My greyhound won't eat, has been to the vet on IV for 6 days. They cannot give me an answer. We have brought her home . She only drinks water. No interest in any type of food. Normal weight around 58lb. Down to 47lb. Don't know what else to do.

Time for a second opinion. Take him to an animal hospital.. they tend to have the ability to perform more tests than the ordinary vet office

I agree.. time for a second opinion. How old is the dog, sex, any other issues (lethargic, bruising, vomiting, etc.)? Six days on IV with no diagnosis is unusual, in my opinion. Did they at least rule some things out? Have they done bloodwork, x-rays, etc?