Vet Confidential

What is feline hyperthyroidism?

This is a condition seen in middle-aged and older cats. Cats have two thyroid glands in the neck, and with hyperthyroidism, one or both develop a small tumor that makes too much thyroid hormone. In most cats, the tumor is a benign type called an adenoma, which vets often refer to as a thyroid nodule. We are not sure why some cats develop hyperthyroidism.

fThe excessive thyroid hormone produced by the adenoma is harmful to the cat, causing an increased metabolism and health problems such as weight loss, heart damage, high blood pressure, anxiety, kidney damage, and elevated liver enzymes. It is very important to control the overactive thyroid before the cat suffers heart failure and other life-threatening consequences.

There are 3 main forms of treatment:

1. Medication.

The most commonly used and successful medication used for hyperthyroid cats is called methimazole, or Tapazole®. Methimazole works very well to lower the thyroid hormone levels. It is essential to realize that methimazole is not a cure: it does not get rid of the adenoma, which in fact continues to grow larger and larger; it simply prevents it from releasing thyroid hormones. If the methimazole is stopped, the hormone levels rise right back up.

Methimazole does have a relatively high rate of side effects. These include physical symptoms such as vomiting, poor appetite, or severe itchiness, but also include potentially dangerous changes on the cat’s blood work. For example, methimazole can cause low white blood cells, anemia (low red blood cells), low platelet cells (the cells that help the blood to clot), or elevated liver enzymes.

Methimazole, and all forms of treatment of hyperthyroidism, can also affect the cat’s kidney function. There is a delicate balance between thyroid levels and kidney function. When a cat’s thyroid level is lowered from being too high to being normal, sometimes this has an adverse effect on the cat’s kidneys. We don’t know exactly what the relationship is.

Perhaps the cat’s kidneys had problems already that the excess thyroid hormone was actually helping in some way. For example, thyroid hormone raises the blood pressure, and this can cause increased blood flow to the kidneys. Maybe when we lower the thyroid hormone level, the kidneys miss all that extra blood flow. Of course, the elevated thyroid hormone level also damages the kidneys over time. We’re just not sure exactly what’s going on, but we do know that it is crucial to carefully monitor a cat’s kidney function when we lower the thyroid hormone level by treating his hyperthyroidism.

It is imperative that cats on methimazole have blood tests done regularly to monitor for side effects. After the medication is started, blood is usually done after about 2 weeks, sometimes sooner if the vet has concerns about the cat’s kidney function. Blood work continues to be checked every 2 weeks until the thyroid hormone level is normal and the vet is comfortable that the kidneys are continuing to function well. A that point, blood work is checked every 3 months or so. It is important that at each recheck, not only is the thyroid hormone level measured, but that the liver, kidneys, red cells, white cells, and platelet cells are evaluated, since all of these can be affected by methimazole. If changes occur, they are usually reversible if caught fast enough, but can be life-threatening if not.

2. Radioactive iodine.

For most cats, radioactive iodine is the safest and most effective way to treat hyperthyroidism. The cat receives an injection of radioactive iodine under the skin. Studies have shown that hyperthyroid cats who are first treated with methimazole, then receive radioactive iodine, live the longest. Since the thyroid is the only part of the body that uses iodine, the iodine selectively “zaps” the thyroid glands.

The advantages of this form of treatment are that in most cases (though not 100%), it is a permanent cure; and that the iodine will target both thyroid glands, which is good because both glands can be affected.

The disadvantages are that radioactive iodine treatment tends to cost between $1000-$2000 dollars, which may be more than some people have on hand; and that because the cat has a radioactive substance in his body that will be excreted in his urine for a period of time, the cat must spend about 1-2 weeks at the facility until it is safe for the owner to have him at home again.

I always recommend that cats be first treated with methimazole, so that the cat’s kidney function can be monitored as the thyroid hormone levels fall. The advantage of methimazole is that if the kidney function suffers, the dose of medication can be adjusted to achieve a balance between kidney function and thyroid hormone levels. For example, a particular cat’s kidneys may function well if his thyroid hormone level is at the upper end of the normal range, but not if the level is at the lower end of the normal range.

Once it is determined that the cat’s kidneys can tolerate a lower thyroid level, the cat can be scheduled for radioactive iodine treatment. If, on the other hand, it turns out that the cat’s kidneys will not tolerate certain thyroid hormone levels, the cat may continue to be treated with methimazole instead, since the thyroid hormone level can be adjusted by increasing or decreasing the dose (with radioactive iodine, you cannot predict or control exactly what the thyroid hormone level will be after treatment, and it can even be lower than normal).

3. Surgery

Surgery is another treatment option. One or both thyroid glands can be removed surgically. The surgery is not painful or invasive since the thyroid glands are located superficially, just under the skin. I usually recommend removing only one thyroid gland at a time, whichever has the adenoma and is making excess hormone. The side effects can be much worse if both glands are removed, including low thyroid levels, and problems with low calcium levels.

