Feline Hyperthyroidism — The Nitty Gritty

by Dr. Jessica Fine, Feline-Exclusive Veterinarian

What are the symptoms of feline hyperthyroidism and what you can do

What is Feline Hyperthyroidism?

Feline Hyperthyroidism is the most common endocrine disorder we see in Senior cats (cats aged 7 and older). It is caused by an overproduction of the hormones produced in the thyroid glands.

Thyroid hormones regulate many normal metabolic processes. You might think of the thyroid glands as the body’s “thermostat;” too little thyroid hormone production and everything slows down to a crawl; too much, and body processes speed up, a little or a lot depending on how elevated the thyroid hormone levels are.

Increased thyroid levels affect every organ in the body, essentially forcing all body systems to run faster than they should. The most common symptoms of feline hyperthyroidism reflect this: eating more, drinking more, increased activity, increased urine output, weight loss (as it becomes impossible for food intake to keep up with the increased rate of degeneration in the body), and often irritability. During the veterinary examination, we often find signs that correlate to these symptoms: increased heart rate, increased respiratory rate, loss of muscle mass, and more.

The ultimate cause of feline hyperthyroidism is still very much a mystery. What we do know is that most hyperthyroidism occurs when benign tumors (thyroid adenomas) develop in one or both thyroid glands. These tumors produce an excess of thyroid hormones. This pushes a kitty’s metabolism into overdrive, causing energy needs and activity levels to go up, and effectively increasing the rate at which the affected kitty ages.

What we don’t know is why these benign tumors appear, although recent findings suggest that hormone-disrupting chemicals that are common ingredients of certain herbicides and other products may be involved.

All cat breeds can develop feline hyperthyroidism. Although it is most commonly seen in middle-aged and older cats, it is becoming more common in cats as young as 18 months.

What and Where is the Thyroid Gland?

The normal feline thyroid gland is very small and consists of two glands, one on either side of the trachea in the neck. It produces two hormones called T3 (triiodothyronine) and T4 (tetraiodothyronine). These two hormones regulate multiple metabolic functions in your cat, such as oxygen consumption, fat metabolism, bone formation and resorption, protein production and more. T3 and T4 hormone production are itself regulated by the pituitary gland, located at the base of the brain, which produces the hormone TSH (thyroid stimulating hormone).

There are several options available for treating feline hyperthyroidism if your cat is diagnosed.

Are Thyroid Tumors Always Benign?

The vast majority of feline thyroid tumors are adenomas, which are indeed benign; they are harmful only in that they cause overproduction of thyroid hormones, not because they are “cancers.” A very tiny percentage of hyperthyroid cats have thyroid adenocarcinomas, which are malignant. Both cause the disorder we call “hyperthyroidism;” While it’s not possible to be 100% sure your cat has the benign form without a surgical biopsy, the odds are very high in your favor.

Symptoms of Feline Hyperthyroidism

The “classic” symptoms of feline hyperthyroidism are caused by an increased metabolic rate, which is a direct result of the overproduction of thyroid hormones. The most common symptoms to watch for at home are:

weight loss
increased appetite
increased drinking
an increased volume of urination (bigger clumps in the litterbox)
increased activity (often, and understandably, misinterpreted as a sign of good health)
vomiting and/or diarrhea
poor/dull hair coat
increased vocalization
irritability
excessive grooming

How Can My Cat Be Checked For Feline Hyperthyroidism?

If your cat is showing any of the symptoms listed above, the first thing to do is to see your feline veterinarian.

Your veterinarian will obtain a history and perform a full physical examination. Since many feline disorders, especially in older cats, can mimic thyroid disease, the information your vet gleans from asking questions and performing an exam is the first step toward a diagnosis.

A definitive diagnosis can only be made through laboratory testing. Your vet will run a panel of laboratory tests, not only to assess the presence or absence of hyperthyroidism but also to look for other possible causes of the symptoms. In addition, there are many disorders that commonly occur concurrently with, or even because of, hyperthyroid disease; these must also be identified in order to understand what the most reasonable approach to your kitty’s problems will be.

