What is the likely diagnosis and recommended management for a 26-year-old with borderline anemia, low serum iron, low transferrin saturation, normal ferritin, normal TSH, and low‑normal vitamin B12?

Iron Deficiency Without Anemia

This patient has iron deficiency without anemia and requires oral iron supplementation along with investigation of the underlying cause. The transferrin saturation of 6% is severely low (normal >20%), confirming absolute iron deficiency despite the ferritin being 72 ng/mL 1, 2.

Diagnosis

• Iron deficiency is definitively diagnosed by the transferrin saturation of 6%, which is markedly below the diagnostic threshold of <20% 1, 2.
• The ferritin of 72 ng/mL falls in the gray zone (46-99 ng/mL), but when combined with transferrin saturation <20%, this confirms iron deficiency 2.
• The hemoglobin of 11.7 g/dL represents borderline anemia (depending on sex; typically <12 g/dL for women, <13 g/dL for men), but this patient has iron deficiency regardless of anemia status 1.
• The low-normal vitamin B12 of 242 pg/mL may contribute to symptoms but is not the primary issue here 1.
• TSH of 2.93 is normal and does not explain the findings 1.

Underlying Cause Investigation

At age 26, the most likely causes are menstrual blood loss (if female), gastrointestinal bleeding, malabsorption, or inadequate dietary intake 1.

• If female with heavy menstrual bleeding: This is a plausible cause and treatment can proceed with iron supplementation while addressing menstrual losses 2.
• If male or postmenopausal female: Bidirectional endoscopy (upper and lower) is mandatory to evaluate for gastrointestinal blood loss 2.
• Test for Helicobacter pylori and celiac disease noninvasively, as both commonly cause iron deficiency 2, 3.
• Review dietary iron intake and consider nutritional consultation 3.
• Assess for NSAID use, inflammatory bowel disease, or other chronic inflammatory conditions 1.

Treatment Approach

Start oral iron supplementation immediately as first-line therapy 1, 2.

Oral Iron Regimen:

• Ferrous sulfate 325 mg daily or every other day is the recommended starting regimen 1.
• Every-other-day dosing improves absorption and may reduce gastrointestinal side effects 2.
• Take on an empty stomach for optimal absorption; if not tolerated, take with meals (preferably with meat protein and 500 mg vitamin C to enhance absorption) 3.
• Approximately 50% of patients experience adverse effects (nausea, abdominal pain, constipation) that may reduce adherence 2.

Follow-Up and Response Assessment:

• Recheck iron studies in 2-4 weeks to assess response to oral iron 2.
• Repeat complete iron panel after 60-90 days of supplementation 4.
• If no improvement or intolerance to oral iron: Switch to intravenous iron 2, 4.

Indications for Intravenous Iron:

• Oral iron intolerance or poor adherence due to side effects 1, 2.
• Malabsorption conditions (celiac disease, post-bariatric surgery, atrophic gastritis) 1, 3.
• Ongoing blood loss that exceeds oral replacement capacity 1.
• Chronic inflammatory conditions (though not present in this case) 1.

Clinical Significance

Non-anemic iron deficiency causes significant symptoms including fatigue, irritability, depression, difficulty concentrating, restless legs syndrome (32-40% prevalence), pica (40-50%), and exercise intolerance 1. These symptoms warrant treatment even without frank anemia 4.

Common Pitfalls

• Do not dismiss iron deficiency because ferritin is "normal": Transferrin saturation is the key diagnostic marker here, and 6% is severely deficient 1, 2.
• Do not delay investigation of underlying cause: While treating with iron, simultaneously investigate the source of iron loss 2.
• Do not use intramuscular iron: There is no role for IM iron injections 4.
• Do not overlook the low-normal B12: Consider B12 supplementation if symptoms persist despite iron repletion, though iron deficiency is the primary problem 1.