The advantage of surgery, if only one gland is removed, is that the cat can usually go home the same day the surgery is performed, and that at some facilities this is less expensive than radioactive iodine treatment. For cats who will be severely affected by being away from home for radioactive iodine therapy, owners may prefer surgery to avoid this. I will say that most cats adjust fine to their stay at the radioactive iodine facility, and after a day or two settle in and eat well.

The disadvantages are that anesthesia is required, which carries risks depending on the cat’s health, and that if only one gland is removed, the other gland may begin to produce excess hormone sooner or later. Just as with radioactive iodine, you cannot predict or control exactly what the thyroid hormone level will be after surgery, and in some cases it will be low, or even still elevated to some degree.

As I explained in the section on radioactive iodine therapy, before thyroid surgery is performed I recommend first treating with methimazole, as a test to see how the cat’s kidneys will function as the thyroid hormone levels drop. The effects of medication are reversible; surgery and radioactive iodine are not.

Use and Misuse of the Free T4 Test
You may have heard of a relatively new blood test called a free T4 test that is sometimes used to diagnose cats with hyperthyroidism, a condition where the thyroid gland becomes overactive. This problem is common in older cats. It’s important to understand the pros and cons of the free T4 test if your cat has it done.

 The cause of hyperthyroidism is a small, usually benign tumor called an adenoma on one or both of a cat’s two thyroid glands, which are located in the neck. Vets can usually feel the adenoma- we call it a thyroid nodule. The adenoma makes extra thyroid hormone that the cat doesn’t need, and this hurts the cat by causing weight loss, heart problems, high blood pressure, anxiety, and problems with organs such the kidneys and liver. We don’t yet know why it happens to a particular cat.

T4 is the thyroid hormone that vets measure to see if a cat is hyperthyroid. Usually, vets measure what’s called the total T4. The newer blood test is called a free T4 test. Unfortunately, the test is not actually free…wouldn’t that be great…it’s called a free T4 because it measures thyroid hormone in the cat’s bloodstream that is floating free (some thyroid hormone is stuck on to proteins in the bloodstream).

The free T4 test was developed because the traditional T4 test (which is called a total T4 since it measures all the thyroid hormone whether or not it is stuck onto proteins) is not high in every cat who has an overactive thyroid gland. Especially in cats who are sick with something else, such as kidney failure, diabetes, or cancer, the total T4 is sometimes normal or even low when in fact the cat does have hyperthyroidism. That’s because when we’re sick, our total thyroid hormone level drops, even if our thyroid glands are normal or even overactive.

However, there are some problems with the free T4 test. The most important problem is that the free T4 may be elevated in cats who are not actually hyperthyroid. In other words, this test is more likely to give a false positive result for hyperthyroidism than the total T4 test. This happens most often in cats who have another health problem. When an animal or person is sick, the proportion of their thyroid hormone that is “free” goes up, so a cat with health problems may have a high free T4 even though their thyroid gland isn’t really overactive.

This may seem confusing, since I just said that your total T4 drops when you’re sick. The deal is that when a cat is sick, their total T4 will get lower, but their free T4 may get higher. Phew!

Another source of confusion is the fact that sometimes weight loss is the only symptom in cats with intestinal problems, such as inflammatory bowel disease (a common condition where the intestines become so inflamed they don’t work properly) or lymphoma (a type of cancer cats often get in the intestines). Since weight loss is a common symptom of hyperthyroidism, vets may suspect these cats are hyperthyroid when in fact their intestines are the problem.

The situation becomes even more confusing when a cat with intestinal problems has the free T4 test and the result is above normal. Lots of times, this is because of the cat’s intestinal problem, not because the cat is hyperthyroid- remember we talked about how the free T4 level is sometimes high in cats with health problems. Sometimes cats with weight loss actually due to intestinal disease are treated for non-existent hyperthyroidism, based on a high free T4 level.

Vets often use the free T4 test when they are not sure whether a cat is hyperthyroid. But honestly, it’s rare that a cat who is significantly hyperthyroid would be difficult to diagnose with this disease. In other words, if hyperthyroidism is causing a cat’s symptoms, it shouldn’t be particularly difficult in most cases to figure out that the cat is hyperthyroid. While a cat with early hyperthyroidism (a small adenoma that is only making a little bit of extra thyroid hormone) may still have serum total T4 within the high normal range, this cat is unlikely to have very bad symptoms, and given a little time, as the adenoma grows, the serum total T4 will indeed rise above normal.

Because of these issues with the accuracy of the free T4 test, total T4 remains the screening test of choice for feline hyperthyroidism. Cats should not be diagnosed with hyperthyroidism based solely on a free T4 measurement. And vets should still be utilizing one of the best tests we have at our disposal: the physical exam. A free T4 blood test should never be sent to the lab without the thyroid ever being palpated to check for an enlarged thyroid nodule! Most hyperthyroid cats will have one.