Cat with Diabetes and Hyperthyroidism? Advice?

My partner's 7 year old cat has just been diagnosed with Diabetes and Hyperthyroidsm. I cant help think Zombie has won the genetic lottery, here- what are the chances of getting two endocrine disorders at the same time? Could this be because he is a tortoiseshell cat? Which is already a genetic... show more My partner's 7 year old cat has just been diagnosed with Diabetes and Hyperthyroidsm.
I cant help think Zombie has won the genetic lottery, here- what are the chances of getting two endocrine disorders at the same time?
Could this be because he is a tortoiseshell cat? Which is already a genetic mutation?

We have already decided that we dont want to treat the Diabetes, as we treated my old dog for a year before she finally succumbed to the disease, and I would never want to put another animal through it.

We were told by a vet this morning that Zombie could live for another couple of years (without treatment), or it could be a few months.
I would prefer to have him euthenased ASAP, rather than put him through the pain of slowly starving to death, but my partner and his family would prefer to wait until he is really sick- I just dont want to put him through that.
What are everyone's thoughts on this?
Update: Zombie is a male cat, so Tortoiseshell is a mutation. Apparently it means that he is a hermaphrodite because only females can have this colouring. Zombie lives with 5 other cats, all but 1 of which are closely related (his mum, brother and sisters) and if we do treat him for Diabetes he will need to be... show more Zombie is a male cat, so Tortoiseshell is a mutation. Apparently it means that he is a hermaphrodite because only females can have this colouring.
Zombie lives with 5 other cats, all but 1 of which are closely related (his mum, brother and sisters) and if we do treat him for Diabetes he will need to be permanently seperated from his family because he cant have any additional food.
I have already treated this disease in one of my pets, and it was horrible watching her slowly die even with regular treatment and vet visits.
My partner does not think it would be fair to his cat to jab him with needles every day, and his family do not want to treat him either.
I would be happy to support him, care for Zombie and give him his daily meds if he wants to treat him, but it is his choice.
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