What blood tests are recommended for diagnosing the cause of hair loss?

Blood Work for Hair Loss

The essential blood tests for evaluating hair loss include serum ferritin, vitamin D, TSH, and zinc levels, as these address the most common reversible nutritional and endocrine causes. 1

Core Laboratory Panel

Order these four tests for virtually all patients presenting with hair loss:

• Serum ferritin is the single most important test, as iron deficiency is the world's most common nutritional deficiency and strongly associates with chronic diffuse telogen hair loss 1, 2

  • Women with both alopecia areata and androgenetic alopecia show lower ferritin levels 1
  • Target ferritin ≥60 ng/mL for adequate hair growth, which corresponds to hemoglobin ≥13.0 g/dL 3
  • Iron deficiency accounts for 70% of female alopecia cases in recent studies 3

• Vitamin D (25-hydroxyvitamin D) should be checked in all patients, as 70% of alopecia areata patients are deficient (<20 ng/mL) compared to 25% of controls 4, 1

  • Lower vitamin D levels correlate inversely with disease severity 4
  • The vitamin D receptor plays a critical role in hair follicle cycling 4

• Thyroid stimulating hormone (TSH) must be checked to exclude thyroid disease, a common reversible cause of hair loss 4, 1

  • If TSH is elevated with low free T4, add thyroid peroxidase (TPO) antibody testing to confirm autoimmune hypothyroidism 4
  • Thyroid disease associates with alopecia areata as a comorbid autoimmune condition 4

• Serum zinc should be measured, as zinc serves as a cofactor for multiple enzymes in hair follicle function 4, 1

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

Additional Testing Based on Clinical Presentation

Add these tests only when specific clinical features are present:

For Women with Signs of Androgen Excess

Order when patient has acne, hirsutism, or irregular menstrual periods:

• Total testosterone or bioavailable/free testosterone plus sex hormone binding globulin (SHBG) to assess for hyperandrogenism 4, 1
• Prolactin level if hyperprolactinemia is suspected 4, 1
• Two-hour oral glucose tolerance test if diabetes or insulin resistance is suspected, as these associate with PCOS-related hair loss 4, 1
• Fasting lipid panel to evaluate for dyslipidemia accompanying metabolic syndrome 4, 1

For Suspected Infectious or Systemic Causes

• Fungal culture is mandatory when tinea capitis is suspected, as incorrect clinical diagnosis is the most common cause of treatment failure 4, 5
• Serology for lupus erythematosus when systemic lupus is in the differential diagnosis 4
• Serology for syphilis when secondary syphilis is suspected (presents with patchy "moth-eaten" hair loss) 4

Critical Pitfalls to Avoid

Do not order extensive laboratory panels when the clinical diagnosis is evident - alopecia areata is typically diagnosed clinically through pattern recognition (exclamation mark hairs, yellow dots on dermoscopy) without requiring laboratory confirmation 4, 1

Do not wait for laboratory results to initiate treatment in certain scenarios - if a patient presents with rapid weight loss, poor dietary intake, or symptoms suggesting severe nutritional deficiency, initiate supplementation immediately 1

Do not overlook the psychological impact - assess for anxiety and depression regardless of laboratory findings, as hair loss causes considerable psychological and social disability 4, 1

Do not confuse normal ferritin ranges with optimal levels for hair growth - the traditional lower limit of normal ferritin (12-15 ng/mL) is far below the 40-60 ng/mL needed for adequate hair growth 3, 2

When Laboratory Testing is Unnecessary

Most cases of alopecia areata do not require laboratory workup - the British Journal of Dermatology states that investigations are unnecessary when the diagnosis is clinically evident 4

Laboratory tests should be reserved for:

• Uncertain or atypical presentations 4
• Diffuse alopecia areata that is diagnostically challenging 4
• When other conditions in the differential diagnosis need exclusion 4

Diagnostic Algorithm

1. Perform dermoscopy first - look for yellow dots and exclamation mark hairs pathognomonic for alopecia areata 4
2. If dermoscopy shows characteristic features - diagnose clinically without laboratory testing 4
3. If presentation is diffuse or atypical - order the core four-test panel (ferritin, vitamin D, TSH, zinc) 1
4. Add targeted tests based on specific clinical features (androgen excess, suspected infection, systemic disease) 4, 1
5. Consider scalp biopsy only if diagnosis remains uncertain after dermoscopy and targeted laboratory testing 4