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Important News About Treating Hyperthyroid Cats

The following information is summarized from a newsletter article entitled “Important News About Treating Hyperthyroid Cats” by Dr. Wackerbarth in the Feline Hyperthyroid Treatment Center's Spring 2010 Newsletter.

Over the past 18 years, Dr Wackerbarth and Dr. Faythe Vaughn have treated over 11,000 hyperthyroid cats. When deciding on how or if to treat a hyperthyroid cat with kidney disease, they look at each cat's:

• Heart Rate

© Dimitri Torterat, CC-by, www.adelieland.eu

• Urine Specific Gravity – This number indicates the kidney's ability to concentrate waste into the urine. (It is a laboratory test that measures the concentration of particles in the urine.)
• BUN/Creatinine Levels – These are blood levels that indicate kidney (renal) function.

When cats are hyperthyroid, their heart rates increase and, with it, the blood supply to the kidneys. This increased blood flow makes it appear that the kidneys are functioning better than they actually are. Specifically, the BUN and creatinine levels that your veterinarian uses to gauge kidney function appear better (lower), but this is not a true gauge of the kidney's health. Once the hyperthyroidism is treated, the heart rate decreases and then the kidney's filtration rate also declines. Cats that have significant pre-existing kidney disease will usually experience an increase in creatinine levels, making it appear that the kidney's are worse.

At the Feline Hyperthyroid Treatment Center (FHTC), they believe that most hyperthyroid cats will have a longer, better quality life with ¹³¹I treatment, and the treatment does not cause any direct damage to the kidneys. However, most senior cats do have some degree of kidney disease. As described above, this kidney disease can be unmasked when the hyperthyroidism is treated. The challenge is determining which cats have significant enough pre-existing kidney (renal) disease to compromise their quality of life after treatment.

It is not possible to determine absolutely the health of a hyperthyroid cat's kidneys, but, after many years, the FHTC has determined that the urine specific gravity (USG) can be used to predict which cats are more likely to develop azotemia (a build up of toxins in the blood) after ¹³¹I treatment.

The FHTC looked at 220 randomly selected hyperthyroid cats that had been treated with ¹³¹I. All of the cats that were selected had complete lab work done both before treatment and at one and three month intervals after treatment, to gauge the effect of the treatment on the kidneys.

The following table shows the relationship of the Urine Specific Gravity (USG) before treatment to post-treatment creatinine levels, a good monitor of kidney function. The first column shows the USG range and the second column shows the number of cats in the study that fell within that range. The third and fourth columns show the number of cats in that range whose creatinine levels were elevated three months after treatment. Columns five and six show the average increase in creatinine levels and the highest level in the group. The final column shows the top of the creatinine reference range (the high end of normal as determined by the reference laboratory where the labs are sent to).

As you can see, the cats with pre-treatment USG ≥ 1.040 had only a 7% chance of developing azotemia (the build up of toxins in the blood) by the three month recheck after ¹³¹I treatment. Also, in all groups, the increase in creatinine levels was fairly modest, ranging from 15 to 35% above the normal reference range. These levels generally don't lead to kidney disease severe enough that your cat will show significant symptoms.

The FHTC concluded that they can use the USG levels to determine which cats are more likely to develop increased levels of toxins in the blood after ¹³¹I treatment. The risk increases with decreased USG levels, but, in fact, azotemia can occur with any USG.

Table 1 – FHTC Study: Comparing Pre-¹³¹I USG to Developing Post-¹³¹I Azotemia

Research shows that most of the drop in kidney filtration rates occurs in the first month after ¹³¹I treatment. Also most of the increase in toxin levels seems to occur in the first month. This suggests that changes in creatinine levels after the first month result from” kidney disease that is progressing (getting worse), rather than pre-existing renal disease being unmasked.

The Methimazole Trial

Some hyperthyroid cats are treated with a medication called methimazole rather than receiving ¹³¹I treatment. This medication can also be used in trial form. It may provide good information about the cat's true kidney function.

A methimazole trial is recommended for cats with elevated creatinine levels. If the cat does well on the medication, the best treatment is probably to continue the methimazole as a life-long treatment. If the kidneys worsen, the medication dose could be carefully decreased. This may allow the kidneys' filtration rate to increase enough to help decrease the creatinine levels and help the cat feel better.

The FHTC also does say, “At the same time, we have seen occasional cats with pre-treatment azotemia that remain stable or paradoxically improve after treatment with ¹³¹I treatment. Their people elected ¹³¹I, despite a guarded prognosis, because the owners or their cats didn't tolerate medication. We evaluate each cat individually, but conclude that there are few absolute contraindications for ¹³¹I treatment in hyperthyroid cats.”

If your cat is hyperthyroid, please discuss with your doctor whether or not a methimazole trial and/or ¹³¹I treatment would be appropriate.