Hyperthyroidism is a very common
endocrine disorder of older (usually >10 years of age) cats. In
fact, it is said to be the most common endocrine disorder in cats
but judging by the large volume of emails I receive asking for help with
management, I would say that it is a toss-up as to which disease is more
The catalyst for the writing of this
webpage is the recent introduction to the veterinarian-prescribed diet market of
Hill's y/d which is an iodine-deficient diet targeted at feline
Please note that Hill's has made false and misleading
statements about this diet that will be addressed
What are the signs of hyperthyroidism?
What are the causes of hyperthyroidism?
is hyperthyroidism diagnosed?
What are the treatment choices of hyperthyroidism?
hyperthyroidism and chronic kidney disease (CKD)
What is hyperthyroidism?
Hyperthyroidism results from an
overproduction of thyroid hormones (mainly T4) from a tumor in the
thyroid gland. This tumor, in over 97% of all cases, is a
benign (non-cancerous) adenoma.
To repeat, hyperthyroid patients do not
have cancer except in very rare (2-3%) cases where a thyroid carcinoma
is the cause of the disease.
An elevation in thyroid hormones increases
the metabolic rate of the body which puts stress on the heart, kidneys,
nervous system, gastrointestinal tract, liver, as well as all other
organs of the body.
Think of how hard your car's engine has to
work and how much fuel it uses if you rev up the motor (increase its
rpms) by stepping on the gas pedal while in neutral.
This is what is happening inside your
hyperthyroid cat's body. Every organ system is under the negative
influence of an abnormal increase in the body's metabolism.
In the analogy above, food is your cat's
fuel and it is being used at a faster rate than normal which results in
an increased hunger and weight loss.
Hyperthyroidism is eventually fatal if
What are the signs of
Since elevated thyroid hormones negatively
affect every organ system in the body, the clinical signs are extremely
varied but include some or all of the following - with an increased
appetite and weight loss being the two most common:
increased appetite (but sometimes
weight loss (fat and muscle) in the
face of a good appetite
increased activity ("acting like a
increased drinking and urination
heart disease - increased heart rate/'pounding' heartbeat
increased respiration rate
hair coat/skin/nail abnormalities
high blood pressure (hypertension) -
Please note that if your cat's eyes look like
big black pools (dilated pupils), this is a sign of possible retinal
detachment which can occur secondary to high blood pressure and will
result in partial or total blindness. Not all
hyperthyroid cats become hypertensive but is a good idea to have
your veterinarian monitor your cat's blood pressure.
What are the causes of
I wish we knew the definitive answer to
Possible causes of hyperthyroidism
Iodine levels in cat food -
either too low or too high. Note that this is a very poorly
regulated nutrient in cat food with levels varying over a 30-fold
PBDEs - PBDEs are fire
retardant chemicals that can be found in high concentrations in some
fish and also in house dust. Since PBDEs are known to affect thyroid function, and
cat foods tend to be high in fish, there may be a causal link
between hyperthyroidism and fish consumption. This is one
of several reasons why I recommend against feeding fish-based diets
Soy in cat food - Soy is a
known disruptor of thyroid gland function. If you have
read my Feeding Your Cat:
Know the Basics of Feline Nutrition which outlines what it means
to be an obligate carnivore, you will understand that soy has no
logical place in cat food. However, soy will increase the
profit margin for pet food companies. Therefore, it is present
in many cat foods - especially those made by Purina.
According to one study, soy was identified
in 60% of all tested cat foods at a level high enough to interfere with
BPA (bisphenol A) - BPA is a
known endocrine disrupter that may play a role in the cause of
hyperthyroidism. BPA is found in the coating of the inside of
some cans of pet food. Generally speaking, the smaller (3
ounces and 5.5 ounces) cans will be less apt to contain BPA than the
larger (12-13 ounce) cans.
How is hyperthyroidism
Your veterinarian will perform a thorough
physical exam including palpating (feeling) both lobes of the thyroid
gland which sit in the throat area near the Adam's Apple. A normal
thyroid gland should not be large enough to feel whereas an adenomatous
thyroid gland will usually be large enough to palpate.
