Yellowing of the toe cap of the big toe may be caused by aging nails, trauma, onychomycosis and psoriasis. 1. Nail aging: The yellowing of the big toe cap may be caused by nail aging. With age, the toenail is prone to aging, which will lead to the phenomenon of thickening and yellowing of the toenail cap, which is a normal phenomenon and does not require special treatment. 2. trauma: toes thumbnail if hit by external damage, it may lead to local blood circulation obstacles, toenails can not get enough nutrition, the color will be yellow. 3. Ringworm: yellowing of the big toe cap may be caused by fungal infection of gray nails, manifested as yellowing of the nails, thickening and other abnormal symptoms, and there is a buildup of debris under the nail and other manifestations. 4. Psoriasis: Patients with psoriasis may also have changes in their nails, which may be characterized by thickening, yellowing, and non-smooth surfaces, etc. Therefore, yellowing of the toe cap of the big toe may also be caused by psoriasis. It is recommended that patients with yellowing of the toe cap of the big toe go to the hospital in a timely manner, after clarifying the cause of the disease under the guidance of the doctor to standardize the treatment.