This may or may not be helpful, but my two kitties LOVE love Fancy Feast wet food. I have tried other wet food brands and they won't touch anything unless it is Fancy Feast. They prefer the "Classic" cans.

Regarding treats, they also are gung ho over "Temptations" treats.
My cat will do anything for just one of these.

Hope this helps out.

Wellness? Blue buffalo?

I'd avoid salmon and tuna. Salmon because it's probably farmed salmon, and tuna for the high mercury content.

Cats with thyroid problems generally can't have fish due to the iodine content. I also have a 16 year old cat with thyroid issues. She is on Science Diet YD. We get it from the vet, but it's not outlandishly expensive: about $60 per month. It controls her thyroid and we don't have to pay for or give her medication.

Try something fishy flavored. My super fussy cat likes Trader Joe's Tuna cat food. It's only 69 cents a can and is not full of fillers, wheat or sugar. After she got used to the fish flavored wet food I then moved her on to chicken and turkey.

Our cat is 16 and has been on thyroid medication for a couple years. He ate IAMS dry food his entire life, but now we're supposed to give him wet food, which he doesn't really like.

He refused all the expensive prescription wet food, so we tried IAMS wet food, which he ate for a while but now just sort of take a few licks and walks away. He's lost a little bit of weight, and while the most recent blood work shows his thyroid is OK, he does have some kidney and minor (borderline) liver issues. He seems happy (still jumps in my laps and purrs), but maybe a little more yeow-ly of late.

Are there any treats, foods, or supplements (fish oil?) I could give him to help him gain weight?