Thank you for the reply.
The diet is a viable option to try in hyperthyroid cats, but they have to eat it and eat it exclusively. If he was eating the dry and is now refusing it, that is not going to be a treatment form that will work for him. On top of that, most cats with HT have normal to voracious appetites so the fact that he is not wanting to eat makes me concerned that there is more going on than just the HT as a cause of his weight loss. It is not uncommon to see a cat that is affected by both HT and inflammatory bowel disease (IBD) and this can make the treatment of both more of a challenge. Either of these can be present and there can be no other abnormalities noted on bloodwork, xray or even ultrasound, although these tests should be run to rule out other conditions that present with the same symptoms. There is a specific GI blood panel that can be requested and if abnormalities noted, can lend support to either pancreatic or intestinal disease. The only way however, to confirm the diagnosis is with a biopsy of the intestine. This can be done via endoscope, but is better performed with an exploratory surgery. The problem with this is that many cat owners are not willing to pursue the diagnosis by these means. It is for that reason that many cats are "suspected" to have IBD or GI cancer as the cause of their weight loss and anorexia once all of the other easy to find things have been ruled out.
I am also suspicious that the symptoms that you first described of losing his balance and turning his head sharply had more to do with him not liking having the transdermal gel spread on his ears. I have seen cats react this way to topical products being put on them, especially if it improved and he is shaking his head. If that is the case, you could try the gel again and see if maybe he would react differently if you split it and rubbed it on both ear flaps or if you took extra time to rub it in well. There are other forms of the drug that can be tried and even though dizziness is a symptom in people, I have not seen it in cats.
I would discuss these with your vet and get their input as far as how to proceed. In the meantime, it is more important to get him to eat whatever it is that he will eat well until you decide how you are going to treat this.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.
My posted replies are for general education only and not meant as a diagnosis. Only after a thorough veterinary examination can a diagnosis for your pet be made and specific treatments be advised or medications be prescribed.