Another health problem (not my week) - reaction to new kitten or something else??

I got a new kitten (5+ months old) a few weeks back. My cat Toffee had a sore trachea last week and we took him into the vet. Just in case it was related to a URI brought in by kitten (seems in hindsight it is not), we gave him an antibiotic but he seemed to be losing weight on it, so since he had enough of it by that point, my vet and I chose to take him off of it.

He is still losing weight (I have a baby scale at home), and is being very fussy about what he will eat (probably the reason for weight loss, he is not eating enough soft food I offer him and is rejecting some flavours, and perhaps is not eating his dry food either - multiple cats so hard to tell). He is throwing up a bit, but not all of the time.

The weight loss is significant enough in a short time that it is either a very serious health issue (I will redo his bloodwork to make sure that his t4 has not changed since last month, but it was only done about a month ago and was middle of normal). Could the stress of new addition kitty to our household make a cat really go off of their food enough for some weight loss to happen? Toffee seems healthy enough other than this weight loss. I should add that he is a very possessive/jealous kind of boy (but he has always had to share me with 4 other cats) so he does have the temperament to perhaps react to a newcomer. The strange thing is that the first day he seemed to adore the newcomer, but now he is more standoff-ish.

I just got some devastating news about my Rexy today (another thread in this section), but there is also this Toffee issue that I need to deal with to, as his weight loss is actually more dramatic. I also had that strange kitten issue too (another thread on here). They more or less seem unrelated but what crap timing to face all of this at the same time.

Any thoughts on Toffee? I am planning on a vet check in a couple of days if this continues and he was seen on Monday of this week, so I've not been neglecting him. But on Monday, the antibiotic might have still been in his system, so the plan was to take him off of it and offer some yummy food.  

Difficult if the vet thinks the antibiotic might have interfered with the bloods to check his thyroid, but also did the vet check other bloods as well to see if anything else was going on?  From what you describe it doesn't sound like he's been overly distressed by your new addition, and possibly more likely to be affected by his peer wbing unwell and general levels of stress in your home.  I think you are going the right thing getting him back to a vet as soon as you can.  Until then try to keep his fluids up and encourage him to eat with anything you can. If that means opening stuff up and being rejected you can always freeze what's left in small portions so you can get it out later to try, or to feed to your other two if they eat similar stuff.  Will he go for stinkier soft food like pilchards or sardines if he's had a bit of a URI and or sore throat?

I hope your vet is able to throw more light on this when they re-open and they see Toffee again.

Oh no, you have a houseful don't you?  Sounds like my house.  I would repeat the bloodwork.  In the meantime make sure he stays hydrated and force feed him if necessary.