Next, a full blood panel, cbc (complete
blood cell count), and urinalysis will be performed to look for any
disease process(es) that could explain any of the clinical signs that
your cat is exhibiting.
It is important that the blood panel
include a total T4 level since this is the most important value
to look at when assessing thyroid gland health. The panel that
your vet chooses may also include a free T4 which is also useful
in diagnosing some hyperthyroid patients but is not as clearly
interpreted as total T4. Free T4 can be falsely elevated for
The panel that I choose to use for my
patients includes both total T4 and free T4 but panels vary from lab to
Thyroid scintigraphy (thyroid scan) is considered the 'gold
standard' for diagnosing mild hyperthyroidism but it is not readily
available to most people. However, as noted below, the best
treatment for hyperthyroidism is radioactive iodine and, ideally, you should
choose a facility that performs thyroid scintigraphy to characterize the
size, shape, and location of the abnormal tissue in order to gain more
knowledge of the disease severity and to also aid in selecting the
proper dosage of I-131.
the treatment options for hyperthyroidism?
In the sections below, you will see links to
Insights into Veterinary Endocrinology written by Dr. Mark Peterson, DVM, DACVIM. Dr.
Peterson is a world renowned expert in feline hyperthyroidism and was
the first veterinarian to recognize this condition in the early 1970s.
He has treated over 10,000 hyperthyroid cats in the past 30 + years and
has lectured countless times to veterinarians from all over the world
regarding topics in endocrinology.
Dr. Peterson and I have collaborated
recently regarding the new Hill's diet, y/d, and on other general
nutrition issues. In combining our
individual strengths (Dr. Peterson's background in endocrinology and
mine in nutrition), Dr. Peterson has written several very informative blog entries.
There are 4 treatment options for
radioactive iodine (I-131) - the 'gold
anti-thyroid medication (e.g.
Hill's y/d (a diet deficient in iodine
- canned and dry)
Note that only
radioactive iodine and surgery address the underlying cause of the
disease which is a tumor.
Anti-thyroid medication and an
iodine-deficient diet do not address the tumor which will continue to
Therefore, representatives from Hill's are making a false
statement when they claim that y/d "restores thyroid
Radioactive iodine -
iodine is the treatment of choice for hyperthyroidism in nearly all
treatment involves an injection of a very small dose of radioactive
iodine that is placed just under the skin. Because the thyroid
gland heavily concentrates iodine, the thyroid tumor will be
In addition to the thyroid tumor being
targeted, any other ectopic (found somewhere other than its normal
location) overactive thyroid tissue will also be destroyed. This ectopic
tissue is often located in the chest cavity but will show up on a
thyroid scan as discussed below.
A small risk (~5%) of hypothyroidism (the
opposite of hyperthyroidism) exists if too much normal tissue is also
destroyed. These cats will need to receive thyroxine
supplementation temporarily or for life.
On the flip side, a small number of cats
(~5%) remain hyperthyroid (and require a second treatment) if not enough of the adenoma is destroyed or
if the patient actually has a carcinoma (cancer) instead of a benign
adenoma. In the case of a carcinoma, a much higher dosage of I-131
is needed to effectively treat the disease.
As noted above, a
thyroid scan (scintigraphy) helps the clinician determine the size, shape,
and location of the abnormal tissue which helps determine the proper dosage
of I-131 to ensure the best possible outcome.
Without a scan, the dosage will only be a
Other treatments are discussed below but I
can say without hesitation that if any of my own deeply-loved cats ever
ended up with hyperthyroidism, this would be the only treatment that I
Update: 6 months after I
wrote the statement above, my Andy had a slight-to-moderate rise in his
T4 and was showing clinical signs of hyperthyroidism - weight loss and
agitation/restlessness - especially at night. I took him
to our local imaging clinic and had him scanned. Right arrow =
normal; Left arrow = thyroid adenoma.
Andy was admitted to the clinic on Sunday.
He was scanned that day and received his I-131 injection on Monday.
He was able to come home on Thursday.
Was he miserable? Yep. Was I stressed and
anxious? You bet. But now I have my Andy back. He has
gained 1.5 lbs and, at 16 years of age, is doing great.
