Why Hair Falls Out — The Complete Science of Hair Loss Types & Causes

Hair loss has many faces, but consumer awareness has collapsed the entire category into "androgenetic alopecia" — the hereditary pattern baldness that dominates advertising. The reality is that diffuse thinning, patchy loss, and progressive recession can each stem from entirely different biological mechanisms, some of which are fully reversible with the right intervention and none of which respond well to treatments designed for a different type. This guide provides the mechanistic foundation for identifying which category of hair loss is actually present — an essential prerequisite for any rational treatment approach.

Type 1: Androgenetic Alopecia — Pattern Baldness

Androgenetic alopecia (AGA) is the most prevalent form of hair loss, affecting approximately 50% of men by age 50 and 40% of women by age 70. The mechanism involves the progressive miniaturisation of genetically susceptible hair follicles in response to dihydrotestosterone (DHT). DHT binds to androgen receptors (AR) in the dermal papilla of follicles in susceptible scalp regions, triggering a progressive shortening of the anagen phase with each successive hair cycle. Over years to decades, the full terminal hair (pigmented, thick, long) transforms through intermediate stages into a fine, colourless vellus hair, and eventually the follicle may become dormant.

The distribution pattern in AGA is diagnostically distinctive. In men, the Norwood scale describes progression from frontal recession at temples → vertex (crown) thinning → confluence of these two regions. Women typically follow the Ludwig pattern: diffuse thinning across the crown and top of the scalp while maintaining the frontal hairline — a pattern that reflects lower absolute DHT levels and higher peripheral aromatase activity converting androgens to oestrogens in female scalp tissue.

Genetic Factors and the Androgen Receptor Gene

The gene encoding the androgen receptor (AR) is located on the X chromosome, which is why maternal grandfather baldness has historically been considered predictive. However, AGA is polygenic — research has identified over 300 genomic loci associated with male pattern baldness, spread across multiple chromosomes. Susceptibility genes include those regulating 5-alpha reductase expression, androgen receptor sensitivity, prostaglandin signalling, and WNT pathway activation. Environmental modifiers including chronic stress, nutritional status, and inflammatory load substantially determine when and how severely genetic susceptibility is expressed.

Type 2: Telogen Effluvium — Stress-Induced Shedding

Telogen effluvium (TE) is a reactive, diffuse hair loss that occurs when a significant proportion of follicles are simultaneously driven into the telogen (resting) phase in response to a systemic trigger. Because the telogen phase lasts approximately 3 months, the dramatic shedding typically begins 2–3 months after the triggering event — creating a diagnostically confusing delay that causes many patients to incorrectly attribute their hair loss to events that occurred after the actual trigger.

Common triggers for acute telogen effluvium include: major surgery, high fever illness (including COVID-19, which produced a high prevalence of post-COVID TE), childbirth (postpartum TE is common, affecting 40–50% of women at 2–4 months postpartum), severe psychological stress, and crash dieting with caloric restriction below 1000 kcal/day. Chronic telogen effluvium — lasting more than 6 months — is typically associated with ongoing systemic factors such as iron deficiency, thyroid dysfunction, or chronic psychological stress.

Type 3: Alopecia Areata — Autoimmune Hair Loss

Alopecia areata (AA) is an autoimmune condition in which T-lymphocytes — specifically CD8+ cytotoxic T cells and natural killer cells — attack the hair bulb and follicle matrix cells. Healthy hair follicles possess a phenomenon called "immune privilege" — they actively suppress local immune activation to protect the rapidly dividing, antigenically distinct cells in the hair matrix. In alopecia areata, this immune privilege collapses. The trigger for collapse is not fully understood but involves genetic predisposition (20% concordance in identical twins suggests substantial environmental contribution), viral triggers, and inflammatory signalling.

Alopecia areata presents as discrete, sharply defined circular or oval patches of non-scarring hair loss. The scalp skin within the patch appears entirely normal — no scaling, redness, or scarring. Pathognomonic "exclamation mark" hairs (short hairs that taper toward the scalp) are found at the margins of active patches. AA can affect any hair-bearing area and ranges in severity from patchy (areata) to complete scalp loss (totalis) to complete body hair loss (universalis).

Type 4: Traction Alopecia — Mechanical Damage

Traction alopecia results from chronic, repetitive tension applied to hair follicles through certain hairstyles. High-tension practices — tight braids, cornrows, weaves, tight buns, ponytails, and locs — exert sustained mechanical force on follicle roots that initially produces reversible inflammation but, if continued long-term, causes permanent follicular scarring. The pattern is diagnostically distinctive: hairline recession in a band-like distribution around the periphery of the scalp, often with a characteristic "fringe sign" (maintained marginal hairline) in early stages.

Traction alopecia is highly preventable and, in early stages, fully reversible. The follicle retains viability as long as the mechanical insult is removed before scarring is established — typically within the first 5 years of onset. Early intervention involves immediate cessation of causative hairstyles and may be supported by topical steroids to reduce inflammation. Advanced traction alopecia with established follicular scarring does not respond to medical treatment and may require hair transplantation.

Type 5: Nutritional Deficiency Hair Loss

Several nutritional deficiencies produce hair shedding and thinning through metabolic impairment of the rapidly dividing hair matrix cells. The most clinically significant are:

• Iron/Ferritin deficiency: Serum ferritin below 30–40 ng/mL is associated with increased hair shedding even without frank anaemia. Ferritin is required by ribonucleotide reductase for DNA synthesis in dividing cells. This is the most common reversible cause of diffuse hair loss in premenopausal women. Target ferritin above 70 ng/mL for hair health.
• Vitamin D deficiency: 25-OH vitamin D receptors are expressed in hair follicle keratinocytes and play a role in anagen initiation. Population studies show consistent correlation between vitamin D deficiency and both androgenetic alopecia and non-scarring alopecia. Target serum 25-OH vitamin D above 50 nmol/L.
• Zinc deficiency: Zinc is a cofactor for 5-alpha reductase and multiple hair follicle enzymes. Deficiency impairs protein synthesis in matrix cells and produces brittle, slow-growing hair. Blood serum zinc measurement is unreliable; clinical diagnosis often requires trial supplementation.
• Biotin deficiency: True biotin deficiency causing hair loss is rare in individuals eating varied diets. The hair loss supplements industry has dramatically overstated the evidence for biotin supplementation in people without deficiency. However, those with digestive disorders affecting biotin absorption may genuinely benefit.

How to Identify Your Hair Loss Type

Pattern (distribution) is the primary differentiating factor: diffuse thinning across the entire scalp suggests TE or nutritional deficiency; patterned thinning following Norwood or Ludwig distribution suggests AGA; discrete circular patches suggest AA; and hairline recession at the periphery in a history of tight hairstyling suggests traction alopecia. A dermatologist can perform trichoscopy (dermoscopy of the scalp) and targeted blood testing (ferritin, thyroid panel, 25-OH vitamin D, CBC with differential) to confirm. Attempting to self-treat without identifying the mechanism is the most common cause of prolonged ineffective treatment.

When to See a Dermatologist

Seek dermatological evaluation if: hair loss has been ongoing for more than 6 months, loss involves discrete patches rather than diffuse shedding, the rate of loss is accelerating, you have other systemic symptoms (fatigue, weight changes, skin changes), you've already tried over-the-counter treatments for 6 months without response, or there is visible scalp inflammation, scaling, or pain. Trichologists (non-medical hair specialists) can provide useful adjunct assessment but cannot order blood tests or prescribe medications. For any hair loss that concerns you, a board-certified dermatologist is the appropriate first specialist.