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Cat Thyroid Center - Click to view larger image
Feline Patients with hyperthyroidism and kidney disease
As cats age, just like humans, they experience a gradual
loss of kidney function. They should be carefully evaluated for any signs
of underlying early kidney disease, e.g. small kidneys on radiographs,
palpable changes in size or structure of kidneys, mineralization in the
kidneys or hyposthenuria and hyperproteinuria. As the disease progresses,
there are measurable changes in their blood levels of urea nitrogen and
Early in renal failure, the kidneys start losing their
ability to concentrate urine. This gradual process can be monitored by
measuring the urine specific gravity with a refractometer. Measurements
should be taken when the cat is not on a diuretic and before the administration
of fluid therapy. A normal feline urine specific gravity should be greater
than 1.035 mg/dL and can range to concentrations greater than 1.050. Aging
alone can take its toll on the kidneys and they begin to fail, producing
a more dilute urine. Thus the cat urinates more frequently and compensates
by drinking more water.
We rely on the BUN (blood urea nitrogen), creatinine,
urine analysis and blood pressure to evaluate and monitor feline kidney
function. Urea is the end product of protein digestion and its concentration
varies with the diet and with kidney function. Creatinine is produced
at a fairly constant rate depending on muscle mass and renal function.
As the cat's kidneys begin to fail, they do not filter these metabolic
byproducts as efficiently. Their concentration in the blood rises. And
the cat begins to drink more water.
Stage 1 Chronic Kidney Disease
Kidney disease usually goes undetected. No apparent symptoms. Usually
caused by an insult to the kidney. No biochemical evidence. No azotemia.
Stage 2 Chronic Kidney Disease
Stage 2 kidney disease may have very subtle signs. These cats will have
a creatinine level between 1.6-2.8 mg/dL. Azotemia does not preclude us
from treating a patient, but we use a lower dose to try to prevent worsening
the condition. Proper diet and fluid therapy remain the mainstays for
renal disease. Click here for more information about feline
Stage 3 Chronic Kidney Disease
By definition, these cats are uremic with elevated BUN levels and Creatinine
levels between 2.9-5.0 mg/dL. They are sometimes nauseated with vomiting
or inappetance and/or lethargy.
Fluid therapy in the hospital or administered at home
by the client at a frequency from once a week to every day has maintained
many cats for years.
We feel the addition of essential fatty acids to the
diet to reduce renal inflammation is beneficial and without any negative
side effects. These are the omega-3 fatty acids.
Concurrent administration of ACE inhibitors has shown
to be beneficial in some patients but is not something that we routinely
recommend. When these drugs are used, the BUN and creatinine levels must
be monitored closely.
Stage 4 Chronic Kidney Disease
These cats are in end-stage renal failure. They have Creatinine levels
in excess of 5.0 mg/dL. Aggressive intravenous fluid therapy, antiemetics,
and adequate caloric intake are essential. Feeding any food at this stage
is far more valuable than trying to encourage a less palatable, scientifically
formulated diet. Patients in stage 4 chronic kidney disease would generally
not be accepted for I-131 therapy.
Some clinicians do a Tapazole trial for a few weeks.
Prescribe Tapazole at 5 mg divided b.i.d. and adjust the dose higher to
reduce the thyroid hormone level to the euthyroid range, ideally between
1 and 2 mg/dL. At this time blood tests are done, including CBC, BUN,
and Creatinine. The CBC should be normal and the BUN and Creatinine may
be higher, but still be within normal limits. Thus if the kidney values
remain stable, the cat will show a similar result when the euthyroid state
is accomplished with radioactive iodine.
This is not a guarantee that the kidneys will remain
healthy for a long time after the treatment, but it is a very good sign.
On the other hand, if both blood tests quickly become abnormally high,
e.g. BUN > 50 and Creatinine >5.0 mg/dL and these values continue
to climb, this patient is not a good candidate for radioactive iodine.
If the patient's blood levels remain elevated but constant, then this
patient would be an acceptable candidate based on all other factors being
In addition to monitoring the Creatinine and Urea nitrogen
levels, it is important to monitor blood pressure and urine protein/Creatinine
Patients with Gallop Rhythm or
We find patients with abnormal heart rhythms and murmurs due to the extremely
high heart rates, sometimes exceeding 300 bpm. This tachycardia can be
controlled with cardiotropic medications such as Atenolol until the radioactive
iodine takes effect. Usually the heart medications can be discontinued
within 10-14 days after treatment and the heart rate will be almost normal.
Sometimes the murmurs improve or disappear, but not always.
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