Tough to say without examination. If localized, it could be due to some degenerative arthritis, or an orthopedic problem. I would consider X-raying the joint and having an orthopedic surgeon evaluate the hand - having a hand specialist take a look at it would be preferable.
If there is consideration for something more systemic (i.e. rheumatoid arthritis for example), then blood tests - such as ANA level, sedimentation rate, rheumatoid factor - can be considered as an initial screen. A referral to a rheumatologist can be considered for further discussion.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
Medical Weblog:
kevinmd_b