Hyperthyroidism In Your Cat
Its Cause And Its Treatment

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Ron Hines DVM PhD.............

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What Is Hyperthyroidism ?

Your cat’s thyroid glands regulate the speed at which your cat’s body metabolism works – much like the accelerator on your vehicle regulates the speed of your car. It does this by producing a hormone called thyroxine or T- 4 that regulates the speed of all body processes. When your cat produces too much of it and its metabolic rate sores, it has become hyperthyroid. Hypothyroidism (low T-4) is quite uncommon in cats. When it does occur, it is usually in a kitten that was born a dwarf. (ref)

Hyperthyroidism is the most common hormone abnormality in cats. It is very rare in dogs. It is a disease of older cats. The average age at which it is first diagnosed is 8-13. Nine out of ten cats that develop hyperthyroidism are over ten years old.

The thyroid is actually a pair of glands in cats. In humans, it is a united two-lobe gland. They are located on the underside of your cat’s neck along its wind pipe (trachea).

Is This A Form of Thyroid Cancer ?

It very rarely is. Less than three percent of the cats that develop hyperthyroidism develop malignant thyroid tumors. In over 98%, the cells in the whole gland, or portions of it, are just producing too much thyroid hormone. The scientific name for what is occurring is "functional thyroid adenomatous hyperplasia".

In about eight out of ten cats with this problem, both thyroid glands are affected.

What Kind Of Cats Develop This Problem ?

Hyperthyroidism is typically a disease of older cats. It can occur at an earlier age - but that is quite rare. We think of pampered cats when we think of this disease. But it could well be that pampered cats just get more frequent veterinary examinations.

Although we traditionally thought that it affected males and females equally often, it may be considerably more common in female cats (ref) , just as hyperthyroidism in humans is more common in women. (ref)

A Bit About Your Cat's Thyroid Gland

Your cat has two distinct thyroid glands on either side of its windpipe midway down its neck (see top right image). In humans, it is a single gland with a left and right lobe. This gland is responsible for regulating the speed of all chemical reactions that occur in your pet's body. This is called your pet's basal metabolic rate. They thyroid gland produced a hormone that it sends to every cell in the body through the bloodstream. This hormone is called thyroxine. The more thyroxine the thyroid gland produces, the higher your cat's metabolic rate and the more calories it burns.

How Does The Thyroid Work ?

When the hormone is first produced by the thyroid, most of it is in a form called T-4 or levo-thyroixine. Before this form can work, it must be converted to T-3 (triiodothyroinine) which is the form that the cat's body cells can recognize. Most of this is done in the liver. (ref)

What Signs Would I See If My Cat Is Hyperthyroid?

When your cat's thyroid glands are over-producing thyroxine hormone, every organ in its body is affected. The pet's kidneys, liver, muscles, heart, nervous and digestive system are all over-stimulated. This leads to a number of physical changes you can see. Rarely does any one cat show all of the listed signs we associate with hyperthyroidism. The signs that do occur all begin very slowly. As time passes, they gradually become more severe.

Weight Loss

The most common complaint that takes hyperthyroid cats to the vet is weight loss. These cats remind me of the pink panther - they are lean in the extreme. Perceptive owners notice that although their cats are losing weight, their appetite is normal or increased. This is because the pet's metabolic rate has accelerated and it is using up food calories just as fast as it can consume them.

Increased appetite

Most hyperthyroid cats are eating more to meet their increased need for calories. You will hear them munching more and complaining when their food dish is empty. However, when they have reached the late stages of this disease, their general health deteriorates to the point that they don't have much appetite.

Occasional cats have a form of this disease called apathetic or masked hyperthyroidism in which they appear listless and apathetic (disinterested). Those cats may have less of an appetite than they once had. Many of these are late cases or cats with other coexisting illnesses.

Increased activity and restlessness

Many hyperthyroid cats are "wired" as if they were taking stimulants. They are overly restless or hyperactive, and they may be more cranky and aggressive. Some have disturbed s sleep patterns.

Poor hair coat

Many hyperthyroid cats appear unkempt. Some no longer groom themselves the way they used to while others over-groom themselves to the point where their hair coat is thin or ragged.

