C6 Radiculopathy: This is the most likely diagnosis because the C6 nerve root is responsible for sensation in the thumb. Compression or irritation of the C6 nerve root in the neck can cause numbness in the thumb.

Carpal Tunnel Syndrome: Although this condition typically affects the wrist, it can cause numbness in the thumb, especially if the median nerve is compressed. However, the question specifies a neck source, making this less likely but still a consideration.
Thoracic Outlet Syndrome: This condition involves compression of the nerves and/or blood vessels that pass into the thoracic outlet (the space between your collarbone and first rib). It can cause numbness in the arm and hand, including the thumb, if the compression affects the nerves.

Spinal Cord Injury or Compression: Conditions such as spinal stenosis, herniated discs, or spinal tumors can compress the spinal cord, leading to numbness in various parts of the body, including the thumb. Missing this diagnosis could lead to severe and irreversible neurological damage.
Stroke or Transient Ischemic Attack (TIA): Although less common, a stroke or TIA affecting the brain's sensory areas could cause numbness in the thumb. Prompt recognition and treatment are crucial to prevent further brain damage.

Pancoast Tumor: A rare type of lung cancer that can compress nerves, including those affecting the arm and hand, leading to numbness in the thumb.
Neurodegenerative Diseases (e.g., Multiple Sclerosis, Amyotrophic Lateral Sclerosis): These conditions can cause a wide range of neurological symptoms, including numbness in the thumb, but are less common and typically present with other systemic or neurological symptoms.