My 17 year old cat has this. She has always been neurotic and a terrible pill-taker, so I tried everything to avoid having to give her the tablets, right up to importing a special herbal medicine for hyperthyroidism from the States. That was unpalatable and didn't work (have 2 bottles if anyone wants to try it) the diet food was refused and she got even thinner, and eventually in despair we tried the tablets. She's calmer and gaining weight, vomiting much less and shouts less at night. Should have done it months/years ago. She hates taking them, but it's a 5 minute sulk, not the previous neurotic retreat under the furniture for days. The only thing is that her temperature regulation has dropped and she spends most of the time sitting on a pretty hot radiator, and lies on me all night. The meds and associated blood tests do add up too. Need to go back for more blood tests next week, but despite the impending bill I'm sure this is the right way to go.
The other thing that might be worth investigating is your cats diet. I was horrified to find out that the diet I was feeding my cats, which I thought was pretty good (no 4% chicken Go Cat) was actually terrible. If you have a Pets at Home near you it might be worth a chat, their staff are well-trained and they can tell you loads. My amateurish interpretation is that cats are obligate carnivores and the more non-meat stuff in their food (rice, cereals, veg, etc) the worse it us for them, and the more non-actual -meat, ie chicken derivatives (feet, feathers, beak) rather than chicken meat, the worse it us. The poorer the food, the more likely they are to have metabolic bad digestive disorders, chucking up or overeating. Cats would naturally eat dozens of small meals a day (think one mouse) rather than 2 big ones, so little and often feeding is better for them. I'm sticking to Applaws, Canagan, Ziwipeak, Natures Nenu, and Lily's kitchen (even Hills and Iams don't measure up) and have definitely seen a difference.