Flat nails may be caused by genetic factors, lack of nutrition, trauma, and other disease factors. 1. Genetic factors: If flat nails appear since childhood, it may be caused by congenital genetic factors. Usually, other people in the family have flat nails, which may cause flat nails by heredity. 2. Lack of nutrition: If there is a long period of malnutrition, lack of calcium, etc., it will lead to a lack of nutrition, and the phenomenon of flat nails may occur. 3. Trauma: If a single nail has flat nails, it may be caused by poor repair of the nail after trauma, or it may be caused by frequent gnawing of the fingers, which leads to the damage of the nail bed, and so the phenomenon of flat nails occurs. 4. Other disease factors: if you suffer from thyroid disease, psoriasis, etc., may lead to the phenomenon of flat nails. In addition, long-term exposure to alkaline substances or mineral oil may also lead to flat nails. After eliminating environmental and lifestyle factors, it is recommended to consult a doctor in a timely manner to clarify the cause of the problem and follow the doctor’s instructions.