This symptom is most often considered to be caused by a gray nail infection. The causative agent of gray nails is a fungus. When the fungus gets underneath the nail bed, it multiplies and increases the buildup of the nail cuticle, causing the nail to thicken. If this symptom is not controlled in time, the nail will become thicker and thicker. Eventually the nail will fall off. If the patient is an elderly person, it is also possible that the metabolism of the cuticle of the nail slows down, which ultimately leads to the thickening of the nail, or it is possible that the patient has thick nails.