Chronic kidney disease and hyperthyroidism often occur together because they are both common disorders in older cats. Hyperthyroidism tends to artificially increase the blood flow to the kidneys as well as the glomerular filtration rate (GFR). The GFR is a measure of how efficiently the kidney filters the blood. When it is artificially increased, it can mask renal insufficiency; blood tests on a hyperthyroid cat may appear normal despite mild to moderate kidney disease.
Treating hyperthyroidism restores the high serum T4 concentration to normal. This tends to lower kidney blood flow and GFR to normal or low levels, “worsening” kidney function tests. It is important to remember that treating hyperthyroidism itself does not cause chronic kidney disease; the kidney problem was already present before treatment. It was simply masked by the hyperthyroidism.
Chronic kidney disease is reported to be prevalent in 8% of cats >10 years of age and 15% of cats >15 years of age. In contrast, up to 30-50% of hyperthyroid cats have chronic kidney disease. This suggests that hyperthyroidism may be initiating or worsening the chronic kidney disease in these cats. If this is true, one should always try to correct the cat’s hyperthyroidism, even if the cat has known renal disease.
In addition, recent reports indicate that many untreated hyperthyroid cats develop proteinuria (excessive protein in the urine), This proteinuria resolves after successful treatment with radioiodine. In addition, high levels of retinol binding protein, a marker for renal damage, have also been shown to decrease significantly after radioiodine treatment. This too suggests that hyperthyroidism can cause reversible renal dysfunction.
Therefore, it appears that leaving a hyperthyroid cat untreated (or poorly regulated with methimazole) may be detrimental to long-term kidney function. However, treating and curing hyperthyroidism may help to preserve remaining kidney function.
Determining which untreated hyperthyroid cats have clinically significant underlying chronic kidney disease can sometimes be difficult. Methimazole and carbimazole can provide a “preview” of how the cat will be after curing hyperthyroidism. Thus, some veterinarians attempt trial therapy with methimazole or carbimazole to test what renal function might remain after treating the hyperthyroidism. Except for advanced kidney disease, the necessity of this approach is questionable, given that treatment for the hyperthyroidism would still be recommended in most cats. In support of that reasoning, the survival of cats that develop obvious kidney disease is not shorter than those that do not develop kidney disease after treatment of hyperthyroidism.
We will review your cat’s kidney tests and urinalysis prior to admitting your cat to Hypurrcat. If we find any signs of chronic kidney disease, we will begin prophylactic measures to control the kidney disease while your cat is being treated. Even in cats with healthy kidneys, we always repeat kidney tests at the end of the treatment to ensure that kidney function has not deteriorated.
Again, if kidney disease does develop, it is not caused by the radioiodine. It was present before the treatment, but was masked by the hyperthyroidism.
Hyperthyroidism and Kidney Function: A Love/Hate Relationship
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