The Causes of Hair Loss – The 2025 Ultimate Hair Loss Guide

Hair loss, medically termed alopecia, is a common concern that affects individuals across genders and age groups. For some, it is a cosmetic inconvenience; for others, it serves as a visible indicator of underlying medical, genetic, or environmental factors. While conditions such as male pattern baldness (androgenetic alopecia) and patchy hair loss from alopecia areata are readily recognised, diagnosing hair loss can sometimes be complex and multifaceted, involving an interplay of biology, history, and examination.

This article explores the science behind hair growth, the various causes of hair loss, and the strategies healthcare providers employ for precise diagnosis.

The Science of Hair: Biology and Structure

Hair is a keratinised structure originating from hair follicles, which are located in the dermis of the skin. Understanding the anatomy and physiology of hair follicles is essential for grasping the mechanisms behind hair loss.

Hair Follicle Anatomy

The human scalp is home to approximately 100,000 to 150,000 hair follicles, each cycling independently to produce hair. A follicle consists of four primary regions:

1. The Bulb: Found at the base of the follicle, it contains the hair matrix—a cluster of rapidly dividing keratinocytes responsible for hair shaft formation.
2. The Suprabulbar Region: Located above the bulb, this area transitions into the isthmus.
3. The Isthmus: A critical region between the insertion points of the arrector pili muscle and the sebaceous gland.
4. The Infundibulum: The most superficial segment, connecting the follicle to the skin surface.

Hair production occurs in the hair shaft, a layered structure comprising:

• Medulla: The innermost layer, often absent in fine hairs.
• Cortex: The thick middle layer, rich in keratin and melanin, determining hair strength and colour.
• Cuticle: The outermost protective layer, composed of overlapping keratinocytes.

The Role of Stem Cells in Hair Growth

The bulge region near the arrector pili muscle insertion is home to hair follicle stem cells. These cells play an integral role in regenerating follicles and maintaining the hair growth cycle. Damage to these stem cells, often observed in cicatricial (scarring) alopecias, leads to permanent hair loss.

Types of Hair Follicles

Human hair follicles produce two major types of hair:

1. Terminal Hair: Thick, pigmented hairs found on the scalp, eyebrows, and pubic regions. These follicles extend deep into the subcutaneous fat and produce hair strands with diameters exceeding 0.06 mm.
2. Vellus Hair: Fine, unpigmented hairs covering much of the body, produced by follicles located in the reticular dermis.

The transition between these hair types occurs during puberty and certain pathological conditions. For example, in androgenetic alopecia, terminal hairs are progressively miniaturised into vellus hairs.

The Hair Growth Cycle

Hair growth follows a lifelong cycle involving three primary phases. Unlike synchronised cycles in some animals, human hair cycles independently across individual follicles, ensuring continuous coverage of the scalp.

1. Anagen (Growth Phase): This is the active growth phase, lasting 2–6 years on the scalp. Approximately 90% of scalp follicles are in this phase at any given time. Terminal hair grows at a rate of about 0.3 mm per day, whereas eyebrow hair grows more slowly due to its shorter anagen phase.
2. Catagen (Regression Phase): This transitional phase lasts around three weeks. During catagen, hair production ceases, and the follicle shrinks as the dermal papilla retracts upward.
3. Telogen (Resting Phase): Lasting 2–3 months, this phase involves the detachment of fully keratinised “club hairs.” These hairs are eventually shed, with normal daily shedding ranging from 50 to 150 hairs.

After telogen, the follicle re-enters anagen, initiating the production of a new hair shaft. Disruptions in this cycle, such as prolonged telogen in telogen effluvium, can result in diffuse hair thinning.

Causes of Hair Loss: Classifications and Mechanisms

Hair loss, or alopecia, can be classified into three major categories based on underlying mechanisms: scarring alopecia, non-scarring alopecia, and structural hair disorders.

1. Cicatricial (Scarring) Alopecia

Scarring alopecias are characterised by irreversible follicular destruction due to inflammatory or traumatic processes. Follicular stem cell damage leads to permanent hair loss.

• Lymphocytic Cicatricial Alopecias: Conditions such as lichen planopilaris and discoid lupus erythematosus cause inflammation around the follicular unit.
• Neutrophilic Cicatricial Alopecias: Disorders like folliculitis decalvans involve purulent inflammation, often linked to bacterial infections.
• Mixed Cicatricial Alopecias: Acne keloidalis nuchae and erosive pustular dermatosis involve overlapping inflammatory patterns.

2. Non-Scarring Alopecia

Non-scarring alopecias preserve follicular structures, allowing for the possibility of regrowth.

