What Causes Hair Loss?
Here are some of the things that can cause hair loss in teens:

Illnesses or medical conditions. Endocrine (hormonal) conditions, such as uncontrolled diabetes or thyroid disease, can interfere with hair production and cause hair loss. People with kidney and liver diseases and lupus can also lose hair. The hormone imbalance that occurs in polycystic ovary syndrome can cause hair loss in teen girls as well as adult women.
Medications. Some medications that have hair loss as a side effect may be prescribed for teens. These include acne medicines like isotretinoin, and lithium, which is used to treat bipolar disorder. Diet pills that contain amphetamines can also cause hair loss. Chemotherapy drugs for cancer are probably the most well-known medications that cause hair loss, but some cancers including leukemia and lymphoma can cause hair loss even before treatment begins.

Alopecia areata (pronounced: air-ee-ah-tuh). This skin disease causes hair loss on the scalp and sometimes elsewhere on the body. It affects 1.7% of the population, including more than 4 million people in the United States. Alopecia areata is thought to be an autoimmune disease, in which the hair follicles are damaged by a person's own immune system. (In autoimmune diseases, the immune system mistakenly attacks healthy cells, tissues, and organs in a person's body.) Alopecia areata usually starts as one or more small, round bald patches on the scalp and can progress to total hair loss, although total hair loss only happens in a small number of cases. Both guys and girls can get it, and it often begins in childhood. The hair usually grows back in 6 months to 2 years, but not always.

Trichotillomania (pronounced: trik-o-til-uh-may-nee-uh). Trichotillomania is a psychological disorder in which people repeatedly pull their hair out, often leaving bald patches. It results in areas of baldness and damaged hairs of different lengths. People with trichotillomania usually need professional help from a therapist or other mental health professional before they can stop pulling their hair out.

Hair treatments and styling. Having your hair chemically treated, such as getting your hair colored, bleached, straightened, or permed, can cause damage that may make the hair break off or fall out temporarily. Another type of baldness that results from hair styling can actually be permanent: If a person wears his or her hair pulled so tightly that it places tension on the scalp, it can result in a condition called traction alopecia. Traction alopecia can be permanent if the style is worn for a long enough time that it damages the hair follicles.

Poor nutrition. Poor eating can contribute to hair loss. This is why some people with eating disorders like anorexia and bulimia lose their hair: The body isn't getting enough protein, vitamins, and minerals to sustain hair growth. Some teens who are vegetarians also lose their hair if they don't get enough protein from non-meat sources. And some athletes are at higher risk for hair loss because they may be more likely to develop iron-deficiency anemia.
Disruption of the hair growth cycle. Some major events can alter the hair's growth cycle temporarily. For example, delivering a baby, having surgery, or getting anesthesia can temporarily stop the hair growth cycle. (Because the hair we see on our heads has actually taken months to grow, a person may not notice any disruptions of the hair growth cycle until months after the event that caused it.) This type of hair loss corrects itself.

Male-pattern baldness. Among adults, particularly men, the most common cause of hair loss is androgenetic (pronounced: an-druh-juh-neh-tik) alopecia, also called male-pattern baldness. This condition is caused by a combination of factors, including hormones called androgens and genetics. In some males, the hair loss can start as early as the mid-teen years. It can also occur in guys who take steroids like testosterone to build their bodies.