Yes, I've had a number of hyperthyroid kitties and with one old boy that was a major question. As the vet probably explained, the overactive thyroid makes the kidneys work overtime, so if the kidneys aren't working right, the thyroid is basically making them work. But the hyperthyroidism has its own consequences, so it's really a balancing act. We struggled between keeping his thyroid somewhat controlled, but keeping his kidneys going. It's tricky.