If you have ever taken your nail polish off your nails and noticed a white streak or white lines or spots on the top of the nails, there are 2 main culprits. The first is that the nail may simply be damaged and dry due to chronic use of polish, as the chemicals can damage the nail. The other possibility is that it is due to a toenail fungus. Some nail fungus can cause a yellow-brown and thickened appearance to nails, but the nail fungus called T. mentagrophytes causes a superficial white discoloration across the top of the nail.