Fast heart rate

It is very common for hyperthyroid cats to have an abnormally fast heart beat. Your cat's normal relaxed heart rate at home should be 140 to 200 beats per minute. It will often be faster at the animal hospital due to fear. Many cats with hyperthyroidism have heart rates of over 200 even when they are relaxed at home.

Increased drinking and Increased Urination

This is also common occurrence in hyperthyroidism. Your cat's increased thirst is due to the increased thyroxine in its system. It's increased urine production is due to its increased water intake.

Vomiting

We do not know why some cats with hyperthyroidism vomit. It occurs in hyperthyroid humans as well. (ref) Perhaps it is due to the increased amounts of food they eat, or perhaps to the direct effects of their high thyroxine levels on stomach motility and portions of the brain.

Diarrhea

The increased level of thyroid hormone in hyperthyroid cats causes their intestines to be more active. This is why many of these cats have bulky or loose stools. The odor of your cat's litter box may be considerably worse than it used to be.

Panting or Difficulty breathing

Cats that are hyperthyroid generate more body heat and may pant as they try to dissipate it. They are more sensitive to heat than they once were. If they get to a point in the disease where their heart is weakened, panting and difficulty breathing is more likely due to problems getting enough oxygen.

Weakness and Listlessness

In later stages of hyperthyroidism multiple factors often cause cats become debilitated and weak. Muscle tremors, wasting, an anemic meow and generalized weakness can all be symptoms of advanced hyperthyroidism. Supplemental vitamin B-6 has helped somewhat with this problem in humans. (ref)

Low Grade Fever

The high metabolic rate of hyperthyroid cats sometimes causes them to have a
mildly elevated rectal temperature (103 F, 39.4 C). But a rectal temperature of 103 can easily occur during a visit to your veterinarian simply due to the stress of the visit.

Lumps and nodules in the neck in the area of the thyroid glands

In healthy cats, the lobes of the thyroid gland cannot be felt with one’s fingers when you examine your cats neck. In hyperthyroid cat at least one lobe is often larger than it should be and can be felt. You or your veterinarian may be able to detect this or small, pea-sized nodules, within the glands. Many older cats do have lumps in their thyroid glands but not all of them are hyperthyroid - yet. If your veterinarian detects any mass in the thyroid area, it is prudent to run the T-4 test and a blood calcium level. Some of these masses turn out to be located in the parathyroid glands that are adjacent to the thyroids. The parathyroid glands are involved in regulation of body calcium.

What Tests Will Tell My Veterinarian If My Cat Is Hyperthyroid ?

If your veterinarian is suspicious that your cat might have hyperthyroidism, he/she will order a T-4 blood test. If T-4 levels in your cat are markedly higher than they should be for its age group, the diagnosis has been made.

Sometimes, however, the T-4 levels are on the borderline between normal and abnormal. In those cases, when initial thyroid tests are normal but signs are very typical of hyperthyroidism, your vet may order a more sophisticated test called a T-3 suppression test. This test requires giving your cat a tablet three times a day for two days before the blood test. Positive results with this test will identify many hyperthyroid cats that are missed when only the T-4 test is run. Hyperthyroidism has become so common in older cats that many veterinary hospitals routinely run yearly T-4 tests on all older (8yrs+) cats.

There is still a sizable group of hyperthyroid cats that we have trouble identifying with either of these two tests. The cat's level of T-4 and T-3 fluctuate during the day and sometimes the sample is taken when they are lower and in the normal range. Repeated T-4/T-3 tests usually identify these cats. Other diseases can also artificially lower T-4 levels.

If the results of both tests are still inconclusive but the results of other tests indicate liver, heart, kidney or muscle problems, or if no other disease is found that could account for the symptoms your pet is having, a thyroid scan needs to be performed. In this test, your cat is given an injection of a small amount of a radioactive element called technetium. Then an image similar to an x-ray is taken to find out where the material traveled. It has an affinity for glands and tends to concentrate there. (ref) If more is seen in the thyroid image than there should be, the cat is hyperthyroid. The test allows the radiologist to actually see your cat's thyroid gland and locate areas of problem within it. This can pick up the few cats with malignant thyroid tumors or those that might be cured surgically. It will also identify the few cats that have misplaced thyroid tissue somewhere else in their bodies.

