There are several possible causes for the swelling of the thumb: 1. tumor, if there is a tumor in the bone or soft tissue of the thumb, there can be swelling of the thumb, and this should be diagnosed by a relevant examination, ultrasound or MRI. 2. stenosing tenosynovitis, patients with stenosing tenosynovitis can have hard nodules on the palmar side of the thumb, at the metacarpophalangeal joint, and can have swelling of the thumb resulting in swelling of the thumb. 3. 3, osteoarthritis, osteoarthritis can lead to joint enlargement and swelling of the thumb. 4, gout, if there is gout stone formation in the thumb, it can lead to swelling of the thumb. 5, rheumatoid, rheumatoid affects the joints, it can lead to joint enlargement, deformity, pain and other symptoms.