Medicine: Hyperthyroidism:

The thyroid glands are two glands located next to the windpipe near the throat. These glands are very important glands, as they produce hormones that regulate many functions of the body, especially metabolism. Hormones are little proteins that are released into the blood stream and travel to various places in the body to do their work. Thyroid problems come in two forms. Too low of thyroid hormone level is called hypothyroidism. It is a problem of dogs and was discussed in a previous article. Too high of thyroid level is called hyperthyroidism. It is very rare in dogs, but is actually quite common in cats. In fact, it is the second most common disease that we see in cats over 10 years old (kidney disease is the most common.) Because this is predominantly a cat disease, most of this article will be devoted to the disease in cats.

Hyperthyroidism occurs when too much thyroid hormone is created and released from the thyroid glands. It can also be caused from too much oral thyroid replacement hormone for hypothyroid patients. The reason that the gland secretes more hormones is because the gland enlarges and becomes over active. Generally, this is from benign (non-cancerous) growth. Although not a cancer, this overactive thyroid gland tissue can cause life-threatening problems, which will be discussed later. This overgrowth can be from 1 gland, both glands, or from a piece of abnormal thyroid tissue elsewhere in the body. This thyroid tissue in an abnormal location is called ectopic thyroid tissue. About 20% of cats with hyperthyroidism have disease in only one gland. At this point, it is not well understood why the thyroid tissue becomes abnormal. In humans there are special antibodies that stimulate the thyroid gland, which is called Grave’s Disease. These antibodies are absent in cats. It could be genetics, viral, or environmental (such as dietary). We just don’t know. In very rare cases, the thyroid glands can become cancerous (less than 5% in cats). Generally these cancers are not functional (don’t increase thyroid hormone production) but they can. Cancer is the main cause of hyperthyroidism in dogs (other than over treatment of hypothyroidism).

Generally, the signs of the disease are directly related to the function of the thyroid hormone excess. It is rare that you will notice a thyroid mass at home. The classic signs of hyperthyroidism are weight loss, increased appetite, and hyperactivity/restlessness. Behavioral changes are a very common cause of why the cat or dog was brought to the office. Not all patients show the same signs. All of the signs include:

• weight loss
• increased appetite
• restlessness/hyperactivity
• diarrhea
• patchy hair loss
• matted fur/unkempt coat
• excessive grooming
• increased thirst and urination
• vomiting
• aggression
• anorexia
• increased heart rate
• muscle tremors
• weakness
• lethargy/depression
• Dehydration
• Head-down stance

Many of these signs can be attributed to many diseases. Therefore if any of these symptoms occur, a through history and examination are imperative. If caught early enough, the disease can be diagnosed before symptoms occur. On examination, there are many factors that make us suspicious of this disease. They include:

• weight loss
• increased heart rate
• galloping heart murmur
• small kidneys
• palpable thyroid masses in neck

From examination and history, we can get a suspicion of the disease. If we are suspicious, then we run blood and urine testing. The blood and urine tests test for more than the thyroid activity. Remember that this is a disease of older cats and they can have other concurrent diseases we need to know about. Also, the symptoms may not fit classical signs of hyperthyroidism and other causes of the symptoms need to be ruled out. Thyroid disease specific tests include:

• T4 test: This measures thyroid hormone in the blood. It is called a screening test. This is a good test to find hyperthyroidism in cats that are showing symptoms. Its advantage is it that it is relatively inexpensive. Its downside is that the levels fluctuate a little over time and milder cases, or in cases where the cat has another disease, it could be artificially lowered.
• FreeT4 test: This is a highly reliable test (if performed by a special means called Equilibrium Dialysis). It measures the thyroid hormone not bound to proteins. This level fluctuates much less. Unfortunately, the test is more expensive to perform. We generally run this test on all cats that we are suspicious of thyroid disease, or for asymptomatic cats that tested slightly high on the T4 test.
• Ultrasound: the thyroid glands can be scanned with ultrasound. This is a good means of determining whether a thyroid mass is cancerous or not in dogs, and what treatment option is best. Generally, it is not performed in cats.
• Nuclear thyroid imaging: This test can detect thyroid disease in patients with symptoms but normal T4 and freeT4. It can also identify if one or both glands are involved. It can also identify ectopic thyroid tissue. It can also sometimes determine whether or not a lesion could be cancerous. It is the best test for hyperthyroid disease. Basically, an intravenous injection is given which is quickly and preferentially taken into the thyroid tissue. The brighter the color on a special scan, the more of the compound was taken up by the thyroid tissues. The more of the compound taken up, the more active the tissue. The downsides of this test are cost and convenience. The test is more expensive. It requires very specialized and expensive equipment. Because of this, the test is only performed in very few facilities outside of veterinary teaching hospitals. In our area, we are quite fortunate, as there is a facility in Tustin, which can perform this test. In some areas, people have to travel hundreds of miles with their cats to perform this test.
  

Hyperthyroidism is one of the few curable diseases we have in cats. The disease can be 100% curable with minimal side effects. It has already been stated that without treatment, the disease is fatal. But should every cat be treated? The answer is no. Remember that older cats are the victims of this disease and they may have other diseases going on. Also remember that this is the second most common disease. Kidney disease is more common. Hyperthyroidism can mask kidney disease and make it less severe. In fact, 30% of cats with thyroid disease may have detectable kidney disease. Treating the hyperthyroidism in a cat with kidney disease can actually cause the cat to go into kidney failure. It can take months for the effects of hyperthyroidism to cause enough damage to the body to become life threatening. So we are very careful about treating the disease with cats that also have kidney disease.

My cat has hyperthyroidism and a heart murmur. What does this mean?