The technetium test requires equipment that most veterinarians do not have. If your veterinarian is suspicious that a borderline high or normal T4 test is due to the pet having another medical problem that masks hyperthyroid tests, your vet may decide to have an Equilibrium Dialysis Free T-4 test run before sending you to a facility large enough to run the technietium test. The Equilibrium Free T-4 test often identifies hyperthyroid cats that are sick with multiple diseases that give a falsely-low Free T-4 reading.

Why Did My Cat Develop Hyperthyroidism ?

We are not certain yet. But here are some things we do know:

Veterinarians did not notice that house cats were developing hyperthyroidism in numbers until after 1979. (ref) Curiously, that was about the time that whales and herring gulls in the St. Laurence Seaway, and cormorants in Tokyo Bay began developing similar thyroid problems. Both these areas are highly polluted with industrial chemicals. There are two chemicals that have been suspected as being the root of this problem. The most recent ones are called Polybrominated diphenyl ethers (PBDEs). Since the 1970s they have widely contaminated our environment. PBDEs are flame retardants used in building materials, electronics, furnishing, motor vehicles, airplanes, plastics, polyurethane foams and textiles. PBDEs are found in particularly high concentrations in fish that are high up the food chain and, therefore, in seafood-flavor cat foods. (ref)

The second possible culprit is a chemical called bisphenol-A (BPA) which is used to coat the inside of cat food cans. We know from a 2000 EPA study that cats that eat canned foods – particularly those that have fish flavor, are more susceptible to hyperthyroidism. (ref) The authors theorized that it might be the BPA can lining that accounted for this - but at the time the article was written, we did not appreciate the link between fish, high PBDE levels and hyperthyroidism.

It may be that, with time, pet foods will be screened for PBDEs and similar toxic substances. But for now, do not feed your cats canned or dry, fish-flavored cat foods. If you feed your cats fish, avoid fish like salmon and whitefish that are known to concentrate this chemical. Not all cats that develop hyperthyroidism eat fish products. The EPA veterinarians who conducted the study pointed out that PBDE's have become so common in our homes that it is impossible to avoid some exposure to them.

It may also turn out ,at it often does, that a combination of environmental exposure and unique sensitivity, rather than a single factor that is involved. Some veterinary scientists have explored the role that iodine levels in cat food might have in contributing to hyperthyroidism and thyroid function (ref 1, ref 2) and even soy protein in cat food has been examined . (ref) You can read more about the possible role of iodine in thyroid disease in cats here.

Why Might My Cat's Kidneys Be Affected By Hyperthyroidism ?

Veterinarians have known for a long time that many hyperthyroid cats have masked kidney problems as well. We do not know if hyperthyroidism is the cause of reduced kidney ability to cleans the body or if this is just a separate disease occurring in older cats. We do know that hyperthyroidism allows these cats to keep there blood flushed clean of waste better. This is because hyperthyroid cats usually drink and urinate more. Their abnormally high thyroid hormone levels and the abnormally high blood pressure that sometimes accompanies hyperthyroidism actually help the pet's kidneys do their work. Once hyperthyroidism is controlled or cured, and the pet's water consumption returns to normal, it's underlying kidney problems often become more apparent - or worse.

If your veterinarian suspects that your cat has significant kidney damage, it is wise to treat the cat with methimazole (Tapazole USA) ,carbimazole (Neomercazole UK,etc) for a while to see how its kidneys perform when its thyroid is under control - before contemplating surgery, radioiodine treatment or life long medication. Starting with a low dose, lower than many veterinarians currently use, and increasing the dose very slowly over many weeks is the best way to approach this. In this ways, the pet's kidneys may have time to adjust. If the cat's BUN and creatinine rise unacceptably high, it is probably better to maintain the cat at some level of hyperthyroidism rather than return its thyroid hormone levels to normal.

Why Might My Cat's Heart Be Affected By Hyperthyroidism ?

