The unevenness on the nail of the thumb may be due to nail dystrophy, which can be caused by a variety of factors, such as eczema, especially chronic eczema, which causes local nail dystrophy, and the nail may become uneven. It can also be seen in thimble nails, which can be caused by complex reasons, such as psoriasis or lichen planus, and other types of chronic skin diseases. It can also occur as a result of a localized fungal infection, especially nail fungus.