Patterns of hair loss

One of the commonest forms of hair loss in women (and men) is a condition called telogen effluvium, in which there is a diffuse (or widely spread out) shedding of hairs around the scalp and elsewhere on the body.

This is usually a reaction to intense stress on the body's physical or hormonal systems, or as a reaction to medication.

The condition, which can occur at any age, generally begins fairly suddenly and gets better on its own within about six months, although for a few people it can become a chronic problem.

Because telogen effluvium develops a while after its trigger, and causes generalised thinning of hair density rather than a bald patch, women with the condition can easily be diagnosed as overanxious or neurotic.

Fortunately, it often gets better with time. Telogen effluvium is a phenomenon related to the growth cycles of hair.

Hair growth cycles alternate between a growth phase (called anagen, it lasts about three years) and a resting phase (telogen, which lasts about three months). During telogen, the hair remains in the follicle until it is pushed out by the growth of a new hair in the anagen phase.

At any one time, up to about 15 per cent of hairs are in telogen. But a sudden stress on the body can trigger large numbers of hairs to enter the telogen phase at the same time. Then, about three months later, this large number of hairs will be shed. As the new hairs start to grow out, so the density of hair may thicken again.

Many adults have had an episode of telogen effluvium at some point in their lives, reflecting episodes of illness or stress.

Another common type of hair loss in women is androgenetic alopecia, which is related to hormone levels in the body. There's a large genetic predisposition, which may be inherited from the father or mother.

Androgenetic alopecia affects roughly 50 per cent of men (this is the main cause of the usual pattern of balding seen as men age) and perhaps as many women over the age of 40.

The cause of hair loss in this condition is a chemical called dihydrotestosterone, or DHT, which is made from androgens (male hormones that all men and women produce) by the action of an enzyme called 5-alpha reductase.

People with a lot of this enzyme make more DHT, which in excess can cause the hair follicles to make thinner and thinner hair, until eventually they pack up completely.

Another important cause of hair loss in women is a condition called alopecia areata, an autoimmune disease that affects more than two per cent of the population. In this, the hair follicles are attacked by white blood cells. The follicles then become very small and hair production slows down dramatically, so there may be no visible hair growth for months and years.

After some time, hair may regrow as before, come back in patchy areas, or not regrow at all. The good news is that in every case the hair follicles remain alive and can be switched on again; the bad news is that we don't yet know how to do this.