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My name is***** and I am sorry to hear that Otis has both hyperthyroidism and renal disease. This is a common problem in old kitties and there are several options out there for treatments.
First, have you considered treatment for the hyperthyroidism with the medication called Methimazole? This drug suppresses the function of the tumor cells in the thyroid gland. It can be tapered to partially control the hyperthyroidism and still support kidney function.
If that is not an option, there is a diet made by Hill's called Y/D. This diet is very restricted in Iodine and therefore reduces the amount of thyroid hormone the diseased gland can produce. This diet is also protein restricted so it supports renal function as well.
Unfortunately, this diet is not the tastiest, so some cats won't eat it. The next best choices are the renal support diets. Royal Canin, Hill's and Purina all have various recipes that you can try to see which one she likes best. They all are basically the same formula, though, reduced protein and phosphorus and increased potassium are their primary features.
Does this help? What other questions do you have?
How is Otis? Were you able to find a food that will be suitable for his nutritional needs? Do you have any other questions?