what would cause severe discoloration of the thumb on the area behind the nail

Hello Anon

Thank you for writing!

Do you recall hurting this area at all? Are you in any pain?

There are numerous other reasons why nail pigment banding occurs: trauma to the nail, medication, systemic diseases, vitamin deficiencies and other skin conditions which may also appear as banding/discoloration in more than one finger.

Nail "moles" (under the nail on the skin) could be a cause.

There is also some very minor risk that discoloration could mean skin cancer.

Our advice is to seek a medical evaluation.

Best,
Susan