Osteoarthritis (OA): This is the most common cause of joint pain in older adults. The thumb joint, particularly the carpometacarpal joint, is a common site for OA due to its high mobility and stress. The patient's age and the location of the pain make OA a highly probable diagnosis.

Rheumatoid Arthritis (RA): Although less common than OA, RA can cause thumb joint pain, especially if it is part of a larger pattern of joint involvement. The diagnosis would be supported by symptoms such as morning stiffness, swelling, and similar pain in other joints.
Tendinitis or Tenosynovitis: Inflammation of the tendons or their surrounding sheaths can cause pain in the thumb, particularly with movement or use. This could be related to overuse or repetitive strain injuries.
Ganglion Cyst: A benign lump that can develop near joints or tendons, causing pain and discomfort. These are common in the wrist and hand area.

Infection (Septic Arthritis): Although less likely, an infection within the joint is a medical emergency that requires prompt treatment to prevent serious complications, including joint destruction and systemic infection.
Gout: While more commonly associated with the big toe, gout can affect any joint, including those in the thumb. It presents with sudden, severe pain and swelling, often accompanied by redness and warmth.
Fracture: Trauma to the thumb or hand could result in a fracture, which might not always be immediately apparent, especially if it's a stress or hairline fracture.

Psoriatic Arthritis: A form of arthritis that occurs in some people with psoriasis, which can cause joint pain, including in the thumbs.
Ligament Sprain or Tear: While more common in younger, more active individuals, a sprain or tear in the ligaments supporting the thumb joint could occur in older adults, especially after a fall or direct injury.
Tumor (Benign or Malignant): Very rarely, joint pain can be the first symptom of a bone or soft tissue tumor in the thumb or hand area.