Chronic kidney disease (CKD) is a common cause of illness in cats — especially in older cats — and unfortunately the incidence is increasing. In 1990, for every 1000 cats admitted to veterinary teaching hospitals, there were 45 cases of CKD. In 2000, the number increased to 96 cases per 1000 admissions. In my feline specialty practice, CKD remains the most common illness we encounter, and the most common reason for euthanasia.
Hyperthyroidism is the most common glandular disorder in cats. It is mainly a disease of elderly cats, with cats typically being around 13 or 14 years of age at the time of diagnosis. Hyperthyroidism occurs when the thyroid gland, located in the neck, secretes an excessive amount of thyroxine, the main thyroid hormone in cats. (Thyroxine is commonly abbreviated as T4.) Left untreated, hyperthyroidism can lead to heart failure and dangerous high blood pressure.
CKD is incurable. With the exception of a kidney transplant, it is difficult or impossible to improve kidney function in cats with CKD. The focus of treatment is to delay the progression of the disease, improve the cat’s quality of life and hopefully extend a cat’s survival time. This is usually achieved through a variety of dietary and drug interventions. There are many treatment options for hyperthyroidism. In fact, depending on the treatment option, hyperthyroidism may be curable.
Why they happen together
Because both diseases are primarily found in older cats, it is not uncommon for both disorders to be present in a cat simultaneously. In fact, most published reports indicate that about 10 percent 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,” explains 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, so 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.”
Unfortunately, no test exists
At the moment, there is no simple test that can predict with any certainty how the kidneys will respond after thyroid treatment. In everyday veterinary practices, measurement of the serum creatinine level remains the best estimator of kidney function. The ability to produce strongly concentrated urine is also a good indicator of kidney function. Cats with well-concentrated urine — and with creatinine levels that fall within the laboratory reference range — are likely to have normal kidney function and are less likely to have problems after the hyperthyroidism is corrected.
There are currently four methods of treatment for hyperthyroidism: medicine, food, surgery and radioactive iodine. The first treatment, medical therapy, is perhaps the most common. The prescription drug methimazole, given twice daily, will control hyperthyroidism in nearly all cats. The most recently developed treatment is the feeding of an iodine deficient diet (Hill’s Prescription Y/D). However, neither of these methods — medication and food — are curative. If the medication administration is discontinued or if the cat stops eating the special diet, the hyperthyroidism will recur. Therefore, treatment with medication or food will be lifelong.
The other two methods of treatment, surgery and radioactive iodine, are curative. Surgery to remove the thyroid gland has fallen out of favor and is rarely performed, mainly due to the risks of anesthesia, the cost of the surgery and the potential for post-operative complications. Radioactive iodine involves no anesthesia, is safe and effective, and it is generally accepted to be the treatment of choice for hyperthyroidism.
Take a slow approach
How does this figure into the approach to treatment? “The key to treating these cats is to go slowly,” says Dr. Stone, “especially in cats that we already know have kidney disease.” If treating with medication, this means starting with a very low dose, and monitoring the T4 level and the kidney toxin level closely. “Rapidly dropping the T4 level with high doses of methimazole can abruptly reduce the blood supply to the kidneys. It’s best to start with a judicious dose, and gradually bring the thyroid level to normal, while closely monitoring the kidney values to be certain they don’t rise too high.”
Dr. Stone’s concern for cats with pre-existing kidney disease is prompted by reports that show that the survival time of cats with previously diagnosed CKD is significantly less than that of cats whose CKD was unmasked after thyroid treatment.
It has been recommended that if cats with hyperthyroidism are to undergo surgery or radioactive iodine treatment, they be treated with thyroid medication initially, to see what effect thyroid treatment will have on the kidney, since surgery and radioactive iodine therapy are irreversible. This is especially advisable in cats with pre-existing kidney disease, since these cats, as noted above, tend to fare worse after treatment.
If giving medication won’t be possible (for example, the cat experiences side effects from the medication; the cat won’t allow administration of the medicine; or the owner’s schedule is such that consistent administration of the medicine is not possible), a curative treatment (radioactive iodine or surgery) should still be performed, as long as it is clearly explained to the owners that there is a chance that the CKD may worsen after treatment.
Not treating the hyperthyroidism is not an option; untreated cats may go into heart failure and/or develop high blood pressure, which can lead to blindness, neurological abnormalities and a hastening of the progression of the CKD.
Chronic kidney disease and hyperthyroidism are two of the most common disorders diagnosed in geriatric cats. Studies are ongoing to try to identify which cats are most likely to develop kidney-related problems after hyperthyroid treatment, and how to improve the survival time of hyperthyroid cats with pre-existing kidney disease. — Arnold Plotnick, DVM, DACVIM