Diagnostic Approach to Thinning Hair
The diagnosis of thinning hair requires clinical examination focusing on hair distribution pattern, dermoscopy to identify characteristic features, and targeted laboratory testing to exclude systemic causes—with the specific diagnostic pathway determined by whether the pattern is diffuse, patchy, or localized to the crown. 1
Initial Clinical Assessment
Pattern Recognition
Diffuse thinning at the crown with frontal hairline preservation indicates androgenetic alopecia (pattern hair loss related to DHT sensitivity), which can be diagnosed clinically without extensive testing 1
Patchy hair loss with well-demarcated areas suggests alopecia areata, an autoimmune condition where T lymphocytes attack hair follicles 1, 2
Exclamation mark hairs (short broken hairs around expanding patches) are pathognomonic for alopecia areata and confirm the diagnosis when present 1, 2
Dermoscopy as Primary Diagnostic Tool
Dermoscopy is the single most useful non-invasive tool to differentiate between diffuse alopecia areata, telogen effluvium, and androgenetic alopecia 1
Yellow dots, exclamation mark hairs, and cadaverized hairs on dermoscopy indicate active alopecia areata 1
Regular round yellow dots with black dots are characteristic dermoscopic features that distinguish alopecia areata from other causes 1
Laboratory Testing Algorithm
When Laboratory Tests Are Indicated
Laboratory investigations are unnecessary in most cases when alopecia areata or androgenetic alopecia can be diagnosed clinically with characteristic patterns and dermoscopic findings 1, 2
Testing should be reserved for uncertain diagnosis, atypical presentation, or when systemic disease is suspected 1, 2
Essential Laboratory Panel
For patients with endocrine therapy-induced alopecia or unclear etiology, the following differential diagnoses must be ruled out through laboratory testing: 3
Thyroid function: TSH and free T4 to exclude hypothyroidism, which commonly causes hair loss 3, 1
Iron stores: Serum ferritin (iron deficiency is the most common nutritional deficiency worldwide causing chronic diffuse telogen hair loss) 1
Vitamin D level: Deficiency (<20 ng/mL) shows strong association with hair loss, with 70% of alopecia areata patients deficient versus 25% of controls 1
Serum zinc: Levels tend to be lower in alopecia areata patients, particularly those with resistant disease >6 months duration 1
Folate level: May contribute to hair loss when deficient 1
Hormonal Assessment (When Indicated)
In women with signs of androgen excess (acne, hirsutism, irregular periods), additional testing is warranted: 1
- Total testosterone or bioavailable/free testosterone levels 1
- Sex hormone binding globulin (SHBG) 1
- Consider evaluation for polycystic ovary syndrome (PCOS) 1
- Prolactin level if hyperprolactinemia is suspected 1
Additional Diagnostic Procedures
When to Perform Scalp Biopsy
Biopsy is indicated for difficult cases, early scarring alopecia, or diffuse alopecia areata that is diagnostically challenging when dermoscopy is inconclusive 1
Fungal culture is mandatory when tinea capitis (scalp ringworm) is suspected, as incorrect diagnosis made on clinical grounds alone is the most common cause of treatment failure 1
Serology for Systemic Disease
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
Practical Clinical Algorithm
Step 1: Examine hair distribution pattern and perform dermoscopy looking specifically for yellow dots and exclamation mark hairs 1
Step 2: If characteristic dermoscopic features are present, diagnose clinically without biopsy (alopecia areata) or based on pattern (androgenetic alopecia) 1
Step 3: If dermoscopy is inconclusive, obtain targeted laboratory tests: TSH, ferritin, vitamin D, zinc, and folate to exclude telogen effluvium triggers 1
Step 4: If diagnosis remains uncertain after laboratory testing, perform scalp biopsy for definitive histopathologic diagnosis 1
Common Pitfalls to Avoid
Avoid ordering excessive laboratory tests when the diagnosis is clinically evident with characteristic patterns and dermoscopic findings 1
Do not overlook dermoscopy as a valuable non-invasive diagnostic tool that provides information not visible to the naked eye 1
Consider the psychological impact of hair loss, which may warrant assessment for anxiety and depression, as alopecia can cause considerable psychological and social disability 1
Remember that 34-50% of alopecia areata patients recover within one year without treatment, making observation a reasonable option for limited disease 1
In patients with endocrine therapy-induced alopecia, differential diagnoses must be actively excluded through the laboratory panel described above, as this presentation can mimic female-pattern hair loss 3