The yellowing of the toenails may be caused by exogenous coloring, or it may be caused by nail dystrophy, onychomycosis and so on. 1. exogenous coloring: if the nail area coated with iodine vodka or tincture of iodine, yellow nail polish may lead to yellow nails, which is a normal physiological phenomenon, generally clean the local nails will disappear after the performance of yellowing. 2. Nail dystrophy: Nail dystrophy may be caused by heredity, malnutrition, physical injury and so on. Patients will not only have yellow and black nails, but also have nail transverse grooves, brittle nails, nail longitudinal grooves, nail peeling and other manifestations. Therefore, patients with nail dystrophy will have symptoms of yellow toenails. 3. Ringworm of the nails: ringworm of the nails is due to the infection of dermatophytes, yeast and other pathogens. The patient will appear nail thickening, nail gray and yellow turbidity, the surface of the nail plate is uneven and so on. If the patient’s toenails have onychomycosis, there will be a yellowing of the nails. There are other reasons for yellow nails (such as yellow nail syndrome, drug side effects), for exclusion of exogenous coloring caused by yellow nails, the patient needs to consult a doctor in time, the doctor will be clear after the cause of the disease to choose targeted measures for treatment.