Many people worry about the hospital stay
and the stress that it will cause for the cat. Yes, it is
stressful but the benefits far outweigh the difficult few days and it
sure beats having to give a pill (methimazole) twice daily, for the rest
of the cat's life, which is a
medication that can have significant side effects and does not address
the tumor. More on this drug below.
carbimazole is a newer drug that is
rarely used and not readily available in the United States.
These drugs are known as 'anti-thyroid'
medications since they interfere with the thyroid gland's production of
I will say at the outset of this section
on anti-thyroid drugs that I am not a fan of putting any chemical into
my own body, or that of my own cats or patients. For every
positive action that a drug exerts on a living being, there are plenty
of side effects that are very often overlooked by the doctors
prescribing the medications.
There are many cats that do well on these
drugs but side effects are not uncommon and include nausea,
vomiting, and severe allergic/itchy facial skin reactions and, less
frequently, life-threatening liver dysfunction, bone marrow disease, and
While these drugs effectively lower the thyroid
hormones and ameliorate the clinical signs of hyperthyroidism, they
do not address the underlying tumor and, therefore, the tumor
will continue to grow with the risk that it
will become a malignant cancer over time.
As the tumor continues to grow, the
dosage will often need to be raised which means that the patient must be
monitored with periodic blood tests for life. The frequency will
depend on the patient but as we all know, trips to the veterinarian are
expensive and very stressful for the patient.
Other health factors may necessitate
lowering the dosage. Therefore, lab monitoring is very important
for these patients.
If methimazole is chosen over I-131, I strongly prefer
the transdermal preparation which is a cream that is applied to the ear.
This formulation results in less gastrointestinal (vomiting/diarrhea)
side effects when compared to the pill form.
Plus, it is a much more humane and less
stressful way to administer the medication.
If the pill form is used, please see my
article and never, ever 'dry pill' any medication for any cat or dog.
All pills or capsules need to be 'chased' with food or at least 4 mls of water
via a syringe.
Or, better yet, use Pill Pockets as
discussed on the Pilling Cats page.
At first glance, I-131 may appear to be a
more expensive treatment but in many cases it turns out to be less
expensive when compared to lifelong treatment with methimazole when the cost of the
medication and the continued monitoring are considered.
Also, the hassle of twice-daily medication
must be considered.
In the end, however, I-131 is closer to
being a true cure for this disease than methimazole since it
addresses the underlying tumor.
Note that if your cat has been on
methimazole and you now want to have him treated with I-131, you may have to
withdraw the methimazole for ~1-2 weeks prior to I-131 treatment.
For more information on these drugs,
please see Dr. Mark Peterson's discussion
to remove the thyroid adenoma has fallen out of favor since I-131
became available. This is true for several reasons:
1) It is impossible to differentiate normal tissue
from tumor tissue. If both lobes of the gland
are involved and removed, this will result in hypothyroidism
necessitating lifelong treatment with thyroxine.
2) The parathyroid gland which sits right
next to the thyroid gland can be damaged or accidentally removed. The parathyroid gland is
critical for calcium balance in the body and if damaged or removed, serious
complications, including death, can result.
3) General anesthesia is necessary
which is always a risk but even more so in an older patient with
hyperthyroidism considering how many organs are negatively affected by
this disease. Severe heart disease can be present in these patients
especially if they have been hyperthyroid for a long time. Any
level of heart
disease will increase the risk associated with anesthesia. For
this reason, if surgery is the treatment of choice, it is recommended to
stabilize the patient with anti-thyroid medication prior to the surgery.
4) Some hyperthyroid cats have
ectopic (not in the normal location) thyroid adenoma or carcinoma tissue located in the chest cavity
which the surgeon will not be able to remove. Plus, the surgeon
will not even be aware of its presence unless a thyroid scan was
here for a more comprehensive chart comparing the pros and cons of
each of the 3 treatments discussed above.
This iodine-deficient diet is the newest
addition to the Hill's 'alphabet prescription diet' product line and it is targeted at the feline
hyperthyroid patient. As stated above, representatives of the
Hill's company have made false and misleading statements about this
diet that will be addressed
y/d is formulated to be deficient in
iodine with the simple goal of providing less 'building material' for
the patient's thyroid tumor and normal glandular tissue to make thyroid
hormones (T3 and T4). Iodine is an important component of these hormones
withT3 needing 3 iodine atoms and T4 containing 4 iodine atoms.
