Causes
People typically lose about 100 hairs a day. This usually doesn't cause noticeable thinning of scalp hair because new hair is growing in at the same time. Hair loss occurs when this cycle of hair growth and shedding is disrupted or when the hair follicle is destroyed and replaced with scar tissue.

Hair loss is typically related to one or more of the following factors:

Family history (heredity). The most common cause of hair loss is a hereditary condition called male-pattern baldness or female-pattern baldness. It usually occurs gradually with aging and in predictable patterns — a receding hairline and bald spots in men and thinning hair in women.
Hormonal changes and medical conditions. A variety of conditions can cause permanent or temporary hair loss, including hormonal changes due to pregnancy, childbirth, menopause and thyroid problems. Medical conditions include alopecia areata (al-o-PEE-she-uh ar-e-A-tuh), which causes patchy hair loss, scalp infections such as ringworm and a hair-pulling disorder called trichotillomania (trik-o-til-o-MAY-nee-uh).
Medications and supplements. Hair loss can be a side effect of certain drugs, such as those used for cancer, arthritis, depression, heart problems, gout and high blood pressure.
Radiation therapy to the head. The hair may not grow back the same as it was before.
A very stressful event. Many people experience a general thinning of hair several months after a physical or emotional shock. This type of hair loss is temporary.
Certain hairstyles and treatments. Excessive hairstyling or hairstyles that pull your hair tight, such as pigtails or cornrows, can cause a type of hair loss called traction alopecia. Hot oil hair treatments and permanents can cause inflammation of hair follicles that leads to hair loss. If scarring occurs, hair loss could be permanent.
Risk factors
A number of factors can increase your risk of hair loss, including:

Family history of balding, in either of your parent's families
Age
Significant weight loss
Certain medical conditions, such as diabetes and lupus
Stress
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https://www.mayoclinic.org/diseases-conditions/hair-loss/symptoms-causes/syc-20372926