At any given time, about 85% to 90% of the hairs on the average person's head are actively growing (the anagen phase) and the others are resting (the telogen phase). Typically, a hair is in the anagen phase for two to four years, then enters the telogen phase, rests for about two to four months, and then falls out and is replaced by a new, growing hair. The average person naturally loses about 100 hairs a day.
In a person with telogen effluvium, some body change or shock pushes more hairs into the telogen phase. Typically in this condition, about 30% of the hairs stop growing and go into the resting phase before falling out. So if you have telogen effluvium, you may lose an average of 300 hairs a day instead of 100.
Telogen effluvium can be triggered by a number of different events, including:
Major physical trauma
Major psychological stress
High fever, severe infection or other illness
Extreme weight loss
Extreme change in diet
Abrupt hormonal changes, including those associated with childbirth and menopause
Hypothyroidism or hyperthyroidism
Because hairs that enter the telogen phase rest in place for two to four months before falling out, you may not notice any hair loss until two to four months after the event that caused the problem. Telogen effluvium rarely lasts longer than six months, although some cases last longer.
Although losing a great number of hairs within a short time can be frightening, the condition is usually temporary. Each hair that is pushed prematurely into the telogen phase is replaced by a new, growing hair, so there is no danger of complete baldness. Because hair on the scalp grows slowly, your hair may feel or look thinner than usual for a time, but fullness will return as the new hairs grow in.