• Alopecia Areata: This autoimmune condition causes patchy hair loss, often progressing to more severe forms such as alopecia totalis (scalp) or alopecia universalis (body).
• Telogen Effluvium: Triggered by physiological stressors such as surgery, childbirth, or acute illness, this condition results in a disproportionate shift of follicles into the telogen phase.
• Androgenetic Alopecia: Commonly known as male or female pattern baldness, this hereditary condition involves the miniaturisation of follicles under androgen influence.

3. Structural Hair Disorders

Structural disorders affect the hair shaft’s integrity, leading to breakage rather than shedding. These may be genetic or acquired through physical or chemical damage.

• Genetic Disorders: Conditions like trichothiodystrophy and monilethrix cause brittle, fragile hair.
• Acquired Disorders: Practices such as excessive heat styling, chemical treatments, or tight hairstyles can damage the hair shaft, leading to trichorrhexis nodosa or traction alopecia.

Diagnosing Hair Loss: A Comprehensive Approach

Accurate diagnosis begins with a detailed patient history, clinical examination, and appropriate diagnostic tests.

Patient History

A thorough history provides critical insights into the aetiology of hair loss.

1. Duration and Progression: Is the hair loss acute, chronic, or episodic? Sudden onset suggests conditions like telogen effluvium, while slow progression is typical of androgenetic alopecia.
2. Pattern and Distribution: Localised loss (e.g., patches in alopecia areata) differs from diffuse thinning (e.g., telogen effluvium).
3. Associated Symptoms: Itching, pain, or burning may indicate inflammatory conditions.
4. Hair Care Practices: Chemical treatments, heat styling, and tight hairstyles are potential culprits in traction or acquired structural alopecia.
5. Medical and Family History: Hormonal imbalances, autoimmune diseases, and genetic predispositions often play a role in hair loss.

Clinical Examination

Physical assessment involves evaluating the scalp, hair density, and shaft characteristics.

1. Visual Inspection: Identifying inflammation, scaling, or follicular ostia absence helps differentiate scarring from non-scarring alopecia.
2. Hair Pull Test: This simple test evaluates active shedding. Extracting more than six hairs from a grasp of 50–60 suggests ongoing hair loss.
3. Trichoscopy: Dermoscopy aids in visualising follicular and shaft abnormalities, such as “yellow dots” in alopecia areata or miniaturised follicles in androgenetic alopecia.

Advanced Diagnostic Tools

1. Scalp Biopsy: Histological analysis distinguishes between scarring and non-scarring alopecia. It is particularly useful in cicatricial cases.
2. Microscopic Examination: Assessing hair shafts reveals structural abnormalities like trichorrhexis nodosa or pili torti.
3. Laboratory Tests:
   • Thyroid Function Tests: To rule out hypothyroidism or hyperthyroidism.
   • Serum Ferritin: Low levels indicate iron deficiency, a common cause of telogen effluvium.
   • Hormonal Panels: Assessing androgens in cases of androgenetic alopecia or polycystic ovary syndrome (PCOS).
4. Trichograms and Phototrichograms: These techniques evaluate hair cycle phases and density changes over time. Phototrichograms, enhanced by software like TrichoScan, offer detailed quantitative data.

Patient Support and Treatment Pathways

Hair loss can significantly impact an individual’s self-esteem and emotional well-being. Addressing these psychological effects is as important as treating the underlying condition.

1. Education and Counselling: Patients should be informed about their condition, potential outcomes, and treatment options.
2. Referrals: Complex cases, such as extensive alopecia areata or unresponsive scarring alopecias, may require dermatology consultation.
3. Support Groups: Organisations like the National Alopecia Areata Foundation and the British Hair and Nail Society provide resources and emotional support.

Treatment Modalities

• Medical Treatments:
  • Topical Minoxidil: Widely used for androgenetic alopecia and alopecia areata.
  • Oral Finasteride: Effective in men with androgenetic alopecia by inhibiting dihydrotestosterone (DHT).
  • Corticosteroids: Administered topically or intralesionally for autoimmune alopecia.
• Non-Medical Interventions:
  • Platelet-Rich Plasma (PRP) Therapy: Emerging as a promising option for hair regrowth.
  • Hair Transplantation: Effective for advanced androgenetic alopecia.
• Lifestyle Modifications: Addressing stress, improving nutrition, and avoiding damaging hair practices can enhance overall hair health.

Conclusion

Hair loss is a multifactorial condition requiring a nuanced, evidence-based approach. By understanding the intricacies of hair biology, recognising diverse aetiologies, and employing comprehensive diagnostic tools, healthcare providers can deliver effective treatment while addressing the psychological impacts of hair loss.

With advancements in medical therapies and diagnostic technologies, the future holds promise for improving outcomes for individuals experiencing hair loss. For patients, seeking timely professional advice and adopting healthy hair care practices are critical steps towards achieving and maintaining optimal hair health.