The premise of this diet is simply to
starve the thyroid gland of iodine but here is where the issue of
'tunnel vision' nutrition comes into
the thyroid gland of iodine, the entire body - including the thyroid
gland itself - is also negatively affected by
If anybody reading this has not read
least the first part of my Feeding
Your Cat: Know the Basics of Feline Nutrition article, I will
briefly summarize it here because, before reading my comments about y/d, it is important to understand what
constitutes a healthy diet for a cat.
In a nutshell, the
cat is an obligate carnivore which means:
They are designed by nature to get
their protein from other animals (meat), not plants (grains/potatoes/other
They lack the enzymes necessary to
efficiently process carbohydrates.
They have a low thirst drive
and are designed to get their water with their food. This
means that there is no dry food that is suitable to feed to a cat.
Their normal prey is ~70% water and in
times of drought, they could actually survive on the moisture in their prey.
Canned foods are ~78% water and dry foods are only ~8-10% water.
Several studies have shown that the water
intake and urine output of cats fed canned food are double that of dry
food-fed cats. This is taking into consideration all sources of water
from the food and the water bowl.
Please understand that a cat on a dry food
will drink much more water from a bowl than a canned food-fed cat but he
will not make up the deficit caused by the feeding of a water-depleted
diet of dry food.
Considering that water is one of the most
important nutrients for every living creature, it is easy to understand
why it is much more physiologically sound to feed canned food diets to
cats rather than dry food.
See Urinary Tract Health for some graphic pictures of what happens to
cats like Opie when fed a water-depleted diet.
Summary statement regarding y/d:
I would only use this diet if:
I-131 was not available or the client could not afford
the treatment, or
the patient was not
a good candidate for surgery, or
the patient had shown an
intolerance to transdermal methimazole.
In other words, I see y/d as a
last resort and certainly not a first-line treatment.
y/d leaves the whole body in a
state of iodine deficiency. This is an excellent example of
tunnel vision nutrition - i.e.- only paying attention to the thyroid
gland, in this case, and not the rest of the body. Safety
studies have not been conducted to assess the long-term damage to
the body when left in an iodine-deficient state.
Most of our knowledge of iodine centers
around the thyroid gland but there is strong evidence pointing to iodine
as playing an important role in immune health, adrenal function, breast
health, and also having antioxidant properties.
y/d does not address the disease
process (tumor) that is causing the hyperthyroidism and the
tumor will continue to grow. Therefore, it does not
"restore thyroid health" as claimed by Hill's.
The longer the tumor is allowed to grow,
the higher the chance of it becoming a malignant carcinoma.
y/d is too low in protein
for many cats
especially considering the muscle-wasting state that hyperthyroid
cats are already in. (See Dr. Peterson's article
y/d is too high in carbohydrates.
(See Diabetes) Note that many
hyperthyroid cats have sub-clinical diabetes. Feeding them a
high carbohydrate diet could very well tip them over the edge into
canned y/d: 24% carbohydrate calories
dry y/d: 23% carbohydrate calories
mouse: 1-2% carbohydrate calories
dry y/d has an extremely unhealthy
ingredient list for an obligate carnivore and it is
water-depleted. (See Urinary Tract
Corn Gluten Meal, Animal Fat (preserved
with mixed tocopherols and citric acid), Whole Grain Corn, Soybean Mill
Run, Dried Egg Product, L-Lysine, Chicken Liver Flavor,.....
Please read the
above ingredient list with your cat's obligate carnivore status in
mind. Also understand that your hyperthyroid cat needs a high
amount of high quality protein since the disease is causing
his body to 'eat' (catabolize) his own muscle mass.
Corn is not a "high quality"
protein for a carnivore.
