I know it seems cruel, but I adopt the 'down the hatch' approach. I know not all cats are the same, but Lazarus knows what is coming. I cup his head with one hand, with the same arm holding him in place. Tilt his head back. I have the pill ready in the other hand, held between my thumb and index finger. I use the middle finger to open his mouth from the side and drop the pill down his throat - then pass the food over so hopefully he we eat to wash the pill down.
I have to give him a thyroid tablet in the morning. In the evening, another thyroid tablet + a blood pressure tablet. I also have to give him Semintra, a kidney med, via syringe, in his mouth. Also, twice a day, I have to give him a hormone via syringe up his nose. It sounds quite daunting, but once you have a routine, it is straightforward. I just make sure I do it all quickly.
You may need to resort to using a pill popper, but better if you can do it by hand. Some pill poppers have a rubber/silicon tip, which is safer I think.
There is no reason thryoid meds should affect appetite (other than to give them a normal appetite once the thyroid is under control. I know some thyroid meds don't agree with all cat. When I adopted Laz about 4 years ago from Cats Protection, they had recently changed him from Vidatlta to Felimazole because Vidalta made him sick. He is 17 or 18 now according to my vet.
Good luck
PS - Well done for adopting an oldie
