At this point I would ask for a referral to an internal medicine vet. You can waste a lot of time and money trying to get a diagnosis from a general practice vet who isn't familiar with liver shunts as I suspect your vet is not. An internist will repeat all the tests anyway.
For example, a bile acids test is always the first step. In fact, all Yorkies should have one done once they reach six months old since shunts are so prevalent in the breed. In the United States, Yorkshire terriers have almost a 36 times greater risk of developing shunts than all other breeds combined.
Portosystemic Shunts FAQ
Fasting is not required for a bile acids test. Also, ultrasounds can be as low as 60% reliable especially on small dogs. Scintigraphy is much more reliable.
PSVA and MVD Research Summary Hepatic Vascular Disorders - WSAVA 2006 Congress