Most hair loss on the planet traces to a single molecule doing a slow, specific kind of damage. Understanding it answers a surprising number of questions at once: why baldness runs in a pattern instead of all over, why the same hormone that thins the scalp can thicken a beard, why finasteride works and "DHT-blocking" shampoos mostly don't, and why the hair at the back of the head is the last to go. Follow the hormone through the follicle and the whole confusing picture snaps into focus.
The hormone is dihydrotestosterone — DHT. It's an androgen, made from testosterone, and it's not a villain by design: DHT is essential in development and does normal, necessary work in the body. Its role in hair loss is a side effect of that normal biology meeting a genetic susceptibility.
Here's the conversion at the heart of it. An enzyme called 5-alpha reductase converts testosterone into DHT, which is a more potent androgen. DHT then binds to androgen receptors inside cells — including the cells around your hair follicles. What happens next depends entirely on where the follicle is and how sensitive it is — which is the key to the whole pattern.
The damage has a name — follicular miniaturisation — and it's gradual, which is why pattern hair loss creeps rather than crashes.
Recall that hair grows in cycles: a long growth phase (anagen), a short transition, and a resting/shedding phase. In a susceptible follicle, DHT binding does two things over successive cycles:
Play that forward over years and a robust follicle producing a strong hair becomes a tiny follicle producing a wispy one — and eventually may stop producing a visible hair at all. That's miniaturisation: not follicles being killed outright, but progressively dialled down until their output is invisible. (This is also why treatment works best early — a miniaturised-but-alive follicle can sometimes be coaxed back, while one that's fully dormant is much harder to revive.)
Here's the elegant part. If DHT circulates throughout the body, why does hair vanish from the crown and hairline while the sides and back hang on — and while beard and body hair often thicken under the same hormone?
Because not all follicles are equally sensitive to DHT — and that sensitivity is genetically programmed by location.
So "pattern" baldness is patterned because your genes assign different DHT sensitivity to follicles in different places. The hormone is uniform; the follicles' response to it is a genetic map. That single fact explains the shape of the whole thing.
Androgenetic alopecia affects both sexes, but it looks different, and the hormonal context differs.
The value of understanding DHT is that it makes every treatment claim legible. Once you know the pathway — testosterone → (5-alpha reductase) → DHT → androgen receptor → miniaturisation — you can see exactly where each intervention acts, and how far its evidence goes. (For the full evidence grading, see what actually works; here we're mapping mechanism to claim.)
Finasteride — blocks the enzyme (strong evidence). Finasteride inhibits 5-alpha reductase, so less testosterone becomes DHT, so susceptible follicles get less of the signal that miniaturises them. It attacks the cause directly, which is why it has strong evidence for men — and why it's a prescription drug with real considerations (a doctor's conversation, not a shelf purchase). Dutasteride is a related, stronger enzyme-blocker used in some cases.
Minoxidil — doesn't touch DHT at all (strong evidence, different mechanism). This surprises people: minoxidil works downstream of the whole hormone story. It doesn't lower DHT; it prolongs the growth phase and improves blood flow to the follicle, partly counteracting the miniaturisation regardless of its cause. That's why it works for shedding types too, and why it pairs well with finasteride — the two hit different points in the chain. It's an OTC drug; results need consistency and reverse if you stop.
Saw palmetto and botanicals — aim at the enzyme, weaker evidence. Saw palmetto is the best-known "natural DHT blocker," proposed to inhibit 5-alpha reductase (reducing DHT by a modest amount in some studies). It targets the right step in principle, but the human evidence is far weaker than finasteride's — a plausible, gentler option, not an equal. Rosemary is proposed to mildly inhibit the same enzyme (plus improve circulation), again on thin evidence.
"DHT-blocking" shampoos — right target, wrong delivery. Here's where the mechanism exposes the marketing. A shampoo may contain an ingredient that can inhibit 5-alpha reductase in a lab dish — but a shampoo is rinsed off in under a minute, which is unlikely to deliver enough active, deep enough, long enough, to meaningfully change follicular DHT. The claim borrows the real pathway; the format can't deliver on it. Same lesson as hair growth serums: a real mechanism on the label doesn't guarantee a real effect in use.
