Hyperthyroidism in cats

Mandy Cooper

Always take your cat to the vet if you notice any changes in them. Usually cats try to hide symptoms as it is in their nature to do so. When you notice something it might have been going on for a while. Take your cat to the vet so and have them fully examined. 

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 car regulates the speed of your car). It does this by producing a hormone called thyroxine that regulates the speed of all body processes. When your cat produces too much of it and their metabolic rate soars, your cat has become hyperthyroid. Hypothyroidism is quite uncommon in cats. When this does occur, it is usually in a kitten that was born a dwarf.

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 between 8 and 13. Nine out of ten cats that develop hyperthyroidism are over ten years old.

The thyroid gland is 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 their wind pipe.

Is this a form of thyroid cancer?

It very rarely is. Less than 3 percent of the cats that develop hyperthyroidism develop malignant thyroid tumors. In over 98 percent, the cells in the whole gland or portions of it are just producing too much thyroid hormone.

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 pampered cats just get more frequent vet examinations. Although it's traditionally thought that it affected males and females equally, often, it may be considerably more common in female cats just as hyperthyroidism in humans is more common in women.

A bit about your cat's thyroid gland

Your cat has two distinct thyroid glands on either side of their windpipe midway down their neck. In humans, it is a single gland with a left and right lobe. The gland is responsible for regulating the speed of all chemical reactions that occur in your pet's body. This is called your cat's basal metabolic rate (the number of calories required to keep your body at it's normal rate). The thyroid gland produces 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 is 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 of levothyroxine. Before this form can work, it must be converted to T3 which is the form that the cat's body cells can recognize. Most of this is done in the liver.

What signs would I see if my cat is hyperthyroid?

When your cat's thyroid glands are overproducing thyroxine hormone, every organ in their body is affected. The cat's kidneys, liver, muscles, heart, nervous and digestive systems 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. Perceptive owners notice that although their cats are losing weight, their appetite is normal or increased. This is because the cat's metabolism 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 late stages of this disease, their general health deteriorates to the point that they don't have much appetite. Occasionally, cats have a form of this disease called masked hyperthyroidism in which they appear listless and 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 are taking stimulants. They are overly restless or hyperactive and they may be more cranky and aggressive. Some have disturbed 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 heartbeat. 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 or vet due to fear. Many cats with hyperthyroidism have heart rates over 200 even when they are relaxed at home.

Increased drinking and increased urination

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

Vomiting

We do not know why some cats with hyperthyroidism vomit. It occurs in hyperthyroid humans as well. Perhaps it's due to the increased amounts of food they eat or perhaps to the direct effects of their high thyroxine levels on stomach motility or 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 get rid of 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 with not getting enough oxygen.

Weakness and listlessness

In later stages of hyperthyroidism, multiple factors often cause cats to become weak. Muscle tremors and a weak meow can all be symptoms of advanced hyperthyroidism. Supplemental vitamin B6 has helped somewhat with this problem in humans.

Low grade fever

The high metabolic rate of hyperthyroid cats sometimes causes them to have a mildly elevated rectal temperature. But a rectal temperature can easily occur during a visit to your vet 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 your  fingers when you examine your cat's neck. In hyperthyroid cats, at least one lobe is often larger than it should be and can be felt. You and your vet 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 vet detects any mass in the thyroid area, it is crucial to fun the T4 test and a blood calcium level. Some of these masses turn out to be located in the parathyroid glands (a potentially serious calcium imbalance) that are next to the thyroids. The parathyroid glands are involved in regulation of body calcium.

How is hyperthyroidism diagnosed?

If you or your vet suspects hyperthyroidism, a thorough physical examination and some blood tests will be required by your vet to confirm the diagnosis. On examination, one or two enlarged thyroid glands can often be felt as a small, firm mass in the neck (these are often the size of a pea or a baked bean in cats with hyperthyroidism).

