BUN = blood urea nitrogen
Chronic renal failure usually occurs due to progressive loss of functioning kidney tissue due to tubulointerstitial degeneration. The cause of such degeneration is currently unknown.
Secondary renal hyperparathyroidism arises for quite a few reasons in association with renal failure. The best understood reason is that the kidneys are unable to excrete phosphorus efficiently. Increased circulating phosphorus inhibits renal synthesis of calcitriol, the active form of vitamin D, effectively increasing PTH (parathormone) concentration = hyperparathyroidism.
A urinalysis is essential to fully understand your cat's prognosis. Please make sure a urinalysis is performed and I then can be more accurate for you vis à vis your cat's prognosis.
A creatinine of 2.4 is elevated. His hyperthyroidism is masking the true health of his kidneys. They're actually functioning better because he's hyperthyroid. Once his hyperthyroidism is controlled, his kidney values are likely to worsen but we can't allow him to be too hyperthyroid as that can damage kidneys.
You wouldn't give phosphorus at home. You would administer a phosphorus binder to your cat - a drug that removes phosphorus from his GI tract before it can be absorbed. Supplemental fluids given subcutaneously at home and a prescription low-protein diet are important as well. Yes, he can live with these conditions but his vet will need to discuss with you how hyperthyroidism is treated and how treating it affects your cat's kidneys. I can't give you a life expectancy until I see the results of a urinalysis and serial measurements of his renal values after treatment is instituted. If he stabilizes with therapy over time, his prognosis is better than if therapy doesn't alter his test results.
The vet misspoke. Methimazole tablets are the usual manner in which we address hyperthyroidism and have a very high rate of success. If you're not satisfied with your current vet, please consider another. You also have the option of having your cat see a specialist veterinary internist as can be found here: www.acvim.
I have to leave my computer for the night but promise to reply tomorrow if need be.