There are often messages from patients saying: I have hair loss, I lose a lot of hair when I wash my hair, what is the cause? This kind of problem is often not clear at once, so I have to slowly talk to you. Hair grows out of the hair follicles in the scalp, and the hair follicles regularly go through the anagen, regression and anagen phases periodically. During the anagen phase, the hair follicles produce new hairs and keep growing, while in the early anagen phase, the old hairs left in the original follicles will be shed voluntarily. The anagen phase of the scalp hair follicle is usually 2-6 years, followed by the regression phase (2-3 weeks) and the resting phase (3-4 months). The hairs surviving in the resting follicles are easily shed under slight tension, which means that these hairs tend to fall off when we wash or comb our hair. That is why we generally have the impression that very little hair tends to fall out the day after washing, because the resting hairs that are not firmly attached have already fallen out when we wash our hair. There are many intrinsic factors that inhibit hair growth and many intrinsic factors that promote hair growth, and the two factors restrain each other to maintain a dynamic balance to ensure that hair follicles can go through the anagen, regression and resting phases in a normal cycle. Once this dynamic balance is disturbed, a disease state will occur, for example, if the factors that promote hair growth are too strong, hirsutism will occur; if the factors that inhibit hair growth are too strong, excessive hair loss will occur, that is, alopecia areata will occur. There are many factors that cause hair loss, including genetics, mental and emotional, malnutrition, endocrine disorders, autoimmune diseases, drugs, infections, inflammatory skin diseases, etc. The condition and prognosis of hair loss varies from cause to cause. Alopecia areata is often divided into scarring alopecia and fee scarring alopecia. Scarring alopecia permanently destroys hair follicles and leads to permanent hair loss. Common causes include trauma, burns or scalds, infections, and inflammatory skin diseases (e.g., discoid lupus erythematosus, lichen planus, etc.). Non-scarring alopecia is possible for hair to grow back after the causative factors have been removed. The following types of non-scarring alopecia are common: androgenetic alopecia, baldness, resting alopecia, anagen alopecia, pulling alopecia, hair pulling fetish, etc. 1. Androgenetic alopecia: On the basis of genetic susceptibility, androgens act on sensitive hair follicles, making the hairs thin and soft, and they fall off without growing, which also results in hair loss on the top of the head and at the hairline. With finasteride and/or minoxidil, hair loss can be slowed down and regrowth occurs to varying degrees. But the hair will fall out again after stopping the drug. 2. Alopecia areata: It is an autoimmune alopecia disorder, in fact, any part of the hair may be involved and shedding may occur. The triggers for the occurrence of autoimmune disorders are complex, some are related to mental emotions, and sometimes high fever, surgery, body blows, vaccinations, other endocrine or autoimmune diseases may trigger baldness, but some people cannot find any triggering factors. Treatment is based on removing the triggering factors and regulating immunity. 3. Resting hair loss: There are many causes. Physiological ones include neonatal hair loss and postpartum hair loss, which can recover naturally. Pathological factors include mental and emotional factors, nutritional deficiencies, endocrine diseases, poisoning, drugs, etc.. Triggers can be recovered after removal. Nowadays, many people live irregularly, often stay up late, overeat, and sometimes skip meals, all of which may lead to resting hair loss. 4, pulling hair loss: common in women with ponytails, hair long-term chronic pulling, can make the pulled hair loss, serious cases can lead to permanent loss. 5, hair pulling fetish: is a psychological disorder, often with anxiety state, often occurring in children. So let me say what causes your hair loss, first of all, we must clarify what the diagnosis is, and then according to the diagnosis in turn to exclude all kinds of causes of the disease, and finally to answer the question.