Feline Hyperthyroidism


The most common medication used to treat feline hyperthyroidism is Tapazole. The advantage to Tapazole is the drug itself is inexpensive. However this can be misleading due to the number of recheck appointments and serial thyroid hormone assays 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. The usual result is that the cat and owner become enemies, with the cat running the opposite way when the owner comes near it. Tapazole also has several undesirable side effects that, although not seen in every case, are all too common. 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.
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. Also, if both glands are removed, there is increased risk of accidentally removing the small para-thyroid glands (causing permanent hypocalcemia), or in 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.
The third option for the treatment of hyperthyroidism is the administration of radioactive iodine. The advantages of radioiodine cure are many. The overall success rate is 96% following a single injection of iodine-131. Recurrence rate is extremely low. The return to a euthyroid state is rapid, as circulating thyroid hormone levels drop precipitously within 48 hours post-administration, and there are essentially