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Hyperthyroid Disease in Cats   J. Kris Hankison, DVM   Stevens Point, WI There are a number of options for feline patients that need treatment for Hyperthyroidism, including radiation therapy and oral medications and liquid medication that is wiped on the ear pinna and is called a transdermal method of dosing the cat.  Be sure to consult with your veterinarian regarding specific treatment and therapy for your cat if hyperthyroidism is a problem.

Hyperthyroid disease in cats is common and is usually discovered in older cats that lose weight but yet eat well, and seem otherwise healthy.  Take a look at Hyperthyroidism in cats as explained by an experienced veterinarian...

“Doctor, is there something wrong with my cat, Tigger? She's losing weight but she can’t be too sick because her appetite is terrific!”

I encourage this concerned pet owner to bring Tigger in for an examination. Further questioning revealed some important clues to solving her health problem. The owner told me that Tigger has seemed restless for several months, wandering around the house at night, yowling and making a terrible racket. The owner also told me that Tigger has had occasional diarrhea and vomiting, and these symptoms have become more frequent.

During the physical exam, several abnormal findings are noted. Tigger is very thin with a poor, dull haircoat. She is very anxious and restless on the exam table and her heart rate is very fast... more than 200 beats per minute! (average is 110 - 140 beats per minute.)  As I continue the examination, I think I can feel a lump in the neck area. At this point, I am pretty sure that I know what is causing Tigger to be ill, and I explain to the owner that some blood tests will help confirm the diagnosis. When the blood test is back, we discover that the Tigger’s thyroid hormone level is more than twice what it should be. We have our answer- feline hyperthyroidism.

Hyperthyroidism is one of the most common diseases of the middle-aged and older cats. It is a disorder that ultimately affects many of the body systems. It is caused by an increase in the amount of thyroid hormones produced by enlarged thyroid glands. First documented 30 years ago, the actual cause of the disease remains a mystery. In most cases, the enlargement in the thyroid gland is caused by a non-malignant tumor called an adenoma. In very rare cases, a malignant form of this disease is seen.

The thyroid glands are located in the front of the neck on each side of the trachea (windpipe). Normally, they are tiny, about ¼ inch long, and difficult to feel through the skin. If the glands begin to enlarge, the veterinarian may be able to feel them. Hyperthyroidism is diagnosed by checking levels of the thyroid hormone in the blood. Since these levels can fluctuate daily, sometimes repeat testing or special thyroid function testing may be necessary for diagnosis.

The most common symptoms of this disease include weight loss, increased appetite, increased thirst, restlessness, unkempt hair with excessive shedding and matting, vomiting and/or diarrhea (although these symptoms are often sporadic). Because of the effects of the thyroid hormone levels on the heart, these patients have a fast heart rate, and may have a heart murmur, high blood pressure, difficulty breathing, and other heart problems.  It is important to note that not all of these symptoms may be present in every cat.  Therefore, any middle-aged to older cat that presents with any of the above symptoms should be screened for hyperthyroidism.

Once hyperthyroidism has been confirmed, there are several treatment options. They include treatment with radioactive iodine, surgical removal of the gland, and treatment with antithyroid medications. The initial choice of treatment is often guided by concern about the patient’s general health status. The veterinarian will want to monitor the kidney, liver and heart functions carefully before, during and after therapy.

For hyperthyroid cats that have no other problems, radioactive iodine treatment or surgery is often recommended. Both these options may provide a cure of the hyperthyroidism and avoid the need for life-long administration of medications.

In areas where radioactive iodine treatment is available, it usually the treatment of choice since this option avoids the risks of anesthesia and surgery.  Radioactive iodine also has the advantage of treating ectopic thyroid tissue (thyroid tissue located somewhere other than the thyroid gland), a condition that may occur in a small percentage of hyperthyroid cats.   This treatment is becoming more readily available as more veterinary referral centers offer this service. However, because this procedure is still somewhat difficult to obtain in some areas, surgical removal of the thyroid gland continues to be an excellent option for treatment of hyperthyroidism in many cats.

