There are many reasons for a depressed horizontal groove in the thumbnail, such as nail dystrophy, systemic diseases, nail lichen planus, fungal infections, and skin disorders. 1. Nail malnutrition: Nail malnutrition can be caused by a lack of micronutrients in the body, and it is usually found in people who are picky eaters, partial eaters, anorexics, and people with chronic consumptive diseases, such as hyperglycemia and hyperthyroidism, etc. Nail malnutrition will affect the nails, causing some of the nails to appear concave, horizontal grooves, and other phenomena. 2. Systemic diseases: severe anemia, or cardiovascular disease, liver and kidney disease, etc., will also show symptoms on the surface of the nails, resulting in a depression in the thumb nail horizontal groove phenomenon, and also accompanied by local symptoms. 3. Nail lichen planus: Nail lichen planus will cause nail depression, horizontal grooves, and the distal end of the nail will also be defective nail plate, the nail will show pus-like changes. 4. Fungal infections: Fungal infections of the nails can also cause nail plate damage, resulting in nail indentation, transverse grooves and other symptoms, accompanied by itching. 5. Skin diseases: some skin diseases can also involve nails, such as psoriasis, which can cause nail depression, horizontal grooves and other phenomena, similar to thimble-like nails. The cause of thumb nail has a depression horizontal groove of the causes are very many, after the symptoms must go to the hospital in time, clear diagnosis, give targeted treatment.