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Do you have the rest of his blood work results like BUN?
Was a urinalysis done? Were stones seen on the ultrasound?
It looks like the bile acids test is incomplete.
There should be two numbers, one before eating and one after.
Do you have the other one or did they not do the full test?
A change in diet can cause symptoms, especially if there was a protein increase or a difference in the type of protein given. Red meat and organ meat in particular produce the most ammonia (toxins) and a liver compromised dog can't process these properly, so they don't feel well, don't want to eat, have neurological symptoms (also called hepatic encephalopathy), etc. Liver compromised dogs are also very prone to developing ammonium urate stones.
A high bile acids test can only tell you that the liver isn't functioning properly. Liver shunt and MVD (microvascular dysplasia or small, inoperable shunting throughout the liver) are just two of the many possibilities. Unfortunately, neither disease is uncommon in Yorkies.
Ultrasounds are only 60-80% accurate when trying to diagnose a shunt. A test that you can do to try and differentiate between liver shunt and MVD is the Protein C Test. It is just a simple blood test sent to Cornell University. Normal numbers (70-80 or higher) usually mean normal or MVD and 70ish or lower usually means a shunt. If it comes back in shunt territory (or maybe even if it doesn't depending on the situation), a scintigraphy should be done. This is where radioactive dye is sent through the body to see if the liver is processing things properly. It is probably the best way to find a shunt. Because of the prognosis, most younger dogs with a shunt should have surgery. The two best places to have it done at are the University of Tennessee or Cornell University.
This is not something that should be let go. He is feeling well because he is on a food that his liver isn't having a hard time processing but it does nothing to fix the problem.
It is best to feed dogs with liver shunt or symptomatic MVD 4-6 times a day, otherwise the liver has to deal with too much at once. While the L/D dry is okay, L/D canned is preferred by some because the moisture is excellent for liver compromised dogs and the ingredients seem to be better. Royal Canin Hepatic LS 14 kibble is also a good choice if you dog isn't allergic to it.
SAMe (Denosyl or Denestra) should be given to prevent further liver cell death.
Milk thistle is sometimes recommended along with vitamin E.
Lactulose binds toxins and they are eliminated in the dog's stool. This is very important if any type of neurological symptoms are present but it may help in any case.
Local vets tend to not be much help in these cases.
They just don't see it that often.
Your vet(s) need to be very careful with the drugs used for the rest of his life if this is LS or MVD. Non-steroidal anti-inflammatories and certain anesthesias in particular should not be used.
However, because high bile acids can mean so many things, it is impossible to say whether this even applies. I would say if he is feeling better on the L/D, it does apply.
LS and MVD are not death sentences and can be managed, but in a lot of cases they are just a bit more tricky to take care of than other dogs.