A bulge on the thumb can be caused by tendon sheath cysts, periungual warts and trauma. 1. Tendon sheath cysts: the etiology is not yet clear, may be inflammatory stimulation, strain and infection and other factors. The clinical manifestation is a superficial subcutaneous hemispherical elevation, commonly found in the hands and feet. 2. periungual warts: its cause is mostly caused by human papilloma virus, mainly manifested as hyperkeratosis around the finger or toenail, dome-shaped papules or plaques and nail deformation and other symptoms, part of the patient will appear on the thumb to grow a bulge of the situation. 3. trauma: if the big toe and the surrounding skin tissue suffered external injuries, resulting in soft tissue contusion, may trigger a local hematoma, at this time the thumb will appear bulging. There are many reasons for the growth of a bulge on the thumb, and it is recommended that the patient seek medical examination to clarify the cause of the disease.