I would never feed a diet like dry y/d to any
cat in my care. It is a more appropriate diet for a horse or a cow to be
eating. Please note that there is no meat
in this food. The only animal-based protein is the dried egg
product which is lower on the list than even the fiber source so there
is next-to-nothing in the way of animal protein in this diet.
y/d canned has a better ingredient
list than the dry and is at least a properly hydrated diet but note
that liver is the first ingredient after water. Liver is high
in vitamin A and a diet of predominantly liver does not represent a species-appropriate
balance of nutrients. However, note that liver is cheap and provides a
higher profit margin for the company.
Water, Liver, Meat by-products, Chicken,
Whole Grain Corn, Rice Flour, Chicken Fat (preserved with mixed
tocopherols and citric acid), Chicken Liver Flavor, Powdered
See Dr. Peterson's article:
Is Hill's y/d a Nutritious Diet for Hyperthyroid Cats?
If you choose to feed this diet, please
feed the canned version and not the dry.
The patient cannot be allowed to
eat any other food which means no treats of any kind. He
must eat only y/d which can pose a problem in multiple-cat
households or if he suddenly stops eating the food.
This diet cannot be fed to
non-hyperthyroid cats. Hill's states that it can be fed to
normal cats if these cats are also fed some regular food.
However, I would not subject your healthy cat to this diet with the
hope that you feed enough food from another source to meet his
iodine needs - especially in light of the fact that there are diets
on the market that are very low in iodine - too low to furnish
enough iodine to make up for the bulk of the diet being
In my strong opinion, Hill's is being
very irresponsible when making the recommendation to:
"supplement (y/d) with 1 tablespoon of regular cat food (dry or canned)
each day to provide additional iodine". Given the wide range of
iodine in commercial cat food, this recommendation will not ensure
adequate iodine in every healthy cat that may be subjected to y/d as the
bulk of their diet.
There is compelling scientific
evidence to show that feeding an iodine-deficient diet can
actually make the hyperthyroid patient worse over time.
When there is not enough iodine in the
body, thyroid hormones decrease. This signals the pituitary gland to
secrete TSH (thyroid stimulating hormone). TSH's action on the thyroid
gland causes it to grow larger (hypertrophy/goiter).
Thyroid Response to Low Intake of
Deficient dietary iodine intake
causes the thyroid gland to make less T4
Plasma T4 levels drop
This results in increased TSH release from the
leading to increased iodide trapping in the
thyroid gland to maintain T3 secretion
resulting in thyroid hypertrophy due to
increased TSH release = GOITER
If you are interested in learning more
about the biochemistry of iodine deficiency and thyroid disease, see Dr.
Does Iodine Deficiency Cause Thyroid Disease in Cats?
Reminder: y/d is an iodine-deficient diet.
Here is an excerpt from Dr. Peterson's article:
"So, based on the trend to lower iodine
levels in cat food over the last two decades, could iodine deficiency be
contributing to the explosion in hyperthyroid cases that we are seeing
In support of that
reasoning, a recent case-control study reported that cats consuming
commercial foods which were relatively deficient in iodine were more
than 4 times as likely to develop hyperthyroidism compared with cats
that ate iodine-supplemented foods."
...and another excerpt under the heading:
"Could y/d make this situation worse?
.....will lowering T4 and T3 secretion
in these hyperthyroid cats lead to increases in circulating TSH and
continued stimulation of thyroid growth and proliferation in some cats?
Will this TSH stimulation cause transformation of benign adenomatous
thyroid tissue to carcinoma, as has been documented to occur in some
The short answer is this: we simply do not know. The long-term safety
studies needed to answer this question have not yet been done.
Because Hill's y/d is not a drug
(although it's certainly being marketed as a replacement for methimazole),
the company is not required to do the these safety studies, and it's
fairly clear that they have no intension of paying to have them done."
Also note this excerpt from an article
that appeared in the Journal of Feline Medicine and Surgery (2010)
entitled Feline Hyperthyroidism: Potential relationship with iodine
supplement requirements of commercial cat foods:
hyperthyroidism resembles toxic nodular goiter (TNG) of humans with
discrete foci of benign adenomatous hyperplasia. The etiology
of TNG includes iodine deficiency and consumption of or exposure to
goitrogens in food, water, and the environment."
The article goes on to discuss evidence
that the current NRC dietary minimum iodine requirement of 1.4 ppm
may be too low and may be contributing to feline hyperthyroidism.
