Due to hormonal changes, irritation or damage, some hair follicles have a shorter growth phase and produce thinner, shorter hair shafts. Your hair goes through a cycle of growth and rest. The course of each cycle varies by individual. But in general, the growth phase of scalp hair, known as anagen, typically lasts two to three years. During this time, your hair grows just less than 1/2 inch (1 centimeter) a month. The resting phase is called telogen. This phase typically lasts three to four months. At the end of the resting phase, the hair strand falls out and a new one begins to grow in its place. Once a hair is shed, the growth stage begins again.
Most people normally shed 50 to 100 hairs a day. But with about 100,000 hairs in the scalp, this amount of hair loss shouldn't cause noticeable thinning of the scalp hair.
Gradual thinning is a normal part of aging. However, hair loss may lead to baldness when the rate of shedding exceeds the rate of regrowth, when new hair is thinner than the hair shed or when hair comes out in patches.
Causes of specific types of hair loss
• Pattern baldness (androgenetic alopecia). In male- and female-pattern baldness, the time of growth shortens, and the hairs are not as thick or sturdy. With each growth cycle, the hairs become rooted more superficially and more easily fall out. Heredity likely plays a key role. A history of androgenetic alopecia on either side of your family increases your risk of balding. Heredity also affects the age at which you begin to lose hair and the developmental speed, pattern and extent of your baldness.
• Cicatricial (scarring) alopecia. This type of permanent hair loss occurs when inflammation damages and scars the hair follicle. This prevents new hair from growing. This condition can be seen in several skin conditions, including lupus erythematosus or lichen planus. It's not known what triggers or causes this inflammation.
• Alopecia areata. This is classified as an autoimmune disease, but the cause is unknown. People who develop alopecia areata are generally in good health. A few people may have other autoimmune disorders, including thyroid disease. Some scientists believe that some people are genetically predisposed to develop alopecia areata and that a trigger, such as a virus or something else in the environment, sets off the condition. A family history of alopecia areata makes you more likely to develop it. With alopecia areata, your hair generally grows back, but you may lose and regrow your hair a number of times.
• Telogen effluvium. This type of hair loss is usually due to a change in your normal hair cycle. It may occur when some type of shock to your system — emotional or physical — causes hair roots to be pushed prematurely into the resting state. The affected growing hairs from these hair roots fall out. In a month or two, the hair follicles become active again and new hair starts to grow. Telogen effluvium may follow emotional distress, such as a death in the family or a physiological stress, such as a high fever, sudden or excessive weight loss, extreme diets, nutritional deficiencies, surgery, or metabolic disturbances. Hair typically grows back once the condition that caused it corrects itself, but it usually take months.
• Traction alopecia. Excessive hairstyling or hairstyles that pull your hair too tightly cause traction alopecia. If the pulling is stopped before there's scarring of your scalp and permanent damage to the root, hair usually grows back normally.
Other causes of hair loss
• Poor nutrition. Having inadequate protein or iron in your diet or poor nourishment in other ways can cause you to experience hair loss. Fad diets, crash diets and certain illnesses, such as eating disorders, can cause poor nutrition.
• Medications. Certain drugs used to treat gout, arthritis, depression, heart problems and high blood pressure may cause hair loss in some people. Taking birth control pills also may result in hair loss for some women.
• Disease. Diabetes and lupus can cause hair loss.
• Medical treatments. Undergoing chemotherapy or radiation therapy may cause you to develop alopecia. Under these conditions, healthy, growing (anagen) hairs can be affected. After your treatment ends, your hair typically begins to regrow.
• Hormonal changes. Hormonal changes and imbalances can cause temporary hair loss. This could be due to pregnancy, childbirth, discontinuation of birth control pills, the onset of menopause, or an overactive or underactive thyroid gland. The hair loss may be delayed by three months following a hormonal change, and it'll take another three months for new hair to grow back. During pregnancy, it's normal to have thicker, more luxuriant hair. It's also common to lose more hair than normal about three months after delivery. If a hormonal imbalance is associated with an overproduction of testosterone, there may be a thinning of hair over the crown of the scalp. Correcting hormonal imbalances may stop hair loss.
• Hair treatments. Chemicals used for dying, tinting, bleaching, straightening or permanent waves can cause hair to become damaged and break off if they are overused or used incorrectly. Overstyling and excessive brushing also can cause hair to fall out if the hair shaft becomes damaged.
• Scalp infection. Infections, such as ringworm, can invade the hair and skin of your scalp, leading to hair loss. Once infections are treated, hair generally grows back. Ringworm, a fungal infection, can usually be treated with a topical or oral antifungal medication.
• Trichotillomania (hair-pulling disorder). Trichotillomania is a type of mental illness in which people have an irresistible urge to pull out their hair, whether it's from the scalp, their eyebrows or other areas of the body. Hair pulling from the scalp often leaves them with patchy bald spots on the head, which they may go to great lengths to disguise. Causes of trichotillomania are still being researched, and no specific cause has yet been found.