I’m in the middle of this too with my 18 year old cat. Also had it with my 20 year old a year and a half ago. He just got sick and gave up.
Zephyr is also 11 on the hyperthyroid test. We got her down to a 5, but the meds were getting her sick. She’s impossible to pill (fights, screams and foams at the mouth). I dosed her food with a liquid compound (tried both fish and chicken) and she refused it most of the time.
I completely stopped the meds when she got down to 6ish pounds (from 10.5) and brought her slowly back up on food.
Stuff I used:
Petalyte (electrolyes with some chicken flavor)
Petromalt
Clam Juice
Parmesan Cheese (grated from the can)
Butler Tartar Control Treats
The key thing was the Royal Canin Recovery formula canned food. Have to get this from the vet, I think Science Diet has one too. Got her eating again and slowly moved her back to the Wellness chicken (no grain) canned.
One thing that helped a lot was mixing and microwaving the food. Tested with the back of a finger for temperature. It probably took 3 weeks to getting her back to eating in substantial quantity.
A word about Hyperthyroidism, since I did a ton of research trying to figure this out.
Zephyr went blind as a result of the uncontrolled HT. She was losing her vision, but the vet had said “old kitty”. Part of what I think drove her weight loss was the sudden onset of complete blindness. She’s adapting now and relying much more on scent and whiskers to navigate.
HT is very deceptive. Basically it overdrives the metabolism, increasing heart rate and BP to the point were complications from those arise. BUT, if you get the HT under control on a relatively old cat, it may reveal kidney issues. Since the HT overdrives the kidneys with increased blood flow, getting the HT under control may drop kidney function and then the cat gets sick from that.
Everything I’ve read about the radiation treatment says it’s the way to go. The return on investment is a year and a half in just raw cost terms, and medicating a cat for life just sucks if you can avoid it. I’d have done it ina heartbeat if it was generally available when my cats were first diagnosed with HT, but at that time the treatment was tapazole pills and neither cat tolerated that.
The dermal compounding sounds like the next best shot, and I may try that next. My vet isn’t confident that Zephyr would be accepted as a radiation treatment candidate because they generally want to demonstrate you can control with meds before approving for the radiation procedure.