Yellowing and thickening of the nails could be gray nails, also known as onychomycosis. It may also be caused by internal diseases and side effects of medicines. 1. Gray nails: yellowing and thickening of nails is caused by fungal infection of the nail bed, which belongs to nail fungus disease. The etiology found may be due to lack of cellular immunity and susceptibility to fungi. 2. Medical disorders: yellowing and thickening of the nails may also be affected, mainly by pulmonary disorders affecting blood circulation, since the nails are the part farthest from the heart and therefore must have a normal blood supply. If the distal blood supply is insufficient when pulmonary heart disease occurs, the nails will thicken. 3. Side effects of medications: Side effects of medications may also lead to thickening and yellowing of the nails, and some lipid-lowering medications have such side effects. Yellowing and thickening of nails may also be caused by other reasons, so the symptoms of yellowing and thickening of nails need to go to the hospital as soon as possible for diagnosis, and cooperate with the doctor for proper treatment.