How could my cat be sick? He acts like a kitten! - Hyperthyroid disease and your cat

THE MANY FELINE FACES OF HYPERTHYROIDISM 

What is hyperthyroidism?

The thyroid is a gland located in the neck. It plays a very important role in regulating the body's rate of metabolism. Hyperthyroidism is a disorder characterized by the overproduction of thyroid hormone (thyrotoxicosis). When excessive amounts of thyroid hormone are in the circulation, the body's metabolism speeds up greatly.

Location of the thyroid gland

 

1: Normal thyroid gland. 2, 3: parathyroid glands. 4: enlarged thyroid gland

Hyperthyroidism is a fairly common disease of older cats. It is estimated that about 5-10% of cats will develop hyperthyroidism in their lifetimes. Although the thyroid gland enlarges, it is usually a non-malignant (benign) change.  Less than 2% of hyperthyroid cases involve a malignant change in the gland.

What does this do to the cat?

The typical cat with hyperthyroidism is middle aged or older; on the average, affected cats are about 13 years of age. The rapid rate of metabolism causes 95-98% of cats with this disease to lose weight. The cat tries to compensate for this with an increased appetite. In fact, 67-81% of these cats have a ravenous appetite and will literally eat anything in sight! Despite the increased intake of food, most cats gradually lose weight. The weight loss may be so gradual that some owners will not even realize it has occurred.  Affected cats usually drink a lot of water and urinate frequently. There may be periodic diarrhea or vomiting, and the hair coat may be unkempt. As the disease progresses, the cat's appetite may decline to the point of anorexia.

In addition to:

• weight loss
• increased appetite
• vomiting
• diarrhea
• unkempt coat

other signs of hyperthyroidism that may be noticed include:

• increased thirst
• increased or more frequent urination, larger clumps in the litterbox, or inappropriate urination outside the litterbox
• increased activity
• increased vocalizations 
• pacing or restlessness
• anxiety
• panting or rapid breathing

How is hyperthyroidism diagnosed?

The disease is most commonly diagnosed by determining the blood level of one of the thyroid hormones; the hormone most frequently measured is T4. Usually, the T4 level is so high that there is no question as to the diagnosis. Occasionally, a cat suspected of having hyperthyroidism will have T4 levels within the range of normal cats. In this case, a second test, called a T3 Suppression Test, is performed. If this is not diagnostic, a thyroid scan can be performed at a veterinary referral center.

Hyperthyroid cat before becoming hyperthyroid, and again at the time of I131 treatment

Is this disease treatable?

Because less than 2% of these cats have cancerous growths of the thyroid gland, treatment is usually very successful.  There are three choices for treatment; any one of them could be the best choice in certain situations.  Many factors must come into consideration when choosing the therapeutic option for a particular cat.

What would happen if I chose not to treat my cat's hyperthyroidism?

Hyperthyroid is a deadly disease if left untreated. The effects that hyperactive metabolism has on the body are progressive, and this is a disease that will not resolve on its own. Over time, a cat that is hyperthyroid will develop problems associated with malnutrition because the overactive metabolism is using up calories and nutrients more quickly than usual. Add to that the likely side effects of vomiting and diarrhea, and an untreated cat will also become severely dehydrated.

The increased heart rate of a hyperthyroid cat will cause the heart muscle to thicken, and many will develop cardiomyopathy (poor muscle contractions) and eventual congestive heart failure. The high blood pressure that often goes hand in hand with hyperthyroid disease can cause damage to many sensitive organs in the body, including the kidneys, the eyes and the brain - as these blood vessels rupture from constant high pressure, blood supply is lost and tissues become unhealthy. This can cause kidney disease, neurologic effects due to damaged brain tissue, and blindness if the eyes are affected.

1) Radioactive iodine.  Probably the safest, and definitely the most effective way to destroy the abnormal tissue is with radioactive iodine (I131) therapy. In 95% of cats treated with I131, the cure is complete and permanent. Radioactive iodine treatment for cats requires one to three weeks of hospitalization at a veterinary clinic licensed to administer radiation therapy, and involves administration of the iodine via injection or oral capsule. The iodine is concentrated in the thyroid gland and destroys the overactive cells. At Exclusively Cats Veterinary Hospital Cats RadioIodine Treatment Center, the cats are treated with an oral dose and isolated for one week at the hospital followed by one to two weeks of isolation within the home.

Here is a vivid demonstration of a cat before and 3 months after treatment.

