What labs should be ordered for a patient presenting with hair loss, considering potential underlying medical conditions such as anemia, thyroid disorders, and hormonal imbalances?

Laboratory Testing for Hair Loss

Order a focused panel including complete blood count, serum ferritin, thyroid-stimulating hormone (TSH), and vitamin D 25-hydroxyvitamin D levels as the essential baseline workup for all patients presenting with hair loss. 1, 2

Essential First-Line Laboratory Tests

The following tests should be obtained in all patients with hair loss:

• Complete blood count (CBC) to evaluate for anemia and systemic disease 2
• Serum ferritin to assess iron stores, as iron deficiency is the most common nutritional deficiency worldwide and a sign of chronic diffuse telogen hair loss, with lower levels found in women with both alopecia areata and androgenetic alopecia 1
• Thyroid-stimulating hormone (TSH) to screen for thyroid dysfunction, as both hypothyroidism and hyperthyroidism commonly cause hair loss 3, 1, 2
• Vitamin D 25-hydroxyvitamin D levels, as deficiency (<20 ng/mL or <50 nmol/L) shows strong association with hair loss, with 70% of alopecia areata patients being deficient versus 25% of controls, and lower levels correlate inversely with disease severity 1, 2

Additional Testing Based on Clinical Presentation

For Suspected Thyroid Disease

• Free T4 should be added if TSH is abnormal (high TSH with low free T4 indicates biochemical hypothyroidism) 3, 1
• Thyroid peroxidase (TPO) antibody testing is warranted if biochemical hypothyroidism is confirmed 3, 1

For Women with Signs of Androgen Excess

When patients present with acne, hirsutism, or irregular periods, consider:

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

For Suspected Nutritional Deficiencies

• Serum zinc levels, as zinc serves as a cofactor for multiple enzymes and plays a role in hair follicle function, with levels tending to be lower in patients with alopecia areata, particularly those with resistant disease >6 months duration 3, 1
• Folate levels may contribute to hair loss when deficient 1

For Specific Differential Diagnoses

• Fungal culture is mandatory when tinea capitis is suspected, as the most common cause of treatment failure in onychomycosis is incorrect diagnosis made on clinical grounds alone 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 Laboratory Testing May Be Unnecessary

Laboratory investigations are unnecessary in most cases of alopecia areata when the diagnosis is clinically evident, as it is typically diagnosed by characteristic dermoscopic findings including yellow dots, exclamation mark hairs, and cadaverized hairs 1. However, testing should be considered when:

• The diagnosis is uncertain or presentation is atypical 1
• Other conditions in the differential diagnosis need to be ruled out 1
• Diffuse alopecia areata is suspected, which may be diagnostically challenging 1

Common Pitfalls to Avoid

• Avoid ordering excessive laboratory tests when the diagnosis is clinically evident, particularly in classic presentations of alopecia areata 1
• Do not delay checking serum ferritin, as iron deficiency is frequently overlooked and is the most common nutritional deficiency contributing to hair loss 1
• Always check vitamin D levels, given the high prevalence of deficiency (70%) in alopecia areata patients and its inverse correlation with disease severity 1
• Never start thyroid hormone replacement before steroids in patients with both adrenal insufficiency and hypothyroidism, to avoid precipitating an adrenal crisis 3
• Do not overlook the psychological impact of hair loss, which may warrant assessment for anxiety and depression, as patients with moderate to severe hair loss are more likely to have accompanying anxiety, depression, and lower quality-of-life scores 1, 4

Algorithmic Approach to Laboratory Testing

1. Start with the essential four tests (CBC, ferritin, TSH, vitamin D) in all patients 2
2. Add free T4 and TPO antibody if TSH is abnormal 3, 1
3. Consider androgen testing (testosterone, SHBG, prolactin) in women with signs of hyperandrogenism 1
4. Add zinc and folate if nutritional deficiency is suspected or in resistant cases 1
5. Order fungal culture before treating suspected tinea capitis 1
6. Consider autoimmune serologies (lupus, syphilis) only when clinical features suggest systemic disease 1