De Quervain's Tenosynovitis: This condition, which affects the tendons on the thumb side of your wrist, is a common cause of thumb pain without swelling, especially when making a fist, grasping, or turning the wrist.

Trigger Thumb: A condition where the thumb catches or locks when bent, often due to inflammation of the tendon sheath, which can cause pain without significant swelling.
Osteoarthritis of the Thumb: Wear and tear arthritis can cause pain in the thumb, particularly at the base, without noticeable swelling, especially in older adults.
Repetitive Strain Injury: Overuse or repetitive movements can lead to strain in the muscles and tendons of the thumb, causing pain.

Gout: Although less common in the thumb, gout can cause sudden, severe pain without initial swelling. Missing this diagnosis could lead to prolonged suffering and potential joint damage.
Infection (Septic Arthritis): Bacterial infection within the joint can cause pain and might not always present with significant swelling initially. Early diagnosis is crucial to prevent joint destruction.
Fracture: A hairline or stress fracture might not cause significant swelling but can be very painful. Missing this diagnosis could lead to improper healing and chronic issues.

Ganglion Cyst: Although typically associated with a visible lump, a ganglion cyst near the thumb could potentially cause pain without noticeable swelling if it's small or in an atypical location.
Tumor: Benign or malignant tumors in the thumb area are rare but could cause pain. The absence of swelling does not rule out this possibility, especially if the tumor is deep-seated.
Neuroma: A benign growth of nerve tissue, like a neuroma, could cause thumb pain, especially if it compresses or irritates nearby nerves.