Pattern hair loss — the most common hair loss there is — comes down to one hormone, DHT, gradually miniaturising follicles that your genes made sensitive to it. That single mechanism explains the pattern (genetically assigned sensitivity by location), the sex differences, why transplanted hair survives, and why the same hormone thins a scalp while filling a beard.
It also makes the treatments honest. Finasteride works because it cuts DHT production at the enzyme; minoxidil works despite not touching DHT, by acting downstream — which is why they're stronger together. Botanical "DHT blockers" aim at the right step with weaker evidence, and "DHT-blocking" shampoos mostly can't deliver on a real mechanism because they wash off. If you understand the pathway, you can read any hair-loss product's claim and know roughly where it acts and how much to believe it — and for a real hair-loss plan, that understanding is best taken to a dermatologist, not a shampoo aisle.
DHT (dihydrotestosterone) is an androgen hormone made from testosterone by an enzyme called 5-alpha reductase. In people genetically susceptible to pattern hair loss, DHT binds to receptors around scalp follicles and gradually "miniaturises" them — shortening each growth cycle and shrinking the follicle, so hairs get progressively finer, shorter, and paler until they may stop being visible. The follicles aren't killed outright; they're dialled down over years, which is why pattern hair loss is gradual and why treating it early works better.
Because DHT sensitivity is genetically assigned by follicle location. Follicles at the hairline and crown are, in susceptible people, highly sensitive to DHT and miniaturise, while follicles at the back and sides are typically DHT-resistant and survive. This is also why hair transplants work: DHT-resistant hair moved from the back to a balding area keeps its resistance. The hormone reaches all your follicles; it's their genetically programmed response that creates the pattern.
Women with pattern hair loss usually show diffuse thinning — a widening part and reduced overall density — often with a preserved frontal hairline, rather than the receding hairline typical in men. The hormonal picture is more complex in women, other factors contribute, and menopause can bring pattern thinning to the surface as the balance of oestrogen to androgens shifts. Partly because of this complexity, finasteride (a DHT-focused drug) is not the standard first-line approach for women that it is for men.
Mostly not, and the mechanism explains why. A shampoo might contain an ingredient shown to inhibit 5-alpha reductase in a laboratory, but a shampoo is rinsed off within a minute — unlikely to deliver enough active, deeply or long enough, to meaningfully lower DHT at the follicle. The claim borrows a real biological pathway, but the rinse-off format can't realistically act on it. A leave-on product has a better chance than a shampoo, but for genuinely interrupting DHT, the evidence sits with drugs like finasteride, not cosmetics.
Two proven drugs, working at different points. Finasteride (prescription) inhibits 5-alpha reductase so less DHT is made — attacking the cause directly, with strong evidence in men. Minoxidil (over-the-counter) doesn't lower DHT at all; it works downstream by prolonging the growth phase and improving follicle blood flow, which is why it complements finasteride. Botanical "DHT blockers" like saw palmetto target the right enzyme but have much weaker evidence. Because these are medical decisions (finasteride especially), the right path for a real hair-loss plan is a doctor or dermatologist rather than self-treating.
This article is neutral educational reference explaining hair-loss physiology, graded on the evidence. It is not medical advice, a diagnosis, or a treatment recommendation. Finasteride and dutasteride are prescription medicines and minoxidil is an over-the-counter drug — all are matters for a pharmacist or doctor, and no dosing is given here. Botanical options are discussed as evidence, which is weaker than that for the drugs. For assessment and treatment of hair loss, consult a qualified dermatologist or trichologist.
A credentialed reviewer (PharmD / PhD / MD) will be named before this entry is finalised. Until then, treat it as a working draft. Last updated 2026-07-14.
How we separate evidence levels: our methodology.
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