Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that your fellow suffering from kidney failure is losing muscle mass despite your best efforts to support him.
I understand that he is taking Azodyl and Epakitin due to elevated levels of BUN (blood urea nitrogen).
I also understand that his T-4 levels are within normal limits. Is his T-4 high normal though? Cats that have kidney failure and hyperthyroidism may have T-4 levels that are within normal limits, but if the T-4 is high normal then they probably are hyperthyroid too. The combination of the two lead to a high normal T-4 with weight loss and elevated liver enzymes. These kitties can benefit from a low dose of methimazole (Tapazole). It is a balancing act to suppress the thyroid a little, so as to decrease his metabolic rate to a more acceptable level, but not enough to suppress his appetite.
Does he have high blood pressure? If he hasn't had his blood pressure checked that should be done.
Does he have high levels of protein spilling in his urine? If he has any protein loss in his urine, and no sign of a urinary tract infection then a test called a urine protein/creatinine ratio is needed. This will tell us if he is spilling protein.
Cats with kidney disease often are hypertensive and spill protein in their urine. Being hyperthyroid will exacerbate the problem. It is hard to maintain weight when spilling needed protein out through the kidneys. Adding either enalapril or benazapril could decrease his blood pressure and decrease protein spill through his kidneys.
Ideally cats with kidney disease should be fed very high quality protein, and avoid poor quality proteins. The kidneys are responsible for filtering protein, and keeping it in the blood. If we feed a diet excessively high in protein, especially poor quality, the kidneys must work harder than usual. Feeding a renal diet usually helps by not overworking the kidneys. In some cases to stimulate a cat that is eating poorly, or with excessive weight loss we recommend adding a small quantity of high quality protein to the meals. White meat chicken, cooked whitefish, scrambled eggs are all possible sources of high quality protein.
Best of luck with your fellow. Please let me know if you have any further questions.