A thorough diagnostic profile for this situation will include:

A Complete Blood Count (CBC), to look for anemias, infections, immune system function, and more.
A comprehensive Chemistry Panel to evaluate the health of major organs such as the kidney and liver. This panel also tells us about multiple electrolyte levels, which are often adversely affected by hyperthyroidism as well as a host of other common disorders.
A Urinalysis is absolutely necessary for an accurate evaluation of kidney function, but also surprisingly provides a great deal more information on the state of the rest of the body.
A thyroid test is the definitive test for hyperthyroidism and is always part of a diagnostic panel for a cat with suggestive symptoms, or any cat who is both ill with an undiagnosed disorder and over 6 years of age.
A pro-BNP is a relatively new diagnostic blood test, which correlates strongly with stress or stretch of the feline heart muscle. Hyperthyroidism always puts a great deal of stress on the heart and frequently induces the development of actual heart diseases, some of which can be quite severe. While a proBNP is not always a part of a preliminary assessment for thyroid disease, it’s certainly a very useful test for cats whose thyroid levels have been shown to be elevated.

Effective and rational patient management for feline hyperthyroidism requires all of the information provided by the comprehensive diagnostic panel discussed here. We must evaluate the entire cat; otherwise, we are likely to remain unaware of other disease processes going on at the same time. The presence of multiple disease processes is far more the rule than the exception for cats with hyperthyroidism, and many of those common concurrent problems can completely sabotage any chance of therapeutic success unless we’re aware of them and addressing them also.

On occasion, cats who in fact are hyperthyroid but also have other diseases will have test results that show normal thyroid hormone levels. This is called “Sick Euthyroid Syndrome.” It happens because the non-thyroid disease often has the effect of decreasing thyroid production. This syndrome can make a sick cat with a normal thyroid have a low thyroid test result; it can also make a slightly hyperthyroid cat with a second illness appear to have a normal thyroid. More sophisticated thyroid testing is necessary for these cats to determine if they have feline hyperthyroidism.

How Will My Cat Be Treated For Hyperthyroidism?

There are several options available for treating feline hyperthyroidism if your cat is diagnosed. Your veterinarian should discuss all the options available, and help you choose which would be right for your individual situation. Not all options are equally good for all hyperthyroid cats; the presence of other disorders, especially those of the kidney or heart, can strongly influence what the most reasonable course is likely to be.

The “Big Three” best options for treatment of feline hyperthyroidism are:

#1 – Medical Management. This is usually done with a drug called methimazole, an antithyroid agent which decreases the thyroid gland’s hormone production. Methimazole is available orally in many different forms or can be made into a topical gel preparation called a ‘transdermal’. Other antithyroid drugs exist, but methimazole has long been recognized as the safest and it is effective. It is, however, a control mechanism, not a cure.

Methimazole is nearly always the first treatment for any newly diagnosed hyperthyroid cat. Methimazole helps us get the problem under control immediately, reducing or removing the ongoing system-wide damaging effects of the disease. Re-evaluation after a short period of methimazole treatment also gives us the opportunity to assess whether our patient is eligible for the more curative approach of I-131 treatment.

Cats receiving methimazole as long-term therapy require periodic laboratory monitoring (on average, every 3-6 months, varying by patient) to ensure that the disease is controlled, and to monitor for side-effects of the drug. Neither the monitoring nor the medication is inexpensive, which can render this choice more expensive over time than the other options.

Rarely, a cat will become allergic to methimazole. This is an immediate contraindication for further therapy; a different approach must be taken for such cats.

#2 – Radioactive Iodine Therapy. Also called I-131, this option is nearly always curative, making it the treatment of choice for any eligible cats. I-131 is safe, effective, and non-invasive. Additional treatment is only very rarely required.

If your cat has other concurrent disorders, especially of the kidney or heart, he or she may not be a good candidate for this procedure. The cost of I-131 treatment should also be taken into consideration, although for many cats, the cost over time for medical management can be far higher than for I-131. Your feline vet will work with you to make the most reasonable decision for you and your cat.

