White toenails can develop for several reasons.
Trauma, such as when an object is dropped on a toenail, often causes bleeding under the nail because of blood vessels being broken. This would cause a black toenail. If the trauma does not cause broken blood vessels, a white spot may appear under the nail. The spot will slowly grow out with the normal growth of the toenail.
Sometimes white lines appear within the toenail. These may be caused by recurring trauma, such as when a runner wears shoes that are too small and the toe hits the end of the shoe.
White lines may also occur due to a medical illness or trauma that has occurred elsewhere in the body, causing protein to be deposited within the nail bed.
A fungal infection that affects the outermost layer of the toenail may cause a bright white discoloration of the toenail.
A white area close to the nail fold (the lunula) varies in size from one person to another. This is a normal aspect of the nail.
It is recommended that you see a foot and ankle surgeon for the diagnosis and possible treatment of white toenails.