What laboratory tests are recommended to determine the underlying cause of hair loss?

Laboratory Testing for Hair Loss

In most cases of clinically evident alopecia areata, laboratory investigations are unnecessary and diagnosis should be made clinically, but when the diagnosis is uncertain, atypical, or when evaluating diffuse hair loss, targeted laboratory testing should focus on nutritional deficiencies (ferritin, vitamin D, zinc), thyroid function (TSH), and hormonal evaluation in women with signs of androgen excess. 1

When to Order Laboratory Tests

Laboratory testing should be reserved for specific clinical scenarios rather than ordered routinely 1:

• Uncertain or atypical presentations where the clinical diagnosis is not straightforward 1
• Diffuse alopecia that is challenging to diagnose clinically 1
• When systemic diseases need to be excluded from the differential diagnosis 1
• Women with signs of androgen excess including acne, hirsutism, and irregular periods 1

Essential Laboratory Tests by Clinical Scenario

Nutritional Deficiency Evaluation

Serum ferritin is the single most important nutritional test, as iron deficiency is the most common nutritional deficiency worldwide and a marker of chronic diffuse telogen hair loss, with lower levels found in women with both alopecia areata and androgenetic alopecia 1:

• Vitamin D levels should be checked, as 70% of alopecia areata patients have deficiency (<20 ng/mL or <50 nmol/L) versus 25% of controls, with lower levels correlating inversely with disease severity 1
• Serum zinc levels tend to be lower in alopecia areata patients, particularly those with resistant disease lasting >6 months 1
• Folate levels may contribute to the evaluation, though the evidence is less robust 1

Thyroid Function Assessment

TSH should be checked to rule out thyroid disease, which can cause hair loss 1:

• High TSH with low free T4 indicates biochemical hypothyroidism 1
• Thyroid peroxidase (TPO) antibody testing is warranted if biochemical hypothyroidism is confirmed 1
• This is particularly important given the association between alopecia areata and autoimmune thyroid disease 1

Hormonal Evaluation in Women

For women presenting with signs of androgen excess, the American Academy of Family Physicians recommends 1:

• Total testosterone or bioavailable/free testosterone levels 1
• Sex hormone binding globulin (SHBG) levels 1
• Prolactin level if hyperprolactinemia is suspected 1
• Two-hour oral glucose tolerance test if diabetes or insulin resistance is suspected 1
• Fasting lipid and lipoprotein levels to evaluate for dyslipidemia 1
• Consider evaluation for polycystic ovary syndrome (PCOS) in appropriate clinical contexts 1

Infectious and Systemic Disease Evaluation

Fungal culture is recommended when tinea capitis is suspected based on clinical presentation with scalp inflammation and scaling 1:

• Serology for lupus erythematosus when systemic lupus is in the differential diagnosis 1
• Serology for syphilis when secondary syphilis is suspected (presents with patchy "moth-eaten" hair loss) 1

When Skin Biopsy is Indicated

Skin biopsy should be performed in specific situations 1:

• Difficult cases where the diagnosis remains unclear after clinical evaluation 1
• Early scarring alopecia or when scarring is suspected 1
• Diffuse alopecia areata that is challenging to diagnose clinically 1

Common Pitfalls to Avoid

Avoid ordering excessive laboratory tests when the diagnosis is clinically evident, as this increases costs without improving outcomes 1:

• Alopecia areata with typical patchy, non-scarring hair loss and dermoscopic findings (yellow dots, exclamation mark hairs, cadaverized hairs) does not require laboratory confirmation 1
• Do not overlook dermoscopy as a non-invasive diagnostic tool that can provide valuable diagnostic information without laboratory testing 1
• Remember to assess psychological impact, as hair loss may cause considerable psychological and social disability warranting evaluation for anxiety and depression 1

Algorithmic Approach

1. Start with clinical diagnosis: Examine pattern, distribution, and dermoscopic features 1
2. If typical alopecia areata: No laboratory testing needed 1
3. If diffuse hair loss or uncertain diagnosis: Check ferritin, vitamin D, zinc, and TSH 1
4. If signs of androgen excess in women: Add testosterone, SHBG, and consider PCOS workup 1
5. If scalp inflammation/scaling: Obtain fungal culture 1
6. If systemic disease suspected: Add targeted serology (lupus, syphilis) 1
7. If diagnosis remains unclear: Proceed to skin biopsy 1