Horizontal depressions on the thumbnail may be related to bad hand habits, skin lesions on the thumbnail and its surroundings, lack of vitamin A and other nutrients, and certain digestive diseases (such as intestinal parasites) can also lead to horizontal depressions on the thumbnail. 1. Bad hand habit: if the patient usually uses the thumb habitually, it may make the thumbnail suffer too much force, which will trigger the horizontal stripe depression on the thumbnail. 2. Thumbnail and its surrounding skin lesions: when the thumbnail itself or its surrounding skin lesions, such as finger skin eczema and so on caused by insufficient supply of nutrients to the nails, it may cause the thumbnail with horizontal lines on the depression. 3. Lack of vitamin A and other nutrients: vitamin A and other nutrients are closely related to the normal growth of the thumbnail, and when the body lacks such nutrients, it may cause malnutrition of the thumbnail, resulting in the appearance of horizontal stripe depressions. 4. Digestive system diseases: Digestive system diseases, such as parasitic diseases, can cause the body to absorb nutrients, resulting in nutrient deficiencies and the appearance of horizontal indentations on the thumbnail. It is recommended to consult a doctor in time to clarify the cause of the disease and standardize the treatment.