Thyroid and high protein diets

Hello, I suspect my 13yr old cat has hyperthyroidism and he is having blood tests today to find out. I am feeding him Blink cat food which is high in protein as I understood he needs more protein. Today my vet said he should not be fed a high protein diet due to his age. Could someone tell me whether this is correct please? Thank you for your time and expertise.

Hello!  I can see how you figured that if a cat was getting thin, they must need more protein to help them to put on weight and build up muscle.   However, if a cat has hyperthyroidism, then the weight loss is not to do with insufficient protein in the diet;  it is to do with the metabolic rate (the speed of 'burning energy' in the body) being too fast;  as soon as the cat eats, the energy is 'burned' / expended (cats are true carnivores;  protein gives them energy).  These cats are often a little jittery, over-energised, thin, and hungry.  They also tend to be a little uncomfortable.

Thyroid hormones drive the chemical breakdown, or metabolism, of any food eg protein to make energy (the other possibility for food would be to be stored for later on the animal's back as muscle or fat.  In hyperthyroidism, there is too much thyroid hormone, so too much food is 'burned' or broken down; this makes a lot of fuel available in the cells.  The heart beats faster, the liver works harder, the cat has excessive energy but of course, gets thinner and thinner because there is no opportunity to maintain the fat layer.  However, when we treat hyperthyroidism, that rapid metabolism stops.  The metabolic rate slows down again and the cat becomes more likely to put on weight naturally, because they are no longer burning up all their fuel.

Usually, onve the hyperthyroidism is being treated, the cats simply put weight on by themselves.

On the other hand, excessive dietary protein is broken down by the body to produce Urea, which is waste product and gets thrown out of the body through the kidneys.   High levels of urea may damage the kidneys, so high protein food is not to be encouraged at this time.