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Frequently Asked Questions |
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What is a Diplomate? Why did my veterinarian recommend Veterinary Imaging Center for my pet's ultrasound? Not all veterinarians have ultrasound machines available in their practice, and ultrasound requires special training to become proficient. And although ultrasound capability is found in many general small animal clinics, many cases require the animal to be scanned (or re-scanned) by a board certified specialist with advanced training in ultrasound. Many diseases can be diagnosed by your family veterinarian using ultrasound. Occasionally, however, there are questionable findings or the diagnosis is not "clear-cut" and a second opinion is in the best interest of the animal patient. This process is similar to human medicine where, for example, an internist or family practitioner will perform the initial ultrasound examination, but for a number of reasons may then refer you to a radiologist who is specifically trained in ultrasound, to get a second opinion. What is Nuclear Medicine or Scintigraphy? Nuclear medicine or scintigraphy is the use of radioactive substances in studying the physiology of the body—or “how the body works”. This type of study is important to see how the body works, especially for the bones, kidneys, liver and thyroids. What is Feline Hyperthyroidism? Hyperthyroidism is the clinical syndrome that exists when there is overproduction of circulating thyroid hormone, which is the hormone that regulates the body’s metabolism. This means that hyperthyroid cats have very high metabolism. Feline hyperthyroidism is very common in older cats. Because the thyroid hormone affects the whole body’s metabolism and body systems, if this disease goes untreated, multiple organ systems may be affected and damaged. Eighty-seven percent (87%) of hyperthyroid cats will have some degree of hypertrophic cardiomyopathy, which is a condition where the heart muscle becomes thickened. Most hyperthyroid cats will have increased heart rates and and irregular heart rhythm. The majority of these cats will have abnormalities on routine chemistry panels (blood analysis) reflecting damage to the liver, kidneys and muscle. What are the symptoms related to Feline Hyperthyroidism? The classic clinical signs of feline hyperthyroidism are a result of the multisystemic effects of increased levels of circulating thyroid hormone. The most commonly seen symptoms of the feline hyperthyroidism are weight loss, loss of the normal hair coat luster and patchy hair loss, normal to increased appetite, irritability and/or restlessness. Hyperthyroid cats also often have intermittent vomiting and diarrhea and increased water consumption and urination. You should also be aware that some hyperthyroid cats may actually have decreased appetites or anorexia. Many hyperthyroid cats will have palpably enlarged thyroid glands (often noted by your veterinarian), however sometimes these thyroid lobes may not be felt, as they are normal sized, displaced into the chest or abnormal area of the neck. Although most cats exhibit many of the clinical signs, it is rare for any individual cat to present with all of the clinical signs. Ninety-eight percent of hyperthyroid disease in cats is benign or NOT malignant. This is due to overactive thyroid tissue (hyperplasia) or functional thyroid nodules or adenomas. Because this disease is usually benign, it carries a good to excellent prognosis. Of the cats with benign disease, more than 60% will have bilateral disease (both the right and left thyroid lobes will be affected) and 1/3 will have only one hyperfunctional gland. Benign hyperthyroidism has been reported in cats as young as 6 years and as old as 22 years (average age is 14 years). There is no sex or breed predilection. The few cats that have a malignant thyroid tumor carry a poorer long-term prognosis. How is hyperthyroidism diagnosed? While most cats with hyperthyroidism are diagnosed based on clinical signs and increased T3 or T4 values (blood levels of thyroid hormone), up to 42% of cats that are proven to be hyperthyroid will have T3 and/or T4 values that are in the normal range. It has been shown that circulating levels of thyroid hormone can fluctuate up to 24% during any 24 hour period and up to 28% over a 2 week period. Thus if a cat is mildly hyperthyroid, a single T4 assay may fall within the normal range at any time. Also, the circulating thyroid levels may be falsely decreased secondary to many other concurrent disease processes (the "euthyroid sick syndrome"). Therefore, the diagnosis of feline hyperthyroidism should not be excluded based on a single normal serum T3 or T4 value. A diagnostic thyroid scan is the most sensitive and specific method to diagnose feline hyperthyroidism. The scan is easily performed, inexpensive and quick. To perform a scan, the cat is injected with a very small amount of a radioactive solution called technetium (99mTc). This radionuclide is an iodine analog and localizes in glandular tissue (thyroid glands, salivary glands, and glands that line the stomach mucosa), thus allowing the thyroid glands to be imaged. In normal euthyroid cats, the amount of uptake in the thyroid glands is equal to that of the salivary glands. In hyperthyroid cats, there is much more uptake in the thyroids when compared to that of the salivary glands. Besides confirming the diagnosis of hyperthyroidism, imaging of the thyroid glands allows us to evaluate the size, location, shape, and pattern of uptake for each thyroid gland, which is highly predictive of benign disease versus that of a malignant thyroid tumor. This is important because circulating thyroid hormone levels cannot differentiate benign versus malignant disease and treatment options and prognosis are drastically different for benign versus malignant thyroid conditions. How can I treat my hyperthyroid cat? Once the diagnosis of benign feline hyperthyroidism is made, there are essentially three treatment options, each of which has advantages and disadvantages. The three options are medical management using anti-thyroid drugs, surgical removal of the thyroid gland(s) and radioactive iodine. 