Carpal Tunnel Syndrome: This condition is the most likely cause of thumb numbness due to compression of the median nerve as it passes through the wrist. The median nerve supplies sensation to the thumb, index, and middle fingers, as well as part of the ring finger. Compression of this nerve can lead to numbness, tingling, and weakness in these areas.

Radial Neuropathy: Although less common than carpal tunnel syndrome for causing thumb numbness specifically, radial neuropathy can lead to numbness on the back of the hand, including the thumb, due to compression or injury of the radial nerve.
Ulnar Neuropathy: Typically affects the little and ring fingers, but in some cases, especially if the compression is at the elbow, it might cause symptoms in the hand that could be confused with thumb numbness, especially if there's a component of median nerve compression as well.
Thoracic Outlet Syndrome: Compression of the nerves that come from the neck can cause numbness and tingling in the hands, including the thumb, though it's less specific and often accompanied by other symptoms like neck pain and weakness.

Stroke or Transient Ischemic Attack (TIA): Although less common, sudden onset of numbness in one thumb (or any part of the body) could be a sign of a stroke or TIA. This is a medical emergency requiring immediate attention.
Multiple Sclerosis: An autoimmune disease that affects the brain and spinal cord, leading to a wide range of neurological symptoms, including numbness. It's crucial to consider this diagnosis, especially in younger patients with recurrent or multifocal neurological symptoms.
Cervical Radiculopathy: Compression of the nerve roots in the cervical spine can cause numbness, weakness, and pain in the arm and hand, including the thumb. This condition can sometimes be overlooked but is critical to diagnose due to the potential for significant disability.

Tumors (e.g., Schwannoma, Neurofibroma): Rarely, tumors affecting the nerves can cause localized numbness, including in the thumb.
Infections (e.g., Leprosy): Certain infections can cause nerve damage leading to numbness, though these are uncommon in many parts of the world.
Vitamin Deficiencies (e.g., Vitamin B12 Deficiency): Can cause neurological symptoms, including numbness and tingling in the hands and feet, though thumb numbness alone would be an unusual presentation.