Thickening and yellowing of big toenails may be caused by fungal infection, aging toenails, and nail malnutrition. 1. Fungal infection: fungal infection caused by onychomycosis can cause the toenail to become cloudy and brittle, which may also lead to thickening and yellowing of the big toenail. 2. Toenail aging: with age, metabolism will gradually slow down, may cause toenail aging, appear big toenail thickening and yellowing. 3. Nail nutritional deficiencies: anemia, vitamin deficiency, and lack of trace elements may also cause thickening and yellowing of the big toenail. Big toe nails become thick and yellow, there may be other reasons, you need to go to the hospital in time to check, clarify the cause of the disease, follow the doctor’s instructions for treatment.