Surgical thyroidectomy is an excellent treatment option unless the cat is an unacceptable anesthetic risk. A very important point here is that age alone is not a contraindication to surgery! This surgery is often done in 15, 16 or even 18 year-old cats. As long as there are no other pre-existing conditions, these cats usually do great, and go on to live into their 20’s.

During surgery, both thyroid glands are examined and all apparent abnormal thyroid tissue is removed and biopsied. In the majority of cases this means that both thyroid glands are removed. The most worrisome post-operative complication comes from inadvertent damage to the very tiny parathyroid glands. These small glands are located close to the thyroid glands and control calcium levels in the body. Monitoring calcium levels in an important part of the post-operative care for these patients.  

Dr. Hankison performed this surgery on his own cat that developed Hyperthyroidism.
The patient is anesthetized, then prepared for the surgery	The Thyroid Glands are located.	The abnormal appearing gland or glands are carefully  removed.	Thyroid glands after removal... time for thyroid hormone supplementation for a while.

Additional complication of surgery can include nerve damage, voice changes and persistence of clinical symptoms. Hypothyroidism (low thyroid hormone levels) may develop in some cats that have had both thyroid glands removed. Clinical signs of hypothyroidism include poor activity, weight gain and hair loss. Treatment involves hormone replacement, which is associated with very little side effects, less expensive than anti-thyroid medications, and less critical if a dose or two are missed. However, the complication of hypothyroidism is less common than you might think. As the surgeon carefully dissects around the vital parathyroid tissue, microscopic amounts of thyroid tissue are actually left behind. In most cases, this tissue is able to produce enough thyroid hormone to supply the body’s needs.

There are effective antithyroid medications available, the most common medication being methimazole (Tapazole). Approximately 15% of patients will experience side effects when taking methimazole. These may range from poor appetite, vomiting, lethargy and skin rashes to the more serious problems such as bone marrow suppression and liver toxicity. In most cats, the side effects are mild and do not interfere with continued treatment. However, periodic monitoring will be necessary while the cat is on the medication.

Methimazole works by inhibiting thyroid hormone production by blocking iodine incorporation into the hormone. It is important to understand that methimazole does not “cure” hyperthyroidism, rather it controls the symptoms by lowering hormone levels. Thyroid hormone levels will usually reach normal levels in 2-3 weeks after starting therapy, and it will need to be given daily to maintain remission. It is very important that this medication be given regularly because the hyperthyroid condition will return when it is stopped.

For those cats that are not candidates for radioactive iodine or surgery, and who can not tolerate methimazole, Ipodate can be a helpful medication. Ipodate was used in human medicine as an organic iodine agent for radiographic contrast studies. Methimazole can be applied topically to the ear pinna and the medication is then absorbed through the skin.  It must be compounded by your veterinarian or a veterinary or human pharmacy.  This is termed a transdermal dosing method and for many pet owners is much easier than making sure a pill is swallowed every day. It has been shown to reduce thyroid hormone levels in people and cats. These reductions are associated with improvement in the clinical signs of the patient and this medication has minimal adverse effects. Unfortunately, it does appear that these beneficial effects may only last a limited time (months). Another problem is that this medication is no longer manufactured and has become increasingly difficult to find.

Propylthiouracil has been used in the past to treat feline hyperthyroidism. Due to a high incidence of serious side effects, this medication is rarely used. If it is used to treat this condition, frequent monitoring is very important.

Although hyperthyroidism is a very serious disease in cats, it does have several very effective treatments. As with any medical condition, early detection can increase the chances of successful treatment.  Geriatric cats (those over 7 years of age) can have periodic blood testing to screen for hyperthyroidism as well as other health problems.  With proper treatment, most cats with hyperthyroidism can live a normal, high quality life. So if your older kitty seems to be eating well but losing weight, has a poor coat and acts somewhat restless... be alert for Feline Hyperthyroidism.