Hi Spottymint, there isnt much i dont know about cat ailments, especially hyperthyroidism and kidney disease. I can get really complex if you want, or i can keep it basic. If your girl is "on the edge" of hypert, i wouldnt do anything yet, hypert should only be treated once the levels of the T4 (thyroid hormone) are pretty high. the meds they use to bring the thyroid down are called Vidalta, Ive seen many problems with this drug, in that the dosage is too high, and many cats go hypOthyroid (under active thyroid) and this is very dangerous, especially to the kidneys.
The thyroid levels have to be be bought down SLOWLY and this is why.......
When a cat (or any species) has hyperthyroidism the whole body is running fast, meaning that the heart is faster, the organs work faster, the digestive system works faster (which is why SOME cats eat like crazy and still loose weight) we also often see diarrhea. Anyway, the most important factor is that the filteration of the KIDNEYS is FASTER. So before any treatment is commenced a blood test is taken, this tells us what level of the T4 (thyroid hormone) is, but it also shows kidney function, and when the filteration of the kidneys is running high, the blood levels (the kidney Urea and Creatinine) numbers look great. However, a word of warning, once treatment is started it has to be at a low dose, this is because the kidneys need time to adjust to the slow down in filteration, and if they dont, then the kidneys can fail. So Vidalta is a high dose drug, often used as a first choice drug by vets at the slightest onset of hyperthyroidism which will bring the thyroid hormone down too fast, sometimes too fast for the kidneys to cope with. So, i wouldnt treat hypert with this drug. I would use the old drug Felimazole which is available in smaller doses, 2.5mg a day. There is quite a bit of blood testing until the levels get right, and that can be costly. But if the kidneys are coping well once the thryoid levels are back to normal, then there are other options of treatment. The 131 radiation treatment is one of them, it is a single injection given under the skin, but the cat has to be kept at the vet facility for 3 weeks because they are radioactive. This is a cure, but it is expensive, i think around £2000 in the UK. The other option is a bilateral thyroidectomy, where they remove the benign tumor on the thryoid, this is not a permanent cure, lasts for around 2 years but it is only a day operation. In the case of cats who are not candidates for either of the above, then medications are the only option, testing every 3 to 6 months and adjusting the dose accordingly. Hope this makes sense.