Depressed thumb nails can be caused by trauma or by diseases such as pemphigus or psoriasis. 1. Trauma: If the thumb has recently suffered from extrusion injury, it is easy to cause nail depression, which can be recovered by itself after a period of time. 2. Patchy baldness: the etiology of this disease is not yet clear, clinically it is believed that it may be related to heredity, autoimmune dysfunction, endocrine disorders, mental nerves and other reasons. Typical symptom is patchy hair loss, but some patients may have nail lesions, which may be manifested as uneven and rough nails, so the depression of thumbnail cover may be caused by patchy baldness. 2. Psoriasis: research shows that the occurrence of this disease is the result of genetic and environmental factors, including obesity, smoking, stress, skin damage, etc. Common symptoms include itchy, dry skin, nail pitting, hyperkeratosis under the nail, etc., which can be manifested as thumb nail cap depression. In addition to this, depression of the thumb nail cover should also be considered as a result of lichen planus, eczema on the hands and other diseases. It is recommended that patients with sunken thumb nails go to the hospital for a complete examination to clarify the cause of the condition.