Spunky losing weight...why? any weight gain tips

Spunky is our 4 year old male rescue from a kitten.  He has been the other way up to 11 lbs, then to 9 lbs and now I'm afraid to weigh him.  He eats...we feed blue buffalo and always have.  No odd color to lips, gums, eats ok, bowel and pee ok.

The problem seems to have started when he had a bout of acne and we gave him two long lasting antibiotic shots over about a month.

We could take him to vet for thyroid blood tests at around $130 to start but since there appears to be no definitive test for that problem? Also the vets around here have limited experience.

Anyway with no other symptoms I don't know where to start...I would like to try a weight gain program?

Thanks

I have very limited experience in this but my first two thoughts are thyroid or dehydration.  Per my post on this forum, my cat seemed completely normal in terms of activity - he was playful, affectionate, not lethargic at all, but was apparently severely dehydrated.

How long ago was the antibiotic shots?  I am guessing it was convenia.  Convenia can stay in the body for a long time and some kitties have adverse reactions to it.  At 4 hyperthyroidism is a very slim possibility.  It usually begins in older kitties.

It was convenia ...shots were given 2-18-2014 and 4-5-2014

Although convenia stays in the body for up to 2 months I don't think it would still be causing weight loss.  The weight loss is usually accompanied by anorexia.  I doubt that it is a thyroid issue, he is still a bit young for that but blood tests for it are pretty accurate.  I do think that he needs to have complete blood work though, there are other things that can cause unexplained weight loss.  I know it is expensive, the last complete blood work I had done for a kitty was this past February and it was just under $300.  Vet costs vary greatly depending on the part of the country you are in.  Vets in more rural areas are also considerably cheaper.

Anorexia has not been the problem. He seems to eat, just not enough.  The dynamics here are interesting as we have 3 rescues in the house.  One is an inside/outside but Spunky has always been our shy one, first to run for cover when the door bell rings, etc.

The arrival of Molly, the UPS cat, seems to have upset the balance, however she had been here several months before the weight loss started.

Spunky will eat frequently and small amounts.  We also give wet cat food and water in the morning, which he eats well.

Of course the issue has to be calories in...calories out...so he is not eating enough or needs a hi-cal food to supplement and off set the weight loss.

So this is what we would like to try first...any suggestions?

Poor Spunky!

It's tough to seem them loose weight, and you should know that unplanned weightloss in non-geriatric cats is almost never calories in vs calories out as the primary source of the problem. I wish it were. Every cat I meet for weightloss that can be measured in pounds is always having bloodwork performed.  There are only a few things on bloodwork that can be called "definitive", but that almost never matters. You just need someone who knows how to interpret the bloodwork relative to how spunky actually looks on his physical exam. If he is actually hyperthyroid, it's a big win, as they can do great (provided he get's treatment early enough....)

k