Unfortunately, when hyperthyroidism is detected late, a substantial number of cats already have some degree of heart damage. In hyperthyroid cats, the added thyroxine causes their heart to beat faster and more forcefully. With time, the left lower side of the heart compensates by becoming thicker. (ref) The condition is a form of hypertrophic cardiomyopathy. Hyperthyroidism is not the only cause of hypertrophic cardiomyopathy. (ref) The best way to diagnose this problem is with a cardiac ultrasound exam and an x-ray. The electrocardiogram (EKG) of cats with thyroid-related heart problems may also be abnormal.

Left untreated, the cat's heart will eventually fail. Most cats that receive treatment for their thyroid problem return to satisfactory heart function. But when the cat is brought in late in the disease, the pet may need heart support before its thyroid problem can be dealt with. (ref) Another complication that occasionally occurs when the cat's heart is failing are blood clots that affect the animals ability to walk (saddle thrombi).

Does Hyperthyroidism Affect My Cat's Blood Pressure ?

Yes, it is very common for hyperthyroid cats to have elevated blood pressure. It is difficult for veterinarians to obtain both blood pressure numbers as is routinely done in humans. But hyperthyroid cats often have systolic pressure above 190mm Hg even when they are relaxed at home. A number of problems can occur when their blood pressure is that high. Tiny blood vessels in the cat's eyes, kidneys and brain can burst under the increased pressure, damaging the organs. Retinal detachment and blindness sometimes occur in these hypertensive cats. Veterinarians have several medications to reduce high blood pressure in cats when it occurs. Often, these drugs are no longer needed once the cat's thyroid problems have been solved.

What Are The Treatment Options Available For My Cat ?

Hyperthyroidism is not a condition you should ignore. It can be treated medically, surgically or radiologically. I will discuss all of them. However, I suggest you have your cat treated radiologically because cats treated that way do so much better than those treated medically and only a few cats are candidates for surgery.

Medications That Help

Two closely related medications, methimazole and carbimazole are very effective in preventing excessive thyroid hormone production. Methimazole is relatively inexpensive. Unfortunately, every cat requires its own special dose and the difference between too high a dose, too low a dose and the right dose is small. So cats going this rout run the larger expense of frequent thyroid hormone level checks to be sure the dose is working correctly. In most cats, twice-a-day dosing is required, but dose frequency ranges from 1-3 times a day. Your cat is safest if the dose is begun low (lower than some veterinarians recommend) and gradually increased until the desired effect is obtained.

The human-formulated form of methimazole is called Tapazole. The veterinary-formulated form is called Felimazole. Methimazole has an awful taste - cats usually hate it (sometimes vomit). Felimazole tablets have a sugar coating that blocks the drug's taste. So if you cat is doing poorly when given generic or human methimazole, either try the coated veterinary tablets or have methimazole compounded as a transdermal ear cream. The problem with compounding is that there is a wide range in the effects of the cream - depending on who prepared it.

Although both drugs do work, I am not enthusiastic about their use in pill form. Too many owners report that their cats hardly eat when on the medication. Vomiting and lethargy are also common and it is s very difficult to get cats to take this bitter pill. Lowering the dose and then increasing it again ,very gradually, sometimes avoids some of the side effects. These bitter medicines must be given for the rest of your cat's life. After a while, most cats will become very displeased with you. The medications can also have a number of serious side effects which include diarrhea, liver damage and bone marrow suppression. A few cats develop skin irritation while taking either drug.

A newer option that may avoid some of the stress of medicating your cat is to have a compounding pharmacy custom-make the medication into a transdermal gel that you apply to your cat's ear (ref). However, there are dangers to this type of administration. The amount of drug that the cat absorbs through the skin of its ear is variable. There is no standardized method for making the gel. The size of area you apply it to, the vascularity of the ear, the amount of irritation the gel ingredients cause, and the characteristics of the methimazole power the pharmacist used all determine the amount and rate that the methimazole enters your cat's body. (ref) So when you change from tablet to gel, you and your veterinarian need to observe your cat carefully until its Free T-4 readings have been in a steady state for some time.

When side effects do not occur or resolve during the first three or four months of medication, cats generally do well for a number of years on either drug. Some cats receive methimazole prior to thyroid surgery to improve their heart function and make them safer candidates for anesthesia. Others receive it prior to radio-iodine therapy to test the true efficiency of their kidneys.