Note that y/d is a very iodine-deficient
diet at 0.2 ppm (range 0.1 - 0.3 ppm) and we know that iodine-deficient
diets cause goiter.
being truthful in their marketing claims? Are they considering
what is best for your hyperthyroid cat?
You be the judge.
Hill's states that this diet "restores
thyroid health". After reading the information above, I hope
that you can see that this is a blatantly false statement.
Hyperthyroidism is caused by a benign
thyroid gland tumor. Feeding a diet that is deficient in iodine
is not halting the growth of the tumor.
In addition, it has been common knowledge
for many years that diets deficient in iodine cause goiter (thyroid
The tumor will keep growing unless
addressed with I-131 or surgery. This continued growth will
only enhance the chances of the benign tumor becoming cancerous.
Therefore, it should be very clear that
this diet does not "restore thyroid health".
Hill's is calling this an
"iodine-restricted" diet and claiming that it is "not
iodine-deficient" which is in direct contradiction to their own
study as well as studies of
other researchers who have investigated the dietary iodine needs of the
To illustrate why their statement is
There are studies showing that the minimum
iodine requirement in cat food should be 1.4 ppm. In fact, the current
NRC requirement states a minimum of 1.4 ppm.
A Hill's study using a corn-based diet
stated that the minimum iodine requirement was 0.46 ppm.
y/d is 0.2 ppm (range 0.1 - 0.3 ppm)
How can Hill's perform a
study coming up with 0.46 ppm as a minimum requirement and then state
that a diet with less than half that amount is "not iodine-deficient"?
Here is what Dr. Mark Peterson has to say
about this issue in his blog entry entitled
Cats with an Iodine Deficient Diet (Hill's y/d): Does It Really Work?
"Hill's y/d is clearly
an iodine deficient diet, containing levels of approximately 0.2 mg/kg
(0.2 ppm) on a dry matter basis, well below the minimum daily
requirement for adult cats (0.46 mg/kg or 0.46 ppm) of food. It's
important to note that the study that established this minimum daily
requirement for cats was done by investigators funded by Hill's Pet
Nutrition. So, claims that this is just a low iodine diet, but not one
deficient in iodine is nonsense, unless one does not believe the results
of that study."
An important issue regarding the
Hill's iodine study is that they fed the test cats a corn-based diet.
No nutrient is an island unto itself. All nutrients are influenced
by other nutrients in the diet. Therefore, it only makes sense to
feed a cat a species-appropriate diet (which corn is not) when
trying to figure out the optimal level of any nutrient in the diet.
Since Hill's is known for manufacturing
feline diets with high levels of species-inappropriate ingredients to
enhance their profit margin, it was not the least bit surprising to
learn that their iodine study involved feeding cats a corn-based diet.
Did this skew the results? We have
no way of knowing but, again, if you are going to conduct any
nutrient study, common sense dictates that you at least feed the test
subjects a species-appropriate diet which, in this case, would be meat -
This statement is appearing in our
"Managing hyperthyroidism is now as easy
as feeding your cat," says Dru Forrester, DVM, MS, Dipl. ACVIM, director
of Scientific & Technical Communication for Hill's Pet Nutrition.
This irresponsible and misleading marketing is going result in far fewer cats receiving the gold standard
treatment of I-131.
It is natural for all of us to want to
take the easy (and cheaper) way out and this inherent desire is exactly
what the Hill's marketing team is focusing on as is evident by Dr.
Forrester's statement above. However, 'easy' is not always
Comments on treating
patients that also have CKD (chronic kidney disease):
Since hyperthyroidism and kidney disease
are common problems in older cats, it should come as no surprise that
many hyperthyroid patients also have concurrent kidney disease. It
is estimated that 30-35% of all hyperthyroid cats also have some level
of kidney disease.
However, the increased metabolic rate of
the hyperthyroid patient can make the kidneys looking better 'on paper.' This is because
an elevated metabolic rate causes an increase in
blood flow/increase in pressure through the kidney which makes it filter
out waste products (e.g., BUN and creatinine) more efficiently.
