How Hair Grows

Hair is produced by an organ in the skin called a follicle. In the human scalp, these follicles occur in natural groups of one to four, called follicular units.
The production of hair is a cyclical process. The anagen phase lasts two to seven years. This is the growing phase. The hair follicle produces “terminal” hair, or full healthy hair.

The telogen phase lasts about three months. This is the resting phase when the hair falls out. The hair is finished growing and is released from the follicle, leaving the follicle empty. Then the anagen phase starts again.

Note: In humans, this does not happen all at once. About 90 percent of the follicles are in the anagen phase and 10 percent in the telogen phase at any given time.

When Hair Doesn’t Grow as It Should
Telogen effluvium is the increased shedding of a portion of the hair after it has entered the telogen phase.

When certain situations occur — like surgery, effects of medications, pregnancy etc. — the growing cycle is modified. The body is saying, in effect, that it needs to put its energy into something other than making hair. So a larger portion of hair production goes into the telogen phase, and you see more shedding of hair than you usually would. This is usually reversible.

Anagen effluvium is the shedding of hair that is in the anagen phase. Since most of the hair is in the anagen phase, this affects almost all the hair. This occurs in chemotherapy, for example.

In androgenetic hair loss in females, the hair loss is usually diffuse, not patterned as it commonly occurs in men. Testosterone, the male hormone that is also present in smaller amounts in females, is converted into DHT by an enzyme called 5 alpha reductase. DHT causes certain follicles to decrease in size and have a shortened anagen or growth phase, lasting months instead of years.

Women have less hair loss than men do, especially in the front, because they have less 5 alpha reductase, which leads to less DHT, and they receive protection by another enzyme called aromatase.

In genetic hair loss, all the hairs gradually decrease in diameter and the hair cycles become shorter before the hair actually disappears. So the appearance of thinning is due to thin hair rather than the total absence of functioning hair follicles.