urine crystals and hyperthyroidism

HELP!!  I have a 13 yr old male cat with a history of urine crystals, where the blockages have been removed twice and put on Royal Canine urinary SO low calorie (at the time he was over weight).  He has developed hyperthyroidism in the last year, changed to Royal Canine urinary SO moderate weight (due to weight loss) and put on a prescription of Felimazole 5 mg twice a day.  He has since lost so much weight that he is skin and bones and absolutely ravenous to the point that he is stealing food from my counter and our plates.  We really would like to switch him to Blue Buffalo true solutions urinary care dry food due to the rising cost of inflation, but I don't know what to do about his hyperthyroidism part of the diet or if we even should switch his food (except to the high calorie formula) .  Any help would be greatly appreciated!!

Hello and thankyou for this interesting question.  It sounds as though there are a lot of issues at play and in this situation, my first thought would be to wonder whether the hyperthyroidism is being controlled because it sounds as though your cat is still showing hyperthyroid symptoms (eating too much), which would normally be controlled by the hyperthyroid medication or, in some cases, by the specific prescription diet intended to do this.  Such hyperthyroid diet is often intended for ongoing feeding and it shouldn't be supplemented by anything else, unless specific hyperthyroid meds are being given instead.  In general and for many cases (remember that I do not know your cats case) it is thought that urinary diets are significantly less important in treating cystitis than other aspects of behavioural control including controlling an animal's stress levels, which treating the hyperthyroidism might well do.  It is possible that a special urinary diet is either unnecessary or of low importance in comparison to the hyperthyroid diet or medication - but the diets are prescription products and your vet would be the best person to discuss the most appropriate food and most cost-effective way to do the best for your cat in this case.   Always let them know honestly about financial constraints because they should help to do the best they can with the situation in front of them, which can sometimes include recommending the most cost effective treatments.  It is also worth reminding them about the urinary situation.   Please do let me know how you get on.

Ps it is also worth saying that this case should be triaged over the phone - please speak to your veterinary team and ask for this as a matter or priority, in order that advice can be given in an appropriately timely manner.  It may make a significant difference.