What are the common causes of hair loss? What is androgenetic alopecia (AGA)?

How does androgenetic alopecia (AGA) occur step by step?

Androgenetic alopecia (AGA) is not a sudden loss of all hair, but a gradual process where hair follicles, under genetic predisposition, become sensitive to the androgen metabolite DHT (dihydrotestosterone), leading to progressive miniaturization of the follicles.

This can be broken down into three changes:

1. Shortened anagen phase: The growth phase (anagen) of each hair becomes shorter, so the hair falls out before reaching its original length and thickness.
2. Thinner and shallower follicles: Affected follicles shrink over successive cycles, turning thick, dark terminal hairs into fine, light-colored vellus hairs, visually appearing thinner, sparser, and unable to cover the scalp.
3. Regional distribution: Follicles sensitive to DHT are mainly concentrated on the forehead and crown; the occipital and temporal areas are relatively unaffected, which is the basis for the donor area in hair transplantation.

Because it is a gradual miniaturization rather than sudden shedding, many people first notice that their hair becomes thinner, appears flat when oily, or the part widens, rather than experiencing massive hair loss.

The above is a neutral educational summary; individual causes of hair loss require evaluation and diagnosis by a physician and are not medical advice.

How to interpret the Norwood and Ludwig hair loss classifications?

Two commonly used classification scales describe the extent and pattern of androgenetic alopecia, serving as a common language during consultations; they describe the current stage, not a value judgment.

Male: Hamilton-Norwood scale (stages I–VII)

Female: Ludwig scale (grades I–III) typically shows diffuse thinning on the crown with widening of the part, but the frontal hairline is usually preserved: grade I is mild thinning at the part, grade II is more noticeable thinning, grade III is extensive thinning over the crown.

Physicians use the classification along with the degree of follicular miniaturization to assess hair density reserve and the adequacy of the occipital donor area.

The above is neutral information; the classification is for description and communication purposes only. Actual evaluation should be based on an in-person consultation with a physician and is not medical advice.

Is hair loss temporary or permanent? How to differentiate?

Distinguishing between hair loss that can recover spontaneously and permanent follicular degeneration is important because the management approaches differ. A common distinction is based on whether scarring is present:

• Non-scarring alopecia: The follicular structure remains intact, theoretically allowing recovery. This includes androgenetic alopecia (progressive miniaturization), telogen effluvium (stress/postpartum/illness/rapid weight loss causing shedding 2–4 months later), thyroid or iron deficiency-related hair loss, and alopecia areata (autoimmune circular bald patches). Once the trigger is removed or controlled, partial recovery may occur.
• Scarring alopecia: Hair follicles are replaced by fibrous tissue and disappear; hair in these areas does not regenerate, and management differs.

For androgenetic alopecia, early stages mainly involve thinning, and when follicles are not yet completely atrophied, there is still room for observation and intervention. Once follicles have been miniaturized for so long that they stop producing visible hair, the area is generally considered permanently bald, which is why physicians discuss whether hair transplantation is needed to replenish follicles.

The above is neutral information; reversibility should be determined by a physician based on individual circumstances and is not medical advice.

What tests are performed when hair loss is suspected?

There are many causes of hair loss, and physicians typically first identify the cause before discussing treatment. Common evaluation methods include:

• History and pattern observation: Rate of hair loss, family history, recent stress/illness/childbirth, medications, weight changes, etc.
• Pull test: Gently pulling a small bundle of hair to see how many come out, providing a preliminary indication of active shedding.
• Trichoscopy/dermoscopy: Observing whether hair shafts within follicular units vary in thickness (an indicator of miniaturization) and the condition of follicular openings.
• Blood tests: Checking thyroid function, ferritin, hormones, etc., as needed to rule out reversible systemic factors.
• Scalp biopsy: Used when necessary to differentiate scarring from non-scarring alopecia.

Common specialties for consultation include dermatology (including hair specialists); hormone-related hair loss may also involve cross-specialty evaluation.

The above is neutral information; tests are arranged by the physician based on individual circumstances and are not medical advice.

How much hair loss per day is normal? Understanding the hair growth cycle

Before determining whether hair loss is occurring, it is important to understand that hair naturally undergoes turnover. Each hair cycles through three phases:

• Anagen (growth phase): The phase during which hair continuously grows; most follicles on a normal scalp are in this phase, which typically lasts several years.
• Catagen (transition phase): A brief transitional phase where the follicle stops growing and begins to shrink, lasting a few weeks.
• Telogen (resting phase): The follicle rests, and the old hair loosens and eventually sheds, after which a new anagen phase begins.

Because follicles are at different stages, losing some hair daily is normal metabolism. A rough reference range is often about 50–100 hairs per day (varies by individual, season, and washing frequency). What matters is a noticeable increase compared to usual, overall thinning, widening of the part, or persistent shedding for months, rather than a single day's count.

Androgenetic alopecia involves miniaturization of follicles, shortening the anagen phase and converting terminal hairs to vellus hairs, which differs from telogen effluvium (where many hairs enter telogen simultaneously and shed in a short period). Differentiation requires physician evaluation.

The above is a neutral educational summary; normal shedding varies by individual and is for conceptual understanding only. Actual assessment of hair loss should be based on an in-person consultation with a physician and is not medical advice.