However, in some cats, there is no thyroid enlargement and this can be because the overactive tissue is present in an unusual site (often within the chest cavity).The diagnosis is confirmed by determination of thyroid hormones in the blood. A blood test looking at thyroxine (T4) concentration is usually all that is required for the diagnosis as this is usually raised in clinical cases. Other lab tests may also be abnormal, for example, live enzymes are commonly increased secondary to hyperthyroidism and assessment of routine blood and urine tests is usually advised to help rule out any other diseases, such as renal failure. Where possible, blood pressure should also be checked in cats with hyperthyroidism and if secondary heart disease is suspected, an ECG (electrical tracing of heart activity) and a chest x-ray or ultrasound might be helpful.

In occasional cases, hyperthyroidism may be strongly suggested on the basis of the clinical signs but blood testing may reveal a normal thyroid hormone (T4) concentration. There are a number of reasons for this and usually, on a repeat test, it will be raised. If not, additional tests may need to be done to confirm or rule out hyperthyroidism.

How is hyperthyroidism treated?

Several treatment options for hyperthyroidism exist, each with advantages and disadvantages.

•        Oral administration of antithyroid medication. Methimazole has long been the drug therapy for feline hyperthyroidism. It is highly effective in correcting the condition, often within two to three weeks. Some cats will suffer side effects though like loss of appetite, vomiting, lethargy and occasionally blood cell abnormalities. Rare but more serious side effects include severe facial itching, blood clotting disorders or liver problems. Most side effects are mild and eventually resolve although some cats may have to stop the medication if the side effects are too severe. Lifelong daily medication is required and CBC and T4 levels need to be rechecked regularly for the remainder of the cat's life.

•        Surgical removal of the thyroid gland. Hyperthyroidism is usually caused by a benign tumour called a thyroid adenoma that involves one or more often, both thyroid glands. Fortunately, most hyperthyroid cats have benign tumors that are easily removed.

•        Radioactive iodine therapy. This is probably the safest and most effective treatment option. Radioactive iodine, given by injection, becomes concentrated in the thyroid gland where it destroys the hyperfunctioning tissues. No anesthesia or surgery is required and only one treatment is usually needed to achieve a cure. Hospitalisation may be prolonged and cats may be kept at the treatment facility for 10 to 14 days until the level of radioactivity in their urine and feces decreases to an acceptable level.

•        Technetium scan. This is a technique that is available at some centers and can be useful in the investigation of some cats with hyperthyroidism. The technique can be used to diagnose hyperthyroidism and also to locate exactly where the abnormal tissue is. With this technique, a very small dose of radioactive chemical is injected into the cat's vein. This is taken up by abnormal thyroid tissue and this can be detected using a special camera. This is a simple, safe and easy procedure that may be recommended in some situations.

Hypertension (high blood pressure) is another potential complication of hyperthyroidism and can cause additional damage to several organs including the eyes, kidneys, heart and brain. If hypertension is diagnosed along with hyperthyroidism, drugs will be needed to control the blood pressure to reduce the risk of damaging other organs. As with heart disease, following successful treatment of the hyperthyroidism, the high blood pressure will sometimes resolve and permanent therapy may not be necessary.

Kidney disease (chronic renal failure) does not occur as a direct effect of hyperthyroidism but the two diseases often occur together because they are both common in older cats. Care is needed where both these conditions are present as the hyperthyroidism tends to increase the blood supply to the kidneys which may improve their function. Blood tests are taken to assess kidney function in a cat with hyperthyroidism may show normal or only mild changes but potentially more severe renal failure may be masked by the presence of the hyperthyroidism.

For this reason, it is usually advisable to start on medical treatment (tablets) initially and to monitor the response with repeat blood and urine tests to look at thyroid function and kidney function. Just occasionally, successful treatment of the hyperthyroidism results in a dramatic decline in kidney function. If this is detected, it may be necessary to reduce the dose of therapy so that the hyperthyroidism is not fully controlled but renal function is not too severely compromised.

Hyperthyroidism (overactive thyroid gland)

T4 (total thyroxine levels)

T3 (active form of thyroid hormone)

Levethyroxine ( a medication prescribed to dogs an cats to treat conditions associated with hyperthyroidism.

Methimazole (used to treat cats with hyperthyroidism in the form of tablets)

2nd chance info

Pets webmd