By far the biggest concern for cats undergoing I-131 treatment is the unmasking of kidney disease. The mechanisms of feline hyperthyroidism can cause or worsen kidney disease, and ironically can also hide if from laboratory evaluation. This is why cats being considered for I-131 therapy must be treated with methimazole first: to normalize thyroid levels and remove those effects. After a short period of treatment, kidney function can be accurately assessed. This step is required by most I-131 providers, because cats with kidney disease past a certain point may go into acute kidney failure as a result of rapid post-I-131 decompensation. Hyperthyroid cats with kidney disease beyond a certain level are not generally accepted for I-131 treatment.

#3 – Surgery. When other options are unavailable, contraindicated, or not tolerated by your cat, surgical removal of thyroid tissue (thyroidectomy) may be an option.

Thyroidectomy requires anesthesia, which is not necessary with the first two approaches. Hyperthyroid cats are at higher risk under anesthesia than the general population, specifically because of the effects of hyperthyroidism on other organs, especially the kidneys and heart. For this reason, cats often need to be treated with antithyroid drugs prior to anesthesia to normalize their thyroid levels and optimize anesthetic safety. Also, since hyperthyroidism can cause serious heart problems not otherwise identifiable, an echocardiogram is recommended for all hyperthyroid cats, but especially for any hyperthyroid cat for whom anesthesia is contemplated.

Thyroidectomy can involve the removal of one or both thyroid glands or even just a part of one. All three surgical variations carry different risks and potential benefits. Many practitioners perform “staged” thyroidectomies, removing one gland, then waiting several weeks before doing the second procedure to remove the other. This has the disadvantage of two anesthetic procedures, but often avoids rapid decompensation issues similar to those that can occur after I-131 therapy, and can also avoid the danger of acute and potentially fatal hypoparathyroidism caused by the simultaneous removal of both parathyroid glands, which are tiny and embedded in one end of each thyroid gland.

Thyroidectomy allows for the possibility of a complete cure but carries no guarantee. Failure to cure can happen for a number of reasons, many of which are out of control of the surgeon, such as abnormal thyroid anatomy, or extra (ectopic) thyroid tissue present at the time of the surgery or appearing afterward. This latter problem also occurs occasionally following I-131 therapy, but can usually be dealt with by a second round of I-131.

Currently, there is no scientific evidence to support the idea that the treatment of hyperthyroidism causes kidney disease in cats.

“Prescription” Anti-Thyroid Diet. This diet is an attempt to decrease thyroid production by feeding an intentionally iodine-deficient diet. Iodine is a necessary ingredient for the production of thyroid hormone. However, the long-term effects of iodine-deficient diets have not, in our opinion, been sufficiently studied, and more than one veterinary endocrinologist opposes this approach.

A second reason we do not favor this mode of therapy is that the available “prescription” dietary products are not, in our judgment, biologically appropriate for cats. The dry version’s first ingredient is corn gluten meal, and the canned version, while marginally better, still contains cornflour and rice sufficient to bring the fiber content up to 3%. If you’ve read any of our nutrition articles, you already know these ingredients and fiber levels (and dry foods, period!) are not what we think is healthy for cats, and we see little reason to potentially induce disease with such a diet when other, safer methods of treatment are available.

That said, this diet does give us an option to consider if all the others fail. The diet is recommended by the manufacturer to be fed exclusively with no other treats or foods. Since this mode of therapy is not curative, the same periodic laboratory monitoring is necessary to ensure that the treatment is working.

What Kind of Ongoing Care/Monitoring Will My Cat Need Post Treatment?

Regardless of the treatment option, you choose for your cat, periodic physical examination, blood pressure measurements, and laboratory monitoring will be recommended. The recommended frequency of this monitoring, as well as the specific tests that comprise that monitoring, will vary based on the needs of an individual patient.

If your cat also has a heart problem, whether independent of or caused by feline hyperthyroidism, regular echocardiograms may be indicated.

