A chronic form with a more insidious onset and a longer duration also exists. Telogen effluvium is a reactive process caused by a metabolic or hormonal stress or by medications. Generally, recovery is spontaneous and occurs within 6 months. Telogen effluvium can affect hair on all parts of the body, but, generally, only loss of scalp hair is symptomatic. Understanding the pathophysiology of telogen effluvium requires knowledge of the hair growth cycle.
All hair has a growth phase, termed anagen, and a resting phase, telogen.
On the scalp, anagen lasts approximately 3 years, while telogen lasts roughly 3 months, although there can be wide variation in these times between individuals. During telogen, the resting hair remains in the follicle until it is pushed out by growth of a new anagen hair. Telogen effluvium is triggered when a physiologic stress or hormonal change causes a large number of hairs to enter telogen at one time. Shedding does not occur until the new anagen hairs begin to grow. The emerging hairs help to force the resting hairs out of the follicle.