Methimazole & Kidney Function in Cats

A hyperthyroid cat's blood flows at a higher rate than that of a normal cat, increasing the glomerular filtration rate through her kidneys. This is the rate of how much blood passes through the kidney's tiny filters. Because of the higher blood pressure, more blood is processed through the kidneys, making them more efficient at their primary job of processing waste.

When a cat is treated for hyperthyroidism, her blood pressure is brought to a more normal level, decreasing the blood flow through her kidneys. When that happens, the previously existing kidney disease is usually discovered. It's relatively common for cats treated for hyperthyroidism to have CKD, Dr. Thomas K. Graves notes in his paper "Feline Hyperthyroidism 2009 (Proceedings)."

It's tempting to believe that treating hyperthyroidism might cause CKD, but that is misleading. Instead, if the cat already had kidney disease, restoring her normal thyroid function can expose the kidney damage that her hyperthyroid covered up.

The preferred treatment for hyperthyroidism in cats is radioactive iodine therapy to eliminate the tissue growth that is causing the thyroid imbalance. A second option is surgery to remove the affected tissue. Although both of these treatments will cure the hyperthyroidism, there is risk that CKD will be unmasked and the cat will then be more compromised by failing kidneys than by an overactive thyroid.

The medication methimazole, though not a cure for hyperthyroidism, offers a flexible alternative for a cat who might have kidney problems. Most hyperthyroid cats who are given methimazole are dosed twice a day, either orally or with a transdermal gel applied to the ear.

The vet can monitor the cat's blood work for glomerular filtration rate readings to see how the kidneys are responding to thyroid treatment. If bringing the thyroid -- and subsequently the blood pressure and blood flow rate -- to a normal level doesn't harm the cat's kidneys, a curative treatment such as radioactive iodine therapy might be considered. If not, the vet can adjust the methimazole level to reach a balance between managing the thyroid and the kidney disease.

It might seem like a choice between two evils: Do you treat your cat for hyperthyroidism or for CKD? The answer: Treat her for both. Allowing your cat's thyroid to run unchecked can be dangerous. "The validity of maintaining a cat in a mildly hyperthyroid state is questionable given that uncontrolled hyperthyroidism, in itself, appears to be damaging to renal function," according to Dr. Mark E. Peterson in his blog "Insights into Veterinary Endocrinology." For cats with severe CKD, keeping a mild hyperthyroid state might give a good short-term result, according to Peterson.

Hyperthyroidism can have serious consequences for your cat. Left untreated, it can diminish heart function and potentially lead to in heart failure. If your cat has CKD and hyperthyroidism, talk to your vet about the best way to manage both.