Another oral medication, ipodate , may prove to be helpful in controlling hyperthyroidism in cats. Unfortunately as of 02/28/06 it was no longer marketed in the United States. A related compound, iopanoic acid , might also have promise. At this time, both treatments would be considered experimental.

A Low Iodine Diet

Iodine is necessary for your cat to produce thyroid hormone. Your cat gets the required iodine from the things it eats. When not enough iodine is present in its diet, it produces less of the hormones involved in hyperthyroidism. Hills Prescription Diets manufactures a a product, y/d, designed to lower your pet's thyroid hormone level by limiting the amount of iodine it consumes. You can read about low iodine diets here.

Procedures That Help

Radioiodine therapy

This is my treatment of choice for hyperthyroid cats. Studies published in 2006 found that cats that were treated with radioactive iodine lived , on average, twice as long (2 vs 4 years) as cats treated with methimazole. (ref) This procedure is safe. But because it involves the handling of radioactive materials, it is only performed at specialized veterinary centers. When the procedure is successful - and it generally is - the pet will need no further treatment or medications.

In this procedure, a single injection of radioactive Iodine-131 is given to your cat subcutaneously or intravenously. Occasionally, a second dose is required. Because iodine migrates to the cat's thyroid gland, the concentration of this special iodine there becomes great enough to kill the overactive cells that utilize iodine to produce thyroid hormone. Usually, enough normal and dormant cells are left to make all the thyroid hormone your cat will need in the future. But about 5% of the cats will need daily, life-long supplemental thyroid hormone in pill form after the procedure.

There have been no serious side effects from this procedure. It is not painful and the cat does not need to be anesthetized. However, the cat will need to stay at the hospital facility until the radiation that is released has subsided (usually 1-2 weeks). (ref) Radio-iodine is also the preferred treatment method for humans who are hyperthyroid. (ref) Occasional cats have misplaced thyroid tissue in other areas of their bodies (ectopic tissue). This treatment eliminates them as a potential problem as well.

How Much Will Iodine Treatment Cost ?

Many cat owners are of the mistaken opinion that radio-iodine therapy is more expensive than treatment with methimaole. It is true that methimazole tablets are relatively inexpensive. But the number of thyroid hormone level checks and repeat visits to the vet will make up for this. Besides, radio-iodine treatment gives consistently better results than managing the condition with medications.

The cost of radio-iodine therapy varies widely between facilities in the United States and Europe. A casual look on the internet in 2009 found the cost running from $785 in Arkansas to $1,700 in the New Jersey/New York area. Costs in Canada are generally a bit lower than in the USA. In Belgium it is a bargain. The University of Gent offered the procedure for 320 ($446). So it is very wise to check a number of facilities within your driving distance when planning this treatment for your cat.

Surgical thyroidectomy

Surgical removal of the diseased portions of your cat's thyroid glands is sometimes another option. But during the surgery, the difficult decision must be made as to how much of the glands to remove. There is no precise way to make this decision. If too much of the cat's thyroids are removed, the cat will become hypothyroid and need lifelong thyroid medications. If too little of the glands are removed, the cat will remain hyperthyroid. Sometimes, it is appropriate to remove all of the thyroid. Cats usually have small islands of thyroid tissue scattered in other locations in their bodies. These "ectopic thyroid cells" can usually produce all the thyroid hormone the cat will need. But they can also lead to the re-appearance of hyperthyroidism months or years later.

The thyroid glands are in a sensitive, crowded location. If the adjoining parathyroid glands are injured during surgery, the cat may have trouble regulating its blood calcium. If nerves in the area are injured the cat may develop eye problems or voice changes. (ref)

Most veterinarians stabilize the cats with methimazole for a number of weeks prior to the surgery. Some, remove a conservative amount of thyroid in a first operation and then monitor the cat's T-4 and blood calcium levels before deciding if a second surgery is required to remove more. Even when performed by an experienced surgeon with excellent post-surgical monitoring, the fatality rate of this surgery can approach 10%. You can expect the cost of this surgery to be equal or more than radio-iodine therapy and to have less predictable results.

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