Pain in the thumb is a common complaint. Usually the pain is worse with use of the thumb, such as pinching against the index finger when opening jars and lifting heavy items.
The location of the pain can give an indication to the likely cause:
Pain at the base of the thumb near the wrist joint
Pain where thumb joins the hand
Does the thumb 'lock' or 'click'?
Pain at the end thumb joint - osteoarthritis of the small hand joints