Why Hair Falls Out: A Doctor's Map of the Causes

Everyone loses hair. You shed somewhere between fifty and a hundred strands a day, and have done since childhood — they collect on the pillow, in the shower trap, on the back of a dark jumper, and almost nobody notices. Hair loss only becomes a problem when the books stop balancing: when more is falling out than is growing back, or when the regrowth comes in thinner than what it replaced.

The trouble is that "my hair is falling out" describes at least four quite different situations, with different causes, different timelines, and very different things worth doing about them. A surprising amount of money and worry is spent treating the wrong one. So before anything else, it helps to have a map.

How hair actually grows

Each follicle runs on a cycle, mostly independent of its neighbours. There's a long growth phase, anagen, lasting two to six years, during which the hair lengthens steadily. Then a brief transition, and a resting phase, telogen, lasting a few months, at the end of which the hair is shed and a new one begins growing underneath. At any moment, the great majority of your follicles are in growth and a minority are resting.

This staggering is why you don't moult. It's also the key to understanding most hair loss, because nearly everything that goes wrong does so by disturbing the cycle — either pushing too many follicles into resting at once, or gradually shrinking the follicles themselves so that each new hair is finer and shorter than the last.

Pattern hair loss: the slow, genetic kind

The commonest cause by a wide margin is androgenetic alopecia — pattern hair loss. In men it produces the familiar receding temples and crown thinning; in women it more often shows as diffuse thinning over the top of the scalp, with the parting widening, while the frontal hairline is usually preserved. It is partly inherited, partly driven by the effect of androgens (notably dihydrotestosterone) on genetically susceptible follicles.

The mechanism is miniaturisation. Affected follicles don't die suddenly; they shrink over successive cycles, producing progressively finer, shorter, less pigmented hairs until eventually they produce almost nothing. This is why pattern loss is gradual and why early thinning can be easy to dismiss — the hair isn't dramatically falling out, it's quietly getting wispier. It tends to be slow, patterned, and symmetrical, which is largely how it's recognised.

Telogen effluvium: the shock shed

The second big category looks alarming but is usually the most reassuring. In telogen effluvium, some bodily stress pushes an abnormally large fraction of follicles into the resting phase all at once. Two or three months later — and the lag is the signature — they shed together, and the person notices handfuls of hair coming out, often diffusely across the whole scalp rather than in a pattern.

The triggers are a roll-call of things the body finds taxing: a high fever or serious infection, major surgery, childbirth, a crash diet or rapid weight loss (including the kind that can accompany the newer weight-loss medicines), significant psychological stress, thyroid disturbance, or starting and stopping certain drugs. The crucial point is that telogen effluvium is a reaction, not a disease of the hair itself. Once the trigger passes, the follicles almost always recover, and the hair regrows over the following months. The hardest part is usually the wait.

The medical causes worth checking

This is the part most online content skips, because it doesn't sell anything. A number of treatable medical conditions show up as hair thinning, and they're worth excluding before assuming the cause is genetic or stress-related.

Iron deficiency is the classic example — low iron stores can drive or worsen diffuse shedding, particularly in menstruating women, and it's checked with a simple blood test (ferritin, alongside a full blood count). Thyroid disease, both under- and overactive, commonly affects hair. Vitamin D status is sometimes relevant. Certain medications list hair loss among their effects. And in women, polycystic ovary syndrome and the hormonal shifts of menopause both belong in the conversation. None of these requires a fancy "hair loss panel" sold online — the relevant tests are ordinary ones a GP can arrange when the history points that way.

The red flags: when it isn't ordinary hair loss

Most hair loss is one of the patterns above, and most of it is benign. But a few presentations point elsewhere and deserve proper assessment rather than a bottle of anything.

Scarring is the one to take seriously. If the scalp looks smooth and shiny where the hair has gone, with the follicle openings lost, that suggests a scarring alopecia — a group of conditions where the follicle is being permanently destroyed, and where early treatment matters because what's lost doesn't come back. Patchy loss — discrete smooth bald patches appearing over weeks, as in alopecia areata, or scaly, itchy, broken-off patches that can signal a fungal infection, especially in children — also needs a look. So does hair loss accompanied by systemic symptoms: a rash, joint pains, marked fatigue, or unexplained weight change, where the hair may be the visible edge of something more general.

A useful rule of thumb: slow, symmetrical, patterned thinning is usually the ordinary kind; sudden, patchy, scarring, or symptomatic loss earns a medical opinion.

Practical takeaways

• Most hair loss is one of two things — gradual genetic pattern loss, or a temporary shed (telogen effluvium) triggered two to three months earlier by stress, illness, childbirth or rapid weight loss.
• Pattern loss works by follicles shrinking over time, which is why it's slow and why early thinning is easy to miss.
• Telogen effluvium is a reaction, not a disease, and the hair usually regrows once the trigger passes.
• Treatable medical causes — iron deficiency, thyroid disease, and in women PCOS or menopause — are worth excluding with ordinary blood tests, not expensive online panels.
• Scarring, patchy, or symptom-accompanied loss is a different category and needs proper assessment.

What this doesn't mean

A map is not a diagnosis. These patterns overlap, more than one can run at once, and the same person can have genetic thinning made suddenly worse by an illness or a crash diet. Identifying which kind of hair loss you have — and whether anything underlying needs checking — is exactly the sort of judgement that benefits from someone examining the scalp and knowing your history, rather than from a self-assessment quiz attached to a shop.

When to seek medical advice

See a clinician if hair loss is sudden or rapid, comes in distinct patches, leaves smooth scarred-looking skin, is accompanied by an itchy or scaly scalp, or comes with other symptoms such as fatigue, a rash or weight change. It's also reasonable to seek advice simply because thinning is distressing you — that's a legitimate reason, and a GP can check the treatable causes and discuss what the realistic options are.

A closing thought

Hair is biologically trivial and psychologically enormous, and the gap between those two facts is where a great deal of misleading marketing lives. The single most useful thing you can do when you notice hair loss is also the least commercial: work out which kind it is, and whether anything underneath needs attention, before reaching for a product aimed at a problem you may not have.

Further reading and sources

• British Association of Dermatologists — patient information leaflets on hair loss, androgenetic alopecia and telogen effluvium
• NICE Clinical Knowledge Summaries — Hair loss in adults and Alopecia
• NHS — hair loss information
• Peer-reviewed reviews of the hair cycle and the differential diagnosis of alopecia