Place · Level 3 · Conditions
Hair Loss
头发有生长周期 · 雄激素性脱发是 DHT 让毛囊一代代变细 (缓慢、有遗传底色) · 休止期脱发是周期被压力/疾病同步打断 (突然、通常可逆) · 分清了才知道什么真有用
Story path
Chapter 1
The hair cycle
The hair cycle
To understand hair loss, first understand that hair is always 'rotating shifts'.
Each hair runs a cycle: a growth phase (anagen, about 3 years on the scalp, hair growing), a transitional phase (catagen, about two weeks), and a resting phase (telogen, about 3 months, the hair paused), before shedding and being replaced by new hair. A healthy scalp has roughly 85% of follicles in anagen and about 15% in telogen (the Telogen Effluvium review).
This gives a key intuition: losing 50-100 hairs a day is normal turnover, not disease. What to distinguish isn't 'am I shedding a lot' but two things — is it 'shedding abnormally much (a problem at the shedding end)', or 'growing less and finer over time (a problem at the growth end)'. These correspond to completely different mechanisms, and completely different management.
The next two scenes cover the two commonest kinds: one where the growth end is weakened generation by generation by a hormone (androgenetic alopecia), and one where the shedding end is synchronized and disrupted by a single shock (telogen effluvium).
Each hair runs a cycle: a growth phase (anagen, about 3 years on the scalp, hair growing), a transitional phase (catagen, about two weeks), and a resting phase (telogen, about 3 months, the hair paused), before shedding and being replaced by new hair. A healthy scalp has roughly 85% of follicles in anagen and about 15% in telogen (the Telogen Effluvium review).
This gives a key intuition: losing 50-100 hairs a day is normal turnover, not disease. What to distinguish isn't 'am I shedding a lot' but two things — is it 'shedding abnormally much (a problem at the shedding end)', or 'growing less and finer over time (a problem at the growth end)'. These correspond to completely different mechanisms, and completely different management.
The next two scenes cover the two commonest kinds: one where the growth end is weakened generation by generation by a hormone (androgenetic alopecia), and one where the shedding end is synchronized and disrupted by a single shock (telogen effluvium).
Chapter 2
AGA · DHT & miniaturization
AGA · DHT & miniaturization
The commonest long-term hair loss is androgenetic alopecia (AGA), in both sexes, more typical in men.
The mechanism isn't 'hair falling out all at once' but thinning generation by generation. Testosterone is converted by 5-alpha-reductase into the stronger androgen DHT (dihydrotestosterone); DHT binds the androgen receptors of genetically susceptible follicles, shortening their growth phase round after round, gradually turning thick terminal hairs into fine vellus hairs (miniaturization), until they're nearly invisible.
Two points. First, genetics decide which areas are susceptible: the male pattern is a receding hairline and crown thinning, while the back of the scalp is usually insensitive — which is why hair transplants harvest a 'donor area' from the back. Second, it's slow and gradual, completely different from the sudden heavy shedding of telogen effluvium in the next scene.
Once you grasp the thread 'DHT weakens susceptible follicles generation by generation', you can see why the genuinely effective drugs (next scene) either lower DHT or extend the growth phase — not the 'root-strengthening shampoo' routine.
The mechanism isn't 'hair falling out all at once' but thinning generation by generation. Testosterone is converted by 5-alpha-reductase into the stronger androgen DHT (dihydrotestosterone); DHT binds the androgen receptors of genetically susceptible follicles, shortening their growth phase round after round, gradually turning thick terminal hairs into fine vellus hairs (miniaturization), until they're nearly invisible.
Two points. First, genetics decide which areas are susceptible: the male pattern is a receding hairline and crown thinning, while the back of the scalp is usually insensitive — which is why hair transplants harvest a 'donor area' from the back. Second, it's slow and gradual, completely different from the sudden heavy shedding of telogen effluvium in the next scene.
Once you grasp the thread 'DHT weakens susceptible follicles generation by generation', you can see why the genuinely effective drugs (next scene) either lower DHT or extend the growth phase — not the 'root-strengthening shampoo' routine.
Chapter 3
Telogen effluvium
Telogen effluvium
Another very common but often frightening loss is telogen effluvium — characterized by 'sudden, diffuse, usually reversible'.
The mechanism: a clear shock pushes a large batch of follicles that were in the growth phase prematurely and in sync into the resting phase. Common triggers include high fever or serious illness, major surgery, the postpartum period (a sharp hormone shift), severe dieting or iron deficiency, intense psychological stress, and certain medications. Under significant stress, about 70% of anagen follicles may shift together into telogen (the Telogen Effluvium review).
Here's a crucial, anxiety-relieving timeline: these synchronized hairs don't fall immediately, but shed in a wave about 2-3 months after the triggering event. So 'I've been shedding so much lately' often points to something that happened months ago (an illness, a surgery, a stretch of extreme dieting, a high-stress period).
The good news is it's usually reversible: once the trigger is removed, hair often regrows within months (full recovery can take more than 6 months). So facing this kind of sudden diffuse shedding, the first step isn't to panic-buy a drug, but to look back for the removable trigger (check iron, check thyroid, improve sleep and diet).
