we had a cat that had diabeties plus all of his organs were enlarged (cant remember what they called it now, and though the enlarged organs affected the diabetes such that we never really got the latter under control, despite daily insulin injections. these he had for over a year and a half (with many vets visits) before he finally secumbed to his illnesses. not had any experience with thyroid problems, but will say if your cat is on repeat medication, you can ask the vet to give you a prescription for the medication, (they may well charge you for this, but you can then use an online pet medicine site (eg petmeds.co.uk) where you will find that the cost of the meds may be up to 50% less.
in the end we were getting the syringes from petmeds (no prescription needed) at half the cost. and the insulin through the vets (we worked out we werent saving a lot buying the insulin straight from petmeds after paying for a consultation and prescription every 6 months (our vet wasnt marking up the meds too much for the insulin- they got their money from all the visits we were making with him).
so its worth checking out, it might even be that you dont need a prescription for the meds, but it could save you a penny or two.
Grendel
