There are many reasons for yellowing toenails, such as fungal infections, aging toenails, malnutrition, and chronic health conditions. 1. Fungal infection: Fungal infection caused by onychomycosis may lead to yellowing and thickening of the toenails, and may also cause the surface of the nails to become uneven and brittle. 2. Aging of toenails: with age, nails may also turn yellow. 3. Nutritional deficiencies, such as lack of vitamins and trace elements, may also cause yellowing of toenails. 4. Chronic diseases: such as bilirubin metabolism abnormalities caused by liver disease, or hemolysis, nephrotic syndrome, etc., may also be manifested in the yellowing of toenails. There may be other reasons for the yellowing of toenails, it is recommended to go to the hospital for examination, to clarify the cause of the disease, and follow the doctor’s instructions for treatment.