Which laboratory tests should be ordered for a 19-year-old woman with iron deficiency anemia and secondary amenorrhea?

Laboratory Evaluation for Iron Deficiency with Amenorrhea in a 19-Year-Old Woman

In a 19-year-old woman with iron deficiency and amenorrhea, you must check a complete blood count with hemoglobin/hematocrit, a full iron panel (serum ferritin, serum iron, transferrin saturation, and total iron-binding capacity), and evaluate for underlying causes of both conditions including pregnancy test, TSH, prolactin, FSH/LH, and celiac disease screening.

Initial Hematologic Assessment

Start with basic hematologic parameters to confirm and characterize the iron deficiency:

• Complete blood count (CBC) including hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and red blood cell distribution width 1
• Reticulocyte count to assess bone marrow response 2

Hemoglobin concentration and hematocrit are late indicators of iron deficiency but essential for determining if anemia is present (Hb <12 g/dL in non-pregnant women) 1. MCV and MCH help identify microcytic, hypochromic anemia characteristic of iron deficiency 3.

Comprehensive Iron Studies

A full iron panel is mandatory, not just ferritin alone 4:

• Serum ferritin: Most specific test for iron stores; <15 μg/L confirms absent iron stores, <30 μg/L indicates low stores 3. However, ferritin is an acute-phase reactant and can be falsely elevated with inflammation 1, 3
• Serum iron and total iron-binding capacity (TIBC): Calculate transferrin saturation 1
• Transferrin saturation: <20% suggests iron deficiency; less affected by inflammation than ferritin 2, 5

The combination of low ferritin (<30 μg/L) and low transferrin saturation (<20%) provides the most reliable diagnosis of iron deficiency 1.

Evaluation for Underlying Causes

Gynecologic and Endocrine Workup

Since amenorrhea is present, you must evaluate for functional hypothalamic amenorrhea and other endocrine causes:

• Pregnancy test (β-hCG): Always rule out pregnancy first
• TSH: Thyroid dysfunction can cause both amenorrhea and anemia
• Prolactin: Hyperprolactinemia causes amenorrhea
• FSH and LH: Distinguish between hypothalamic/pituitary causes (low FSH/LH) versus ovarian failure (elevated FSH) 6

Document menstrual history thoroughly, including duration of amenorrhea, previous menstrual patterns, and any history of heavy menstrual bleeding before amenorrhea onset 7, 8.

Gastrointestinal Evaluation

Even in young women, gastrointestinal causes must be considered 9:

• Celiac disease screening: Tissue transglutaminase IgA with total IgA level 9. Celiac disease causes both iron malabsorption and can be associated with amenorrhea
• Helicobacter pylori testing: Consider if other causes not identified 9

Nutritional Assessment

Given the association between iron deficiency and amenorrhea in young women:

• Assess for eating disorders and energy deficiency: Low energy availability causes functional hypothalamic amenorrhea and iron deficiency 6
• Vitamin D level: Often deficient in those with restrictive eating 4
• Consider vitamin B12 and folate if MCV is not low or if dietary restriction suspected 1

Clinical Context Matters

Common pitfall: Assuming heavy menstrual bleeding is the cause when the patient has amenorrhea. In this scenario, the iron deficiency preceded or coincided with loss of menses, suggesting:

1. Functional hypothalamic amenorrhea from energy deficiency (inadequate caloric intake, excessive exercise, or stress) causing both conditions 6
2. Malabsorption (celiac disease, H. pylori, inflammatory bowel disease) 9
3. Previous heavy menstrual bleeding that depleted iron stores, followed by amenorrhea from severe iron deficiency itself

Diagnostic Algorithm

1. Confirm iron deficiency: CBC, ferritin, iron studies
2. Rule out pregnancy: β-hCG
3. Evaluate amenorrhea: TSH, prolactin, FSH/LH
4. Screen for malabsorption: Celiac serology, consider H. pylori
5. Assess nutritional status: Detailed dietary history, signs of eating disorder, BMI
6. Consider GI evaluation: If no clear gynecologic or nutritional cause, particularly if age >50 years or concerning symptoms, though less urgent in asymptomatic premenopausal women 9

Follow-Up Testing

After initiating iron replacement, recheck hemoglobin in 4 weeks. An increase of ≥1 g/dL confirms iron deficiency as the cause of anemia 1. Continue treatment for 2-3 months after normalization to replete stores 1, 8.

The combination of iron deficiency and amenorrhea in a young woman warrants thorough evaluation for both conditions simultaneously, as they may share a common underlying cause such as energy deficiency or malabsorption 4, 6.