2) Surgery.  Surgical removal of the affected thyroid lobe(s) (thyroidectomy) is also very effective.  Because hyperthyroid cats are usually over 10 years of age, there is a degree of anesthetic risk involved, but not necessarily any more risk than for other cats of similar age.  However, the risk is much less than most people think, as long as the cat is otherwise healthy.  Tests are done before surgery to evaluate the cat and predict the chances for complications. If the disease involves both lobes of the thyroid gland, two surgeries may be required, depending on the surgeon’s choice of procedures.  In many cats, only one thyroid lobe is abnormal, so only one surgery is needed. Possible side effects include possible damage to the parathyroid gland, which is intimately associated with the thyroid gland, possible post-surgical hypothyroidism (under-functioning of the remaining thyroid gland, or as a result of bilateral thyroid removal).

3) Oral medication.  Administration of an oral drug, methimazole, can control the effects of the overactive thyroid gland.  After starting the medication, results can be seen in just 2-3 weeks. Some cats have reactions to the drug, but that number is fairly small (less than 20%). However, the side effects may begin as many as six months after the beginning of treatment and can include vomiting, lethargy, anorexia, fever, and anemia. Less frequently, cats may develop facial itching, clotting disorders and liver malfunctions. Methimazole does not destroy the abnormal thyroid tissue, but rather prevents the production of excess thyroid hormone.  Therefore, the drug must be given for the remainder of the cat's life. If the medication is stopped, hormone production will return to high levels again and the cat's symptoms will return.

Some cats can be particularly difficult to regulate, or may be poorly sensitive to this medication, which can be a downside to choosing this method of treatment. Other cats are difficult to medicate and would be better served by I131 treatment. Methimazole is a medication that some cats metabolize well in a transdermal form, but this option is not tolerated by all cat, and there is some risk to the owner in applying a thyroid-inhibiting medication that is absorbed through the skin, so appropriate handling of the medication is important, and transdermal medication may not be an appropriate choice for cats living in a home with small children. Periodic blood tests must be done to keep the dosage regulated. This type of treatment is appropriate for the cat that is a poor surgical risk due to other health problems.

4) Hill's Y/D diet.  Because the thyroid gland needs iodine to make thyroid hormone, feeding a diet with minimal iodine content should decrease the thyroid hormone levels of the cat. Feeding trials have indicated that after 3 weeks of feeding ONLY this diet - no treats, no human foods - thyroid levels are significantly reduced in most cats. Cats will not become hypothyroid while eating this diet, and a two year study suggests that cats that are NOT hyperthyroid that eat this diet will not suffer adverse effects. However, in practice, we have had a very difficult time getting cats to eat this diet, so while it is an option, it is a pricy (4 pounds of food is about $25 dollars and lasts about 3 weeks for a 7 pound cat) experiment that may not always pan out. There are many people who have had excellent results with this diet change, however, so it is definitely a viable treatment option. This treatment is not curative and the cat will need to remain on this diet for the rest of its life.

How Y/D diet works

If I elect to have surgery for my cat, what is the procedure?

If surgery is the treatment method chosen, the cat is put on methimazole for one to four weeks before surgery. This treatment should cause the ravenous appetite to subside, and your cat will probably gain weight. Some cats also have a very fast heart rate and may be medicated before surgery with another drug, such as Atenolol. After one to two weeks, another T4 level in the blood is measured.

The operation is performed in a sterile operating room and your cat is under general anesthesia. An incision is made along the neck just below the throat and the enlarged thyroid gland is removed. The skin is sutured together.

Your cat is generally hospitalized for one night following surgery and returns home feeling quite well. He or she should eat normally after returning home. 

Can hyperthyroidism occur again?

Recurrence is a possibility in some cats. Recurrence is uncommon after radioactive iodine therapy.  When surgery is performed, the chance of recurrence is slightly greater. It is usually not possible to surgically remove all of the cells from the abnormal thyroid gland. If those remaining cells grow, the disease may recur. However, this occurs less than 10% of the time and usually after 2-4 years.  Another possibility is that one side of the thyroid gland appeared normal at the time of surgery so it was not removed. Then, months or years later, it may become abnormal.

I think my cat is too old for anything but treatment with the oral medication or diet change.  Do you agree?

Many owners of cats with hyperthyroidism are hesitant to have radiation therapy or surgery because of their cat's advanced age. But remember, old age is not a disease. The outcomes following both surgery and radiation therapy are usually very positive, and most cats have a very good chance of returning to an excellent state of health. Some of the cats referred to our hospital in Waterford, Michigan, for radioactive iodine treatment have been as old as 20 years of age, and have done very well!