In a 15-year-old female with three months of low mood, irritability, school refusal, insomnia with early morning awakening, decreased appetite, 6‑lb unintentional weight loss, fatigue, dizziness, PHQ‑9 (Patient Health Questionnaire‑9) score of 14 with passive suicidal thoughts, heavy menstrual bleeding lasting 7–8 days, pale conjunctiva, hemoglobin 10.2 g/dL and MCV (mean corpuscular volume) 74 fL, what are the most likely differential diagnoses and how can symptoms of iron‑deficiency anemia be distinguished from those of major depressive disorder (MDD) using current pediatric/adolescent guidelines?

Differential Diagnosis and Clinical Approach

This 15-year-old presents with overlapping features of both iron deficiency anemia (IDA) and major depressive disorder (MDD), and the most critical clinical insight is that these conditions frequently coexist and exacerbate each other—you must treat both simultaneously rather than attempting to distinguish which came first 1, 2.

Most Likely Differential Diagnoses

Primary diagnoses:

• Iron deficiency anemia (confirmed by Hb 10.2 g/dL, MCV 74 fL, pale conjunctiva, heavy menstrual bleeding)
• Major depressive disorder (PHQ-9 score 14 indicates moderate depression, meets DSM-5 criteria with low mood, irritability, insomnia with early morning awakening, appetite changes, weight loss, fatigue for 3 months) 1
• Comorbid IDA and MDD (most likely scenario given the strong bidirectional association) 2, 3

Secondary considerations:

• Hypothyroidism (can cause fatigue, weight changes, depression)
• Other causes of microcytic anemia (thalassemia trait—less likely given heavy menses and clinical context)

Distinguishing IDA from MDD Symptoms: A Practical Algorithm

Symptoms That Overlap Completely (Cannot Distinguish)

Both conditions cause 4, 2, 5:

• Fatigue and lethargy
• Irritability
• Poor concentration/impaired cognition
• Dizziness
• Exercise intolerance

Critical evidence: A 2025 study specifically in adolescents with heavy menstrual bleeding found that 51% of those with IDA had PHQ-9 scores ≥10, and PHQ-9 scores improved significantly when hemoglobin levels improved (p=0.04) 2. This demonstrates that IDA directly causes depressive symptoms in this exact population.

Symptoms More Specific to MDD

• Early morning awakening (classic melancholic feature)
• School refusal (functional impairment in social/academic domain)
• Passive suicidal ideation ("what's the point")
• Anhedonia (if present—PHQ-9 items 1-2 are most sensitive to depression) 6
• Guilt, worthlessness, psychomotor changes (assess per DSM-5 criteria) 1

Symptoms More Specific to IDA

• Pale conjunctiva (physical sign)
• Heavy menstrual bleeding 7-8 days (clear blood loss source)
• Pica, pagophagia (ice craving—ask specifically)
• Koilonychia, glossitis (in severe cases)
• Tachycardia, dyspnea on exertion (cardiovascular compensation)

Immediate Diagnostic Workup Required

Complete the IDA evaluation:

• Serum ferritin (most specific test; <15 μg/L confirms IDA; <30 μg/L indicates low stores) 7
• Transferrin saturation (low in IDA)
• Reticulocyte count (assess bone marrow response)
• Peripheral blood smear (confirm microcytosis, hypochromia, anisocytosis) 5

Screen for other medical causes:

• TSH and free T4 (hypothyroidism)
• Celiac serology (malabsorption cause of IDA)
• Consider hemoglobin electrophoresis only if ferritin is normal (to rule out thalassemia) 7

Complete the MDD assessment per GLAD-PC guidelines:

• Direct interview with patient and parent separately to assess all DSM-5 MDD criteria 1
• Functional impairment assessment across school, home, peer domains 1
• Comprehensive safety assessment including suicidal ideation, intent, plan, access to means (she has passive ideation—requires close monitoring) 1
• Screen for comorbid conditions: anxiety disorders, ADHD, substance use, trauma history 1
• Collateral information from teachers regarding academic decline 1

Treatment Approach: Treat Both Conditions Simultaneously

Iron Deficiency Anemia Treatment

Oral iron therapy (first-line):

• Elemental iron 3-6 mg/kg/day (typically 60-120 mg elemental iron daily for adolescents) 5
• Ferrous sulfate, ferrous gluconate, or ferrous fumarate
• Take on empty stomach with vitamin C to enhance absorption
• Continue for 3 months after hemoglobin normalizes to replete stores 5

Gynecologic management:

• Refer to gynecology for heavy menstrual bleeding management
• Consider hormonal contraception to reduce menstrual blood loss

Expected timeline: Hemoglobin should increase by 1-2 g/dL within 3-4 weeks; reticulocyte count rises within 5-10 days

Major Depressive Disorder Treatment

Per GLAD-PC guidelines, initial management options include 1:

1. Active support and monitoring (for mild depression, PHQ-9 <10)—NOT appropriate here given PHQ-9=14 and passive suicidal ideation

2. Evidence-based psychotherapy (appropriate for moderate depression):

   • Cognitive behavioral therapy (CBT)
   • Interpersonal therapy (IPT)
   • 6-8 weeks trial

3. Antidepressant medication (SSRI—fluoxetine is FDA-approved for adolescent MDD):

   • Consider given moderate severity (PHQ-9=14), functional impairment (school refusal), and suicidal ideation
   • Black box warning: Monitor closely for increased suicidal thinking, especially first 4 weeks

4. Combination therapy (psychotherapy + medication):

   • Most effective for moderate-severe depression

Safety planning is mandatory given passive suicidal ideation 1:

• Remove access to lethal means
• Establish crisis contacts
• Follow-up within 1 week initially
• Educate family about warning signs

Critical Clinical Pitfalls to Avoid

1. Do not assume all symptoms are "just anemia": The PHQ-9 score of 14, early morning awakening, school refusal, and suicidal ideation indicate true MDD requiring psychiatric treatment 1, 2

2. Do not delay MDD treatment waiting for iron repletion: While treating IDA will improve some symptoms, the 3-month duration and severity suggest established MDD requiring concurrent treatment 2

3. Do not miss the safety assessment: Passive suicidal ideation requires immediate risk stratification and safety planning 1

4. Do not overlook the gynecologic source: Heavy menstrual bleeding needs definitive management to prevent IDA recurrence 2

5. Monitor response systematically: Repeat PHQ-9 every 2-4 weeks and hemoglobin at 4 weeks to assess treatment response 1, 2

Evidence Synthesis

The 2025 study by 2 is particularly relevant: Among adolescents with heavy menstrual bleeding, 51% with IDA had PHQ-9 ≥10, and PHQ-9 scores improved when hemoglobin improved (p=0.04). However, PHQ-9 scores also improved with HMB control regardless of hemoglobin levels (p=0.003), suggesting multifactorial causation.

Multiple studies confirm the bidirectional association 3, 8, 9: IDA increases odds of depression (OR 1.45-1.86), and depression severity correlates negatively with hemoglobin levels (r=-0.429). Female adolescents show particularly high risk (OR 1.52-1.84 for MDD with IDA) 9.

The GLAD-PC guidelines emphasize 1: Depression assessment must include direct interview, functional impairment evaluation, comorbidity screening, and safety assessment—not just screening tools. The PHQ-9 score aids diagnosis but does not replace clinical judgment.