologically while proceeding with steps 2-4
2. Dietary assessment → Identify and correct nutritional deficiencies
3. H. pylori testing → Treat if positive
4. Celiac screening → Serologic testing
5. Bidirectional endoscopy → Especially if postmenopausal, or if premenopausal with inadequate explanation from menses/diet
6. Consider capsule endoscopy or deep enteroscopy → If above evaluation negative and iron deficiency persists

The goal is not just to replace iron but to identify and treat the underlying cause to prevent recurrence and detect potentially serious pathology early 2, 1.