If by the top of your thumb, you mean the side opposite the palm, that would not be coming from carpal tunnel syndrome.
That area is actually innervated by the radial nerve. So, you could have entrapment of the radial nerve, or problems with the C6 nerve root in the spine.
In some patients, who wear a tight wrist watch or bracelet, they can actually compress the radial nerve, causing the back of the hand on the thumb side to go numb.
Carpal tunnel syndrome, classically, causes numbness and tingling in the palmar surface of the thumb, index finger, middle finger, and the thumb side of the ring finger. There may also be pain in the hand, sometimes radiating up the arm. Some patients are awakened by the pain and numbness. And some have a feeling of clumsiness in the hand, and may drop objects. Static grasping can sometimes bring the symptoms on.
Of course, every patient presents in his/her own way, so each presentation is just a little different.
You may want to be seen by an orthopedic or hand surgeon. The surgeon can evaluate your left upper extremity and spine. Good luck.