What Causes Hair Loss? The Science of Androgenetic Alopecia Explained

Short answer: Androgenetic alopecia — the medical term for both male and female pattern hair loss — is driven by a hormone called DHT acting on genetically sensitive hair follicles.^[1][2] It affects roughly half of Caucasian men by age 50 (and about 80% by age 70), and StatPearls estimates that androgenetic alopecia impacts approximately 50% of males and females at some point.^[1]It's progressive without treatment, though how fast it progresses varies enormously from person to person.

If you've started noticing thinning and want to understand what's actually happening — not just what to do about it — here's the biology, how common it really is, how it's diagnosed, and what the research says about how it progresses.

What DHT Actually Does

Dihydrotestosterone, or DHT, is a hormone made from testosterone by an enzyme called 5-alpha reductase. There are two versions of this enzyme — Type 1 and Type 2 — and Type 2 is the one that matters most for hair loss.^[1][3]It's concentrated in the hair follicle itself, along with the prostate and a few other tissues.

DHT binds to androgen receptors far more strongly than testosterone does. When it binds to receptors in the hair follicle, it shortens the follicle's growth phase (anagen). Do that repeatedly, cycle after cycle, and the follicle gradually shrinks — a process called follicular miniaturization.^[1]Over time, follicles that once produced thick, long hair start producing shorter, finer hairs, and eventually hairs so fine they may not even break through the skin's surface. The Propecia prescribing information likewise describes balding scalp as showing miniaturized follicles with increased DHT.^[3]

Here's the part that explains why hair loss shows up in a specific pattern instead of all over the scalp at once: some areas of the scalp have far more androgen receptors and more of the DHT-producing enzyme than others. The vertex (crown) and frontal scalp are highly sensitive. The back and sides of the scalp are comparatively resistant — which is exactly why hair transplant surgeons harvest donor hair from those areas; it's genetically wired to resist the same miniaturization process.^[1]

The Genetic Component

Pattern hair loss runs in families, but not in a simple, single-gene way — it's polygenic, meaning multiple genes each contribute a piece of your overall risk.^[1][6] The most consistently implicated locus is the androgen receptor (AR) gene on the X chromosome, so maternal inheritance can influence risk — though it is one contributor among several, not a single-gene maternal-only cause.^[6] Sons of men with pattern hair loss have been found to have five to six times the relative risk of developing it themselves.^[1]

Genome-wide association studies have also pointed to additional risk loci beyond AR; reviews of AGA genetics describe a polygenic architecture rather than one gene that fully explains male or female pattern hair loss.^[6]

How Common Is This, Really?

Pattern hair loss is genuinely common, not a rare condition:

• About 50% of Caucasian men are affected by age 50^[1]
• About 80% of Caucasian men are affected by age 70^[1]
• Early signs can appear in young adulthood for some men, though exact early-onset rates vary by population and study
• Female pattern hair loss is also common, with incidence rising after menopause; StatPearls notes androgenetic alopecia impacts approximately 50% of males and females overall.^[1]

The rise after menopause makes biological sense: hair follicles are sensitive to estrogen, and the hormonal shift of menopause — less estrogen, relatively more androgen influence — affects the follicle the same way aging affects it in men, just on a different timeline.^[1]

Men and Women Lose Hair Differently

The pattern itself differs by sex, which is part of why diagnosis and treatment approaches diverge too.

In men

Hair loss typically starts with thinning at the temples, followed by thinning at the crown — the classic "receding hairline plus bald spot" pattern most people picture. This is tracked using the Hamilton-Norwood scale, a 7-stage classification running from barely-there recession (Stage 1) to complete loss on top with a thin band remaining around the sides (Stage 7).^[1][4]

In women

The frontal hairline is usually preserved. Instead, thinning happens diffusely across the crown and top of the head, often noticed first as a widening part. Some women develop what's described as a "Christmas tree" pattern — more thinning toward the front along the part line, tapering outward. This is tracked using the Ludwig scale, a simpler 3-grade system from mild to severe.^[1][5]

How It's Actually Diagnosed

For most people, pattern hair loss is diagnosed through a clinical exam — no advanced testing required. A provider will typically look at:^[1]

• History:when it started, how it's progressed, family history, medications, other health conditions
• Pattern and distribution: comparing hair density at the crown/front to the back of the scalp, which stays relatively unaffected
• The pull test:gently pulling a small section of hair close to the scalp. Excess loose hairs can suggest active shedding — but this test is usually negative in straightforward pattern hair loss, since it's a gradual miniaturization process, not active shedding
• Dermoscopy: a handheld magnification tool that can reveal variation in hair shaft thickness and other pattern-hair-loss-specific signs, useful for ruling out other conditions

Blood work — thyroid levels, iron studies — is often used to rule out other contributing factors, especially in women, since thyroid dysfunction and iron deficiency can both cause or worsen hair thinning independent of genetics.^[1]

Conditions that can look similar but aren't the same thing:

• Telogen effluvium — diffuse shedding triggered by a specific event (illness, childbirth, major weight change) 1–6 months earlier, rather than a genetic pattern^[1]
• Alopecia areata — typically shows up as distinct, smooth patches rather than a gradual pattern, and can spontaneously regrow^[1]
• Thyroid-related hair loss — a treatable underlying cause, not androgenetic alopecia itself^[1]

Does It Get Worse Over Time?

Yes — androgenetic alopecia is explicitly described in the medical literature as progressive and chronic. Left untreated, it doesn't plateau on its own.^[1] That said, how fast it progresses varies enormously between individuals — some people see rapid change over a few years, others experience a much slower thinning over decades. Early onset, how much scalp surface area is involved, and family history all factor into how a given case tends to progress.

One nuance worth understanding: there's no sharply defined point in the research where miniaturization becomes permanently irreversible. What the evidence does show is that treatments require consistent, ongoing use — the Propecia label notes that continuous therapy is needed, and withdrawal is associated with reversal of effect within 12 months, which suggests miniaturized follicles don't "reset" to a fully healthy state even when treatment helps; they remain dependent on continuing it.^[3]

Frequently Asked Questions