golden/lab cross

Question
We have a 12 1/2 year old female who's been on soloxine for about 6 years and last month was treated for a UTI. In the past two weeks we have to coax her to take a few bites of food, she's lost 5 lbs, her rear right leg barely operates and she has trouble getting from one place to another. The vet has run CBC, x rayed her everywhere, and done an ECG. She had one small pink 'bump' on her forehead, and I just found another beside her ear.
Our vet now wants to perform an ultrasound....we're thinking it may be time to let her go, but we'd like to have a more definitive reason than 'we can't find anything wrong with her'.
Any ideas/help ??
Thanks

Answer
This is a very tough case.  We spent $1200 having a tumor removed from our 13 year old Lab early this year.  We knew what was wrong, and she is doing fine now, fetching sticks from the river yesterday.  

You don't know what is wrong and if it is anything curable.  Perhaps you might try a second vet, but it could just be a case of paying for the same inconclusive tests again.