I think you should get a second opinion as others have suggested. There may be a surgeon who thinks otherwise-- if you have the means, seek out the Mayo Clinic. There are many doctors out there with different experiences and knowledge banks. Not all are equal. I would double check with your thyroid doc as well- I saw a thyroid doctor in LA who showed me this article:
"Thyroid disease is common, and its effects on the gastrointestinal system are protean, affecting most hollow organs. Hashimoto disease, the most common cause of hypothyroidism, may be associated with an esophageal motility disorder presenting as dysphagia or heartburn."
This might not be the case for you, but make sure you have your tests read by a top expert in endocrinology, because I discovered my internist who was making my thyroid decisions for a year COULD NOT EXPLAIN TO ME HOW THE THYROID WORKED. If your doctor doesn't know about the possible effects of the thyroid on the gut (beyond the standard constipation symptoms), and if they don't know how to read a thyroid panel of Free T3/Free T4 (not just TSH), then find a new doc and investigate this lead as well. It's possible that insufficient meds and/or insufficient transformation of T4 (synthroid) to T3 in your body could be adding to your problems. My LA endo just added Cytomel (T3) to my treatment to see if it would help me... No luck yet, but fingers still crossed.
Again, no guarantee, but definitely worth chasing down that lead until you're 100% positive that's not a factor.
I wish good things for you,