We recommend that all Senior cats (age 7 and up) have an examination, blood pressure, and general Senior screening labs at least twice a year. This frequency may also be sufficient for many hyperthyroid cats post I-131, post-surgery, or those on methimazole who have demonstrated long-term stability. Kitties new to methimazole therapy, or those with other concurrent problems, may require more frequent monitoring. Your veterinarian will work with you to set a monitoring schedule appropriate for your cat’s individual situation.

Even if your kitty has had I-131 therapy or thyroidectomy, it is possible for new thyroid tissue to develop. Periodic monitoring of these patients still includes thyroid levels, to ensure that thyroid tissue hasn’t regrown.

We also strongly recommend that you purchase a baby scale, and weigh your cat weekly at home. Many common feline disorders, including hyperthyroidism, are characterized in part by a long, slow weight loss. Living with a cat and seeing him every day can make it very difficult for us, as cat guardians, to recognize that this is happening. A scale and a spreadsheet can help you identify a weight loss trend long before you otherwise might, and this is most important with older cats since they are at higher risk for so many diseases that behave this way.

Busting the Myth of Feline Hyperthyroid Treatment – It Does Not Cause Kidney Failure

As with most diseases, there are myths that become so pervasive that we take them at face value without checking into the facts. One such myth is the idea that treating feline hyperthyroidism causes kidney disease. Let’s do a little myth-busting to set that record straight.

Currently, there is no scientific evidence to support the idea that the treatment of hyperthyroidism causes kidney disease in cats. Rather, treatment of hyperthyroidism can unmask kidney disease that is already present. Further, feline hyperthyroidism itself (especially via the high blood pressure which hyperthyroidism often induces) can cause or worsen kidney disease.

The state of the hyperthyroid cat is one of a metabolism on overdrive. When this occurs there is an increase in blood flow to the kidneys. This creates a “wash-out” effect, which is what makes kidney function blood tests misleading in uncontrolled hyperthyroid cats. Once the hyperthyroidism is treated successfully, that blood flow inevitably decreases, removing the washout effect and rendering kidney function blood tests more accurate — and of course, elevated over pre-treatment levels. The kidney disease was already there; it’s just that now it’s no longer hidden.

As with most myths, this one contains a grain of truth. It is true that when kidney disease is unmasked, the symptoms of it can become apparent where they were not before. It is true that overly sudden normalization of thyroid levels can induce acute kidney failure, especially in a cat who’s been hyperthyroid but untreated for a long time, or has particularly high thyroid levels. It is also still regrettably true that most cats diagnosed with hyperthyroidism have had it for a long time prior to diagnosis and treatment, which makes these cats more susceptible to such problems. This is a big reason why we advocate for periodic thyroid screening for all older cats — it allows us to find and correct thyroid problems before we’re in this sort of no-win situation. Finally, it is true that a large percentage of hyperthyroid cats also have kidney disease, since both independently are common in older cats. All of this sets up a situation that can easily make it appear that thyroid treatment “caused” a kidney problem, but that is simply not true.

The Latest On Feline Hyperthyroidism Research

For all of you who like to know when some possible light is shed on a particular disease, here’s something to tickle your curiosity.

An article published in the New York Times, based on current ongoing veterinary research, presented some interesting possibilities as to why hyperthyroidism began showing up in the feline population in the late ’70’s when it was not recognized (and quite possibly didn’t exist) before that. Although there are no definitive answers, research links this endocrine disease to a common class of flame retardants that became popular in homes at that time. These flame retardants are in a class called PBDEs (polybrominated diphenyl ethers). They were added to many household articles including couch cushions, carpet padding, and electronics. These PBDEs have a molecular structure that closely resembles that of thyroid hormones, and may mimic or compete with these hormones in cat’s bodies. Similar observations have been made about other chemicals used commonly in our environment, including at least one popular herbicide introduced about the same time as the PBDEs.

Perhaps the most disturbing implication of this research is this: once deployed, these chemicals stay in the environment for very long time periods. The phenomenon of feline hyperthyroidism may be the “canary in the coal mine” for a future similar pandemic in humans.

It is worth reading this article if you are interested in how polluting our environment with man-made chemicals and toxins can interfere with human and animal metabolic processes.

You can find the article on the New York Times site, here.