In fact, most published reports indicate that about 10% of hyperthyroid cats have concurrent CKD at the time of diagnosis. While the treatment of hyperthyroidism is fairly straightforward, and the methods for controlling CKD are well-established, treatment of thyroid disease in cats with concurrent CKD must be undertaken with caution.
“Treatment of hyperthyroidism in cats with kidney disease poses an additional challenge for veterinarians”, says Dr. Michael Stone, a veterinary internist at Tufts School of Veterinary Medicine. “Cats with hyperthyroidism have an increased blood volume. They also have an elevated heart rate. This results in an increased cardiac output. The blood flow to the kidneys is enhanced, and this improves the ability of the kidneys to filter toxins from the blood stream”. Dr. Stone continues, “When hyperthyroidism is treated, the blood flow to the kidney is reduced. In cats with pre-existing kidney disease, this reduction in blood flow to the kidneys can elevate the level of kidney toxins in the bloodstream and make the cat feel poorly.” But there's more to the story. “In some hyperthyroid cats, the increased blood flow to the kidneys helps hide the fact that the cat already has pre-existing kidney disease. Treatment of the hyperthyroidism reduces the blood flow to the kidneys and 'unmasks' the kidney disease.”