What causes hair loss?

What Causes Hair Loss

Hair loss occurs through multiple mechanisms including autoimmune attack on hair follicles (alopecia areata), genetic/hormonal factors (androgenetic alopecia), nutritional deficiencies, stress-induced shedding, infections, medications, and physical trauma to the hair.

Autoimmune Causes

Alopecia areata is mediated by T lymphocytes attacking hair follicles, resulting in patchy, non-scarring hair loss that can affect any hair-bearing area. 1

• Approximately 20% of affected individuals have a family history, indicating genetic susceptibility 1
• The condition associates with other autoimmune diseases including thyroid disease, lupus, and vitiligo 1
• Exclamation mark hairs (short broken hairs) are characteristic findings around expanding patches 1
• Prognosis varies: 34-50% recover within one year, but 14-25% progress to total scalp or body hair loss 1
• Childhood onset and ophiasis pattern (scalp margin involvement) carry poorer prognoses 1

Genetic and Hormonal Causes

Androgenetic alopecia (pattern baldness) is the most common form of hair loss, driven by genetic sensitivity to dihydrotestosterone (DHT). 2

• Affects over 80 million people in the United States 3
• First-line treatment is topical minoxidil 3
• Oral finasteride is available for male patients 3

Nutritional Deficiencies

Multiple micronutrient deficiencies directly contribute to hair loss and should be evaluated in patients with unexplained alopecia.

• Vitamin D deficiency shows strong association with hair loss, with 70% of alopecia areata patients having levels <20 ng/mL versus 25% of controls 1

  • Lower vitamin D levels correlate inversely with disease severity 1
  • The vitamin D receptor (VDR) plays a critical role in hair follicle cycling 1

• Iron deficiency is the most common nutritional deficiency worldwide and causes chronic diffuse telogen hair loss 2

  • Serum ferritin levels are consistently lower in women with alopecia areata and androgenetic alopecia 2

• Zinc deficiency impairs hair follicle function, with serum zinc levels tending to be lower in alopecia areata patients 1, 2

• Folate deficiency may contribute to hair loss 2

Stress-Related Hair Loss

Telogen effluvium is stress-induced shedding where physiologic or emotional stressors push hair follicles prematurely into the resting (telogen) phase. 1, 4

• Triggers include illness, surgery, childbirth, severe emotional stress, rapid weight loss, and nutritional deficiencies 4
• Typically self-limited once the underlying stressor is removed 3
• Hair loss occurs 2-3 months after the triggering event 4

Medication-Induced Hair Loss

Anagen effluvium occurs when medications (particularly chemotherapy) interrupt actively growing hair follicles. 1

• Chemotherapy is the most common cause 1
• Hair loss is typically more rapid and severe than telogen effluvium 1

Infectious Causes

Tinea capitis (scalp ringworm) causes patchy hair loss with scalp inflammation and scaling. 1

• Requires fungal culture for diagnosis 1
• Treatment requires oral antifungal therapy 3
• Signs may be subtle, making diagnosis challenging 1

Physical/Mechanical Causes

Traction alopecia results from chronic tension on hair from tight hairstyles, braiding, or hair extensions. 3

• Treatment involves decreasing tension on the hair 3
• Can progress to permanent scarring if not addressed early 3

Trichotillomania is compulsive hair pulling that mimics alopecia areata. 1

• Distinguished by incomplete hair loss and firmly anchored broken hairs that remain in anagen phase 1
• May coexist with alopecia areata in some cases 1

Systemic Disease-Related Hair Loss

Multiple systemic conditions manifest with hair loss as a prominent feature:

• Thyroid disease (both hypo- and hyperthyroidism) causes diffuse hair thinning 2
• Systemic lupus erythematosus can cause both scarring and non-scarring alopecia 1
• Secondary syphilis presents with patchy "moth-eaten" hair loss 1
• Polycystic ovary syndrome (PCOS) causes androgenetic alopecia in women with androgen excess 2

Environmental and Cosmetic Factors

Exogenous exposures contribute significantly to hair damage and loss. 5

• Heat styling, chemical treatments, and harsh hair cosmetics damage hair structure 5
• Solar radiation and water exposure affect hair texture and integrity 5
• X-irradiation causes permanent hair loss 5

Common Diagnostic Pitfalls

• Do not confuse trichotillomania with alopecia areata: Look for exclamation mark hairs (alopecia areata) versus firmly anchored broken hairs (trichotillomania) 1
• Do not miss tinea capitis: Subtle inflammation and scaling require high clinical suspicion and fungal culture 1
• Do not overlook nutritional deficiencies: Check vitamin D, ferritin, zinc, and folate levels in unexplained hair loss 2
• Do not ignore psychological impact: Hair loss severely affects quality of life, anxiety, depression, and work productivity 3
• Do not order excessive testing when diagnosis is clinically evident: Most alopecia areata cases are diagnosed clinically without laboratory workup 1

When to Investigate Further

Laboratory testing is indicated when:

• Diagnosis is uncertain or presentation is atypical 1
• Diffuse alopecia areata is suspected (may require biopsy) 1
• Signs of androgen excess exist (acne, hirsutism, irregular periods) - check testosterone, SHBG 2
• Systemic disease is suspected - check TSH, lupus serology, syphilis serology 1, 2
• Nutritional deficiency is suspected - check ferritin, vitamin D, zinc, folate 2