We were without internet all yesterday, otherwise I would have posted then. We heard back from the vet and got some results. FB#1's kidneys are just fine and it looks like he has "a little bit of early cardiac disease" - they need to do another thyroid test tomorrow because he's "in the gray zone for hyperthyroidism" which could be affecting the heart. An over-active thyroid would also explain the weight loss. So, it looks like we caught it way early and that we can very likely take care of the thyroid problem with medication and slow down any progression of the heart problem.
Whew! I'm SO glad - he's been his same-old self the past couple of days - appetite is just fine (which makes me blame the thyroid for the weightloss rather than a lack of appetite).
Has anyone's kitty ever had any thyroid problems? Thanks also for the lovely comments!! And Furball #1 thanks you too and asks that you send turkey and ham from now on instead of intangibles like blog comments...he's such a diva.