If she is used to fish flavors, start blending the chicken and turkey flavors with some fish flavor, and gradually decrease the amount of fish until she will readily accept the new flavors. In order to stop the effect of dietary iodine on her thyroid, you would have to eliminate ALL iodine from her diet, every last bit, not just reduce it some, so if fish is what she eats, let her eat fish. It is more important that she get the calories she needs. Hyperthyroidism is like driving a car with the gas pedal pushed all the way to the floor, and trying to control the car with just the gears and the brakes. Her "motor" is running at full speed all the time, even when she is sleeping.
She needs good quality calories, not carbs, to gain weight. What your vet has told you is the same thing as telling a person who is losing weight due to illness, to "eat a lot of candy to gain your weight back." Feed her more. Give her more meals. If you are home during the day, give her lunch as well as breakfast and dinner, and give her a bedtime meal as well. Feed her as much as she will eat.
With the transdermal medicine, you do know that you have to clean her ears constantly, yes? The creme will build up a waxy layer that prevents further doses from being absorbed. We would keep a system like "dose right ear in the morning, clean left ear (wipe with water, or to start, gentle soap then wipe with water), dose left ear at night, clean right ear" to keep ears clean of build up and know which one to dose and which to clean each time.
I'd also look into finding a vet with more experience with hyperthyroidism. Best dosing is twice a day. Ear cleaning is essential. Suggesting high carbs to gain weight is irresponsible, even if the cat isn't diabetic to begin with. And the radioactive iodine is a CURE. We had our cat done, his thyroid tumor was just beginning to go malignant, so they gave him a really high dose to make sure they got it. so, he is now HYPOthyroid, too low, but that is easily fixed with simple thyroid pills, and once his level is regulated, it hasn't changed in years. The iodine treatment is expensive, but it is just once and done. Considering the dosage of the transdermal you are using, you will spend that much money in a fairly short time, and it's just a treatment, not a cure.
Look for and join the Facebook group "Hyperthyroid Cats". They are an awesome group with an amazing amount of knowledge, much like this group is with diabetes.