1. The most common medication used to treat feline hyperthyroidism is an oral medication called, tapazole (methimazole). The advantage to tapazole is the drug itself is inexpensive. However, this can be misleading due to the number of recheck appointments and blood work needed to assure the patient is being dosed properly. Tapazole does have several disadvantages, the most important of which is that it is not a cure for the disease, but merely a treatment. Because of this, the drug must be administered at least once a day (many times more often) for the remainder of the cat's life. As many of you know, this in itself can be a CHALLENGE. Often over time, your cat will “learn” that he/she is about to be pilled and will “disappear” when you head for the pill jar. This may be bypassed because, several pharmacies now are able to formulate methimazole for transdermal use (this can be applied in the ear and absorbed through the skin), but is more expensive. Tapazole also has several undesirable side effects that, although not seen in every case, will occur fairly commonly. These include vomiting, diarrhea, blood dyscrasias, and hepatotoxicity. As stated in the Physicians Desk Reference (PDR) and the package insert, "tapazole is intended to be used to ameliorate hyperthyroidism in preparation for a more definitive treatment" and as such tapazole is not intended to be used as a long term treatment. 2. The second option for the treatment of hyperthyroidism is the surgical removal of the thyroid gland(s). Sub-capsular thyroidectomy has the advantage of being curative, with low rates of recurrence, however there are several disadvantages of thyroidectomy. Once the ventral neck incision is made, a decision must be made to take out only one gland (usually the largest) or both glands. This decision is complicated by the fact that many hyperfunctional thyroid glands are not physically enlarged, thus many hyperfunctional, normally sized glands will not be removed, and the cat will still be hyperthyroid, requiring a second surgery. The largest gland is not necessarily the most active, thyroid hormone producting gland. Also, if both glands are removed, there is increased risk of accidentally removing the small para-thyroid glands (causing permanent hypocalcemia), or at the least damaging their blood supply (causing transient hypocalcemia). Also, if all thyroid tissue is removed, the owner is back to giving at least one pill per day for the remainder of the cat's life, this time thyroid supplementation. Another disadvantage to surgery is that it is not uncommon for an animal to have functional extrathyroidal tissue in abnormal locations (most commonly in the thorax). Obviously, this tissue would be missed from a ventral incision in the neck. Lastly, in the largest study done to date on the "Surgical Treatment of Feline Hyperthyroidism" involving 85 cases, 9% of cats died in the peri-operative period due to complications. 3. The third option for the treatment of hyperthyroidism is the administration of radioactive iodine. This involves giving your cat a single injection (“shot”) of radioactive iodine (iodine-131), which is usually curative. Radioactive iodine is curative 96% of the time, following a single injection of iodine (I131). Recurrence rate is extremely low. The return to a normal thyroid state is rapid, as circulating thyroid hormone levels drop precipitously within 48 hours post-administration, and there are essentially no side effects. Because the normal thyroid cells are suppressed in hyperthyroid animals, when the radioactive iodine is given, it will concentrate in the abnormal thyroid tissue, which is overproducing thyroid hormone. Thus the normal cells are spared and can "turn back on” and make normal amounts of thyroid hormone following treatment. In this way, cats treated with radioactive iodine typically do not need any medication following radioactive iodine therapy. Because the radiation, which has the killing effects (beta particles) that are released from the radioiodine travel such a short distance, there is no effect on the adjacent para-thyroid glands and post-therapy hypocalcemia is not seen. The only disadvantage to the use of radioiodine is that the cat must be hospitalized in a special facility for about 3-5 days, while the animal is excreting the radioactivity. Many pet owners “worry” about their cat during this hospitalization period; however, the vast majority of these cats do “great” while hospitalized. Most of them eat well and are relaxed during their stay; some may actually gain weight while hospitalized. Copyright © 2003 Veterinary Imaging Center. All rights reserved. |
