Some possible causes of pain on the inside base of the thumb include arthritis and carpal tunnel syndrome, according to the Chicago Tribune. Overuse and hand injuries can also cause pain to the basal thumb joint.
Pain on the inside base of the thumb may be caused by arthritis, particularly in patients who are older than 40, as stated by UConn Health Orthopaedic Surgery and Sports Medicine. Arthritis can cause increased pain when the person pinches or grips. Arthritis in the basal joint may be caused by a history of sprains or other thumb injuries. In many patients, the exact cause is impossible to identify.