When the patient's T4 level is returned to
normal - regardless of treatment choice - underlying kidney
disease may be unmasked.
It is very important
to understand that treating the hyperthyroid state did not cause
In addition to the increased blood flow
making the patient's lab values look better than they might end up being
post-treatment for hyperthyroidism, hyperthyroid cats have less
muscle mass than healthy cats and since creatinine comes from internal
muscle breakdown (a normal daily occurrence), a skinny kitty is going to
generate less creatinine - making his kidneys look better than they
Once the treated hyperthyroid cat starts
to regain his muscle mass, the creatinine value may rise - leading the
clinician to erroneously assume that the kidneys have worsened.
Again, the underlying kidney disease is
being masked by the hyperthyroidism but it is critical to understand
that treating a cat for hyperthyroidism does not actually
cause kidney disease.
Quite the contrary.
Recent research has shown that the
negative effects (including the increase in blood pressure at the kidney
level) of an increased metabolic rate may very well contribute
to the development and progression of kidney disease in the untreated
While it may be tempting to leave a
hyperthyroid cat either untreated or undertreated with very low levels
of anti-thyroid medication, this can have a negative effect on kidney
health, as well as heart health.
Many veterinarians shy away from I-131 or surgery versus anti-thyroid medication and Hill's y/d
the first two are permanent and the last two are temporary. Their
argument is that the
medication dosage can be lowered or discontinued and the diet can be
changed if they don't like how the kidney values look once the T4 is
lowered to the normal range.
But the idea of lowering the dosage of an
anti-thyroid medication or altering the diet to let the patient 'run
hot' so that the
kidneys look better is not without its very unhealthy consequences.
Remember that hyperthyroidism is revving
up your cat's 'engine' (metabolic rate) damaging the entire body,
including the kidneys.
The heart is another vital organ that
suffers with this disease since it is basically wearing itself out by its
increased rate of pumping.
I try very hard not to fall into the trap
of 'tunnel vision' medicine. I focus on the whole body and, for
that reason, I will repeat this statement:
I-131 is the only choice I will consider
for my own cats.
If the I-131 unmasked kidney disease that was not apparent when their metabolic
engine was in high gear, I would just deal with the CKD as needed.
Some readers may be familiar with a 'methimazole
(Tapazole) trial' whereby the patient is put on this anti-thyroid
medication to return the patient's T4 level back to normal which often
happens within 2-4 weeks. Once
the T4 level is back to normal, the kidney values (BUN, creatinine,
etc.) are assessed.
This trial is basically showing us what the
kidney values will most likely be after effective I-131 treatment but in
a reversible manner.
Even though this is definitely not the protocol that
I would follow for my own cats - since I am going to treat their
hyperthyroidism with I-131 no matter what their kidney values are - I do
offer it as an option for my clients so that there are no surprises regarding
kidney function after the I-131 treatment.
It is important for some clients to know
what the kidney values may be after the T4 returns to a normal level but
most of them forego the methimazole trial and go right to I-131 when
hearing that this is how I treat my own cats.
I have never heard any client express
regret for skipping the trial.
Note that the experts in this field who I
have spoken with have recently tended toward slightly lower doses of
I-131 for cats with significant kidney disease so that the T4 is
more likely to end up in the upper part of the desired range versus the
lower part. Therefore, please discuss the issue of dosage with the
doctor administering the I-131 to your cat.
It must be understood that it is
impossible to know the exact dosage that will result in your cat's T4
ending up in the higher end of the desired range versus the lower end.
Therefore, there is always a chance that a lower dosage of I-131 may not
be enough to cure the hyperthyroidism and your cat's T4 will still be
too high. In these cases, the treatment will need to be repeated
at additional expense.
Please note that no
medical or nutritional advice will be provided via email. If you
would like to discuss your cat's condition, an appointment for a phone
consultation can be set up but only after your cat's medical records
have been provided for my review.
Updated October 2014
Lisa A. Pierson, DVM
Information on this site is for general informational purposes only
and is provided without warranty or guarantee of any kind. This
site is not intended to replace professional advice from your own
veterinarian and nothing on this site is intended as a medical diagnosis
or treatment. Any questions about your animal's health should be
directed to your veterinarian.