What is the likely cause of hair thinning in a 26-year-old female with normal laboratory results?

Hair Thinning in a 26-Year-Old Female with Borderline-Low Ferritin

The most likely cause of hair thinning in this patient is iron deficiency, evidenced by a ferritin level of 34 ng/mL, which falls below the optimal threshold for hair health despite being within the laboratory's reference range. 1

Primary Diagnosis: Iron Deficiency-Related Hair Loss

Your patient's ferritin of 34 ng/mL is the key abnormality. While technically "normal" by laboratory standards (15-150 ng/mL), this level is suboptimal for hair follicle function:

• Serum ferritin levels are lower in women with alopecia areata and androgenetic alopecia, with iron deficiency being the most common nutritional deficiency worldwide and a sign of chronic diffuse telogen hair loss. 1
• Iron serves as a cofactor for ribonucleotide reductase, the rate-limiting enzyme in DNA synthesis, playing a critical role in tissues with high cellular turnover like the hair follicle matrix. 2
• Multiple studies demonstrate that women with various forms of hair loss have lower mean ferritin levels compared to controls without hair loss. 2

Why Other Common Causes Are Less Likely

Thyroid disease is excluded by the normal TSH of 2.900 uIU/mL (reference 0.450-4.500), which rules out both hypothyroidism and hyperthyroidism as contributors. 1

Vitamin D deficiency is excluded with a level of 47.2 ng/mL, well above the deficiency threshold of <20 ng/mL. This is notable because vitamin D deficiency shows strong association with hair loss, with 70% of alopecia areata patients having levels <20 ng/mL versus 25% of controls. 1

Androgenetic alopecia (female pattern hair loss) remains a possibility but would typically present with:

• Diffuse thinning at the crown with frontal hairline preservation 1
• Often associated with signs of androgen excess (acne, hirsutism, irregular periods) in younger women 3
• The clinical examination pattern would help distinguish this

Alopecia areata is less likely given the presentation as "thinning" rather than patchy loss, though it should remain in the differential. 1

Recommended Management Algorithm

Step 1: Iron Supplementation

• Initiate iron supplementation to raise ferritin levels above 40-70 ng/mL, the threshold associated with improved hair growth in clinical studies. 2
• Iron deficiency remains the most common nutritional deficiency worldwide and a recognized cause of chronic diffuse telogen hair loss. 2, 1

Step 2: Clinical Examination Details to Assess

• Hair pattern: Diffuse thinning at crown with frontal hairline preservation suggests androgenetic alopecia; patchy loss suggests alopecia areata. 1
• Exclamation mark hairs (short broken hairs around expanding patches) are pathognomonic for alopecia areata. 1
• Nail changes (pitting, ridging, dystrophy) occur in approximately 10% of alopecia areata patients. 1
• Signs of androgen excess: acne, hirsutism, which would warrant hormonal evaluation. 3, 4

Step 3: Consider Dermoscopy

• Dermoscopy is the single most useful non-invasive tool to differentiate diffuse alopecia areata, telogen effluvium, and androgenetic alopecia. 1
• Look for yellow dots, exclamation mark hairs, and cadaverized hairs (indicative of alopecia areata). 1
• Hair diameter variation and follicular miniaturization suggest androgenetic alopecia. 5

Step 4: Additional Testing Only If Indicated

• Hormonal evaluation (total testosterone, free testosterone, SHBG) only if signs of androgen excess are present. 3, 4
• Zinc levels may be considered, as serum zinc tends to be lower in alopecia areata patients, particularly those with resistant disease >6 months duration. 1

Critical Pitfalls to Avoid

• Do not dismiss ferritin levels in the 30s as "normal" for hair health—optimal ferritin for hair growth is higher than the lower limit of laboratory reference ranges. 2, 1
• Avoid ordering excessive laboratory tests when the diagnosis is clinically evident; most cases of hair loss can be diagnosed through history, examination, and targeted testing. 1
• Do not overlook the psychological impact of hair loss in young women, which may warrant assessment for anxiety and depression. 1, 6
• Remember that telogen effluvium (stress-induced shedding) can be triggered by various factors including illness, surgery, severe emotional stress, rapid weight loss, and nutritional deficiencies—inquire about recent stressors. 1

Expected Timeline

• Hair follicles have a lag time of 3-6 months, so improvement from iron supplementation will not be immediate.
• Reassess ferritin levels in 3 months and clinical response in 6 months.
• If no improvement occurs despite ferritin optimization, reconsider the diagnosis and evaluate for androgenetic alopecia or other causes. 2