What blood tests should be ordered to evaluate the causes of diffuse hair loss?

Blood Tests for Evaluating Hair Loss

For diffuse hair loss, order serum ferritin (target ≥60 ng/mL), TSH with free T4, vitamin D, and zinc levels as your core panel; additional tests are only needed when specific clinical features suggest androgen excess, autoimmune disease, or systemic illness. 1

Core Laboratory Panel (Order in All Cases of Diffuse Hair Loss)

Serum ferritin is the single most important test, as iron deficiency is the most common nutritional cause of diffuse hair loss worldwide. 2, 1

• Target ferritin ≥60 ng/mL for optimal hair growth (not the standard anemia threshold of 12-15 ng/mL) 1, 3
• Iron deficiency accounts for 70% of female alopecia cases in recent studies 3
• The corresponding hemoglobin for adequate hair growth is ≥13.0 g/dL, higher than the anemia cutoff 3

TSH and free T4 to screen for thyroid disease, which commonly causes hair loss. 1, 4

• If biochemical hypothyroidism is confirmed (high TSH, low free T4), add thyroid peroxidase (TPO) antibody testing 4

Vitamin D level should be checked, as deficiency (<20 ng/mL) shows strong association with hair loss. 1, 4

• 70% of alopecia areata patients have vitamin D <20 ng/mL versus 25% of controls 4
• Lower vitamin D levels correlate inversely with disease severity 4

Serum zinc should be measured, as zinc deficiency impairs hair follicle function. 1, 4

• Zinc levels tend to be lower in alopecia areata patients, especially those with disease resistant for >6 months 2, 4

Conditional Tests (Only When Clinically Indicated)

Androgen testing (total testosterone, free testosterone, SHBG) should only be ordered if signs of androgen excess are present: acne, hirsutism, or irregular menstrual periods. 1, 4

• Do not order routinely in all women with hair loss 1

Complete blood count to detect anemia, particularly when ferritin testing suggests iron deficiency. 5, 6

Fungal culture of scalp scrapings is indicated only when inflammation or scaling suggests tinea capitis. 1, 4, 7

Lupus serology (ANA, anti-dsDNA) should be ordered only when systemic features are present: joint pain, photosensitivity, facial rash. 1, 4

Syphilis serology is warranted only when risk factors for infection exist. 1, 4

Scalp biopsy is reserved for uncertain diagnosis or suspected scarring alopecia after clinical examination and dermoscopy. 1, 4, 7

Critical Pitfalls to Avoid

Do not order extensive autoimmune panels in straightforward cases of alopecia areata or telogen effluvium, as the modest increase in autoimmune disease prevalence does not justify routine screening. 1, 4

Do not use standard anemia thresholds for ferritin interpretation in hair loss—the adequate ferritin level for hair growth (≥60 ng/mL) is much higher than the level that defines anemia (12-15 ng/mL). 1, 3

Do not order androgen testing routinely in all women with hair loss; this is only indicated when clinical signs of hyperandrogenism are present. 1, 4

Do not perform fungal culture routinely—this test is only needed when the scalp shows inflammation or scaling suggestive of tinea capitis. 1, 4

Clinical Context for Test Interpretation

When alopecia areata is clinically evident (patchy loss with exclamation-mark hairs on dermoscopy), investigations are unnecessary in most cases. 1, 4

When telogen effluvium is suspected (diffuse shedding 2-3 months after a triggering event like childbirth, fever, or surgery), the core panel helps identify reversible causes. 5, 6

When female pattern hair loss is suspected (gradual central scalp thinning with preserved frontal hairline), the core panel plus androgen testing (if signs of excess are present) guides management. 5, 8

Psychological stress and iron deficiency are the two most common underlying factors for telogen effluvium, making ferritin testing particularly high-yield. 6

Prognostic Considerations

Disease duration matters for iron deficiency-related alopecia: initiating iron supplementation within 6 months of onset results in better prognosis compared to longer-standing disease. 3

Telogen effluvium is self-limited and resolves in 3-6 months if the trigger is removed or treated, with spontaneous remission in up to 80% of cases with duration <1 year. 7, 5, 6