One of the side effects of too much thyroid hormone is toxicity to heart muscle cells. It can cause a heart muscle disease called cardiomyopathy. This heart muscle disease is actually a leading cause of death in cats with untreated hyperthyroidism. Symptoms of this disease include:

• heart murmur
• increased heart rate/pounding heart
• respiratory distress
• weakness
• acute paralysis

This effect is diagnosed with ECG, chest X-rays, and an echocardiogram (ultrasound of the heart). Thyroid disease is not the only cause of cardiomyopathy in cats, but thyroid levels are always checked on cats with these symptoms. If your cat has cardiomyopathy secondary to hyperthyroidism, special heart medications may be necessary before treatment of the hyperthyroidism. In most instances, once the thyroid disease is treated, the cardiomyopathy is reversible.

There are 3 treatments for this disease: Surgery, medication, and radioactive iodine

Surgery: This is the most invasive treatment, is quite costly, and carries a high rate of recurrence. It can also cause another hormonal disease called hypoparathyroidism if both thyroid glands are removed. Given all of that, it is not a recommended treatment unless 1) you can’t give medication 2) you don’t live near a facility that can give the radioactive iodine. We do NOT perform this surgery on our patients since we have a facility to cure the disease close by.

Medication: Anti-thyroid medication (methimazole) does not cure hyperthyroidism. They block the ability to form functional thyroid hormone. But as soon as the medication is stopped, the hyperthyroidism returns. This medication is used both short term and long term. An advantage of the medication is that, once stopped, the thyroid function returns to normal. For this reason, we use this treatment method preferentially with our very old cats and cats with mild kidney disease. That way, if kidney signs worsen, the medication can be stopped immediately. Some cats take this medication for years. The medication is relatively inexpensive. It is less than ideal for several reasons though. It requires giving your cat oral medication generally twice daily. Anyone who has given a cat medication before knows that it is not fun. Also, monitoring of thyroid levels and for side effects to the bone marrow must be performed regularly. The medication can also cause vomiting, diarrhea, and anorexia. Also, it doesn’t work for all cats. With time, it can become the most expensive form of treatment (sometimes within 1 year).

Radioactive Iodine: This is the most effective means of treatment for hyperthyroidism in cats. Radioactive Iodine (131) is given via a simple injection. Thyroid tissues preferentially take up the iodine. It damages the thyroid hormone producing cells without damaging the surrounding tissues. 80% of cats are cured within 1 week, and 95% within 6 months from this one injection. 2% of cats can become hypothyroid following treatment that will require lifelong thyroid supplement. While non-invasive, hospitalization for about a week is required due to leakage of radioactive waste in the urine. State and federal laws require the cats to stay in the facility until they are no longer leaking a certain level of the iodine. Other than side effects of treating hyperthyroidism (such as kidney disease) and the possibility of causing hypothyroidism, the injection causes no side effects. The downsides are 1) it is not reversible 2) the cat is away from home for 1 week 3) cost 4) availability of the procedure near your home. Again we are quite fortunate that there is a facility in Tustin to help our hyperthyroid cats. At our hospital, unless there are other circumstances (such as kidney disease) we recommend radioactive iodine as the treatment of choice. Radioactive iodine is also effective against thyroid cancers as well.

Thyroid masses in dogs are thankfully very rare. Most of these masses are cancerous unfortunately. Most dogs with thyroid cancers are not hyperthyroid, as the tumor cells are usually not functional (90%). Dogs can get benign (non-cancerous) thyroid disease as well (although this is even rarer than cancer). If a dog has a functional thyroid cancer, or a functional benign thyroid mass, the symptoms are similar to those in a cat. It is important to remember that there are other diseases that can cause similar symptoms to hyperthyroidism and can cause masses on the neck.
For dogs, the most common presentation for a dog with a thyroid mass is that a mass was felt near the throat. If clinical signs from that mass occur, they are usually due to compression of surrounding tissues, like the windpipe (causing breathing problems) or the esophagus (swallowing/eating problems).
Generally if a mass is felt on examination, many tests will be performed. These include: thyroid tests (T4 and free T4), chest and neck X-rays, general blood and urine tests, and ultrasound of the neck. Also a needle aspirate of the mass maybe taken to look at under the microscope or sent to a pathologist to identify the mass.

Benign masses generally are surgically removed. Even if functional, this is usually curative. Cancerous masses maybe completely removed surgically. Unfortunately, these masses are usually very aggressive and can surround arteries, nerves, the windpipe, and the esophagus, all of which may make surgery difficult or impossible. Also thyroid cancers tend to metastasize or spread to other areas, such as the lungs. If a thyroid tumor is especially invasive, surgery may be done to reduce the size of the tumor, relieving symptoms and also allowing time for other treatments to work. Sometimes after surgery, special drugs are used to suppress tumor cells (chemotherapy). Radioactive iodine is very useful in dogs with functional thyroid tumors after surgery. Radioactive cobalt (which is a treatment for thyroid cancer in people) also is useful in combination with surgery and/or chemotherapy. Unfortunately, without very expensive intervention, thyroid tumors in dogs do not carry a good prognosis. The earlier a tumor is caught the better the overall prognosis.

The disease can not be prevented. But the earlier the detection, the more likely a good outcome for both dogs and cats. Annual examination includes a palpation of the thyroid gland areas. Annual lab work for both dogs and more importantly cats includes a T4 test that might find subclinical hyperthyroidism. Knowledge of the symptoms and examination if they occur are also vital for early detection. Armed with this information, hopefully if your cat or dog develops this disease it can be quickly identified and treated, so your pet can live a happy, healthy long life.

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