Topic Overview

There are many types of hair loss. It is often categorized according to when it takes place during the hair growth cycle: anagen, catagen, and telogen.

• The anagen phase is when your hair grows—your hair follicle forms a new hair shaft. Most (90%) of the hair on your scalp is in a growing phase that lasts from 2 to 6 years.
• The catagen phase follows the anagen stage. This is a 1- to 2-week transition stage between anagen and telogen. Less than 1% of your scalp hair is in this phase.
• The telogen stage is the rest stage. At the end of this 3- to 4-month phase, some of your hair falls out. Losing up to 100 hairs a day is normal. When a hair falls out, a new hair is grown in the same hair follicle, and the growing cycle begins again.

Androgenetic alopecia is inherited hair loss. In this type of hair loss, the growth (anagen) cycle becomes shorter and shorter. The hair follicles sprout hairs that are thinner than normal. The hairs become thinner and thinner, and eventually the follicles wither away.

Alopecia areata is a type of hair loss that is caused when the immune system mistakenly attacks hair follicles. This type of hair loss begins to get worse when hair follicles enter the rest (telogen) phase too soon.

Telogen effluvium has many causes. In this type of hair loss, large numbers of hairs enter the resting phase (telogen), which causes shedding and thinning. Usually no more than 50% of the hair is affected, and hair loss may occur up to 3 months after the event that causes it.^{footnote 1}Causes include:

• Mental stress or physical stress, such as recent surgery, illness, or high fever.
• Poor nutrition, especially lack of protein or iron in the diet.
• Side effects of medicines, such as blood thinners (anticoagulants).
• Changes in hormone levels, such as those that occur after childbirth, when taking birth control pills, or at menopause.

Two types of hair loss not related to the hair growth cycle are trichotillomania and traction alopecia.

• Trichotillomania is a compulsive behavior in which a person pulls hair out of the scalp, eyelashes, or eyebrows. There is usually mounting tension before pulling the hair and a feeling of relief afterward.
• Traction alopecia involves hair loss around the edge of the hairline and is especially noticeable around the face and forehead. It is caused by your method of hairstyling. Certain hairstyles can pull hair too tight; for example, tight braids or tight ponytails can cause hair loss.