How do you figure out if tingling in my left hand is coming from the ulnar/median nerve, neck, or thoracic spine? how do you figure out what nerve?

See MD promptly: You need a full exam in the clinic by a physician to figure this out. Tingling in the extremity can be caused by systemic diseases such as diabetes and several other causes. This is why your physician will take a careful history. When tingling is unilateral and fits into the sensory distribution of a peripheral nerve, a physician considers dermatomes to determine which nerve to study. See MD ASAP.

Exam, studies: Each nerve has a specific distribution and so a detailed physical exam goes a long way in determining the likely diagnosis. We then will often use a test called a nerve conduction test which then tests each specific nerve and this can tell specifically which nerve is affected and where as well.