The mechanism: a clear shock pushes a large batch of follicles that were in the growth phase prematurely and in sync into the resting phase. Common triggers include high fever or serious illness, major surgery, the postpartum period (a sharp hormone shift), severe dieting or iron deficiency, intense psychological stress, and certain medications. Under significant stress, about 70% of anagen follicles may shift together into telogen (the Telogen Effluvium review).
Here's a crucial, anxiety-relieving timeline: these synchronized hairs don't fall immediately, but shed in a wave about 2-3 months after the triggering event. So 'I've been shedding so much lately' often points to something that happened months ago (an illness, a surgery, a stretch of extreme dieting, a high-stress period).
The good news is it's usually reversible: once the trigger is removed, hair often regrows within months (full recovery can take more than 6 months). So facing this kind of sudden diffuse shedding, the first step isn't to panic-buy a drug, but to look back for the removable trigger (check iron, check thyroid, improve sleep and diet).
Chapter 4
What works vs hype
What works vs hype
For androgenetic alopecia, the strongest-evidence options are two drug classes, but both need a doctor's guidance (this page gives no dosages).
Minoxidil: mainly topical, chiefly extending the growth phase and improving follicle blood supply. It works for a fair share of people (roughly a third to forty percent respond), has a good safety profile, with local irritation the common side effect. Key point: it doesn't address the root (doesn't lower DHT), the benefit fades after stopping, and it needs long-term use.5-alpha-reductase inhibitors (such as finasteride): act directly on AGA's mechanism by lowering DHT, with strong evidence. But its sexual side-effect signal needs weighing with a doctor (at the lower AGA doses, one pooled analysis found this risk not statistically significant, though individual reports exist and deserve informed consent); women of childbearing potential must not handle crushed tablets (teratogenic risk).Combination therapy generally outperforms monotherapy (Gupta 2022 network meta-analysis).
Hyped by marketing but weak on evidence: biotin only helps when there's a genuine deficiency, with no evidence for 'anti-shedding hair growth' in those who aren't deficient (see biotin-b7); the various 'root-strengthening' shampoos and essential oils mostly stay at the experiential level.
In a line: the earlier AGA is treated, the more is preserved; but for medication, see a doctor first to identify the type and weigh the trade-offs, rather than self-medicating long-term or blindly buying trending products.
Minoxidil: mainly topical, chiefly extending the growth phase and improving follicle blood supply. It works for a fair share of people (roughly a third to forty percent respond), has a good safety profile, with local irritation the common side effect. Key point: it doesn't address the root (doesn't lower DHT), the benefit fades after stopping, and it needs long-term use.5-alpha-reductase inhibitors (such as finasteride): act directly on AGA's mechanism by lowering DHT, with strong evidence. But its sexual side-effect signal needs weighing with a doctor (at the lower AGA doses, one pooled analysis found this risk not statistically significant, though individual reports exist and deserve informed consent); women of childbearing potential must not handle crushed tablets (teratogenic risk).Combination therapy generally outperforms monotherapy (Gupta 2022 network meta-analysis).
Hyped by marketing but weak on evidence: biotin only helps when there's a genuine deficiency, with no evidence for 'anti-shedding hair growth' in those who aren't deficient (see biotin-b7); the various 'root-strengthening' shampoos and essential oils mostly stay at the experiential level.
In a line: the earlier AGA is treated, the more is preserved; but for medication, see a doctor first to identify the type and weigh the trade-offs, rather than self-medicating long-term or blindly buying trending products.
Chapter 5
When to see a doctor
When to see a doctor
Most everyday hair loss is a cycle issue — change what you can first; but a few situations warrant seeing a dermatologist early rather than self-managing.
Sudden patchy / circular loss (round bald patches): could be alopecia areata or another disease, with different mechanism and managementScalp with scarring, redness, pain, or scaling: beware scarring alopecia — once follicles are destroyed it's irreversible, so the earlier you're seen, the more hair is savedHair loss in women with menstrual irregularity, acne, or hirsutism: may relate to elevated androgens / PCOS (see pcos), warranting hormone testingChildren's hair loss, or loss with marked fatigue, cold intolerance, or weight change: check the thyroid (see hashimoto) and ferritin (iron deficiency is a common, reversible cause — see iron)
What it means for you: first identify the type and find the reversible trigger (iron, thyroid, stress, extreme dieting), then talk medication; the earlier proper AGA treatment starts, the more is preserved. Chronic stress is also a common driver (see chronic-stress). This page is education, not a diagnosis; use medication under a doctor's guidance.
Sudden patchy / circular loss (round bald patches): could be alopecia areata or another disease, with different mechanism and managementScalp with scarring, redness, pain, or scaling: beware scarring alopecia — once follicles are destroyed it's irreversible, so the earlier you're seen, the more hair is savedHair loss in women with menstrual irregularity, acne, or hirsutism: may relate to elevated androgens / PCOS (see pcos), warranting hormone testingChildren's hair loss, or loss with marked fatigue, cold intolerance, or weight change: check the thyroid (see hashimoto) and ferritin (iron deficiency is a common, reversible cause — see iron)
What it means for you: first identify the type and find the reversible trigger (iron, thyroid, stress, extreme dieting), then talk medication; the earlier proper AGA treatment starts, the more is preserved. Chronic stress is also a common driver (see chronic-stress). This page is education, not a diagnosis; use medication under a doctor's guidance.