My four year old westie has recently started being sick for no reason. It is like a yellow bile. He is also constantly hungry and thirsty. Last night he bit my daughters 14 yr old friend. Any ideas?

Any change in behaviour would warrant a vet check. Might also be worth taking along a urine sample (collected first thing in the morning if possible) and having bloodwork done to check for things like diabetes etc.

One cause for bringing up bile is having an empty stomach. How long does he go between meals ?. Some dogs bring up bile in the mornings - mine did until we realised why. He was having breakfast at around 8am and tea around 6pm. That meant he was then going without food for 14 hours. We cured it immediately by giving him a small supper. If a dog has a sensitive stomach then getting too hungry isn't a good thing. Anyway, its something to consider. Hope he's ok soon.

I agree that a vet visit is in order.  You definitely need to find out if he has any kidney problems, diabetes, or thyroid problems etc.

How's he doing ?.