De Quervain's Tenosynovitis: This condition, which affects the tendons on the thumb side of your wrist, is a common cause of thumb pain, 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, leading to pain and stiffness.
Osteoarthritis of the Thumb: Wear and tear arthritis affecting the joint at the base of the thumb, leading to pain and limited mobility, especially with gripping or twisting motions.
Gamekeeper's Thumb (Skier's Thumb): An injury to the ulnar collateral ligament of the thumb, typically occurring from a sudden force that causes the thumb to bend away from the hand, leading to pain and instability.

Gout: Although less common in the thumb, gout can cause sudden, severe pain and swelling. Missing this diagnosis could lead to prolonged suffering and potential joint damage.
Infection (Septic Arthritis or Osteomyelitis): Bacterial infections in the thumb joint or bone can lead to severe pain, redness, and swelling, and if not promptly treated, can result in significant morbidity.
Ganglion Cyst: A lump under the skin that occurs near joints or tendons, which can cause pain if it presses on a nerve or joint.

Giant Cell Tumor of the Tendon Sheath: A rare, benign tumor that can cause pain and swelling in the thumb, especially if it compresses nearby nerves or tendons.
Erythromelalgia: A disorder characterized by burning pain, heat, and redness in the hands and feet, which can occasionally affect the thumb.
Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the joints, leading to pain and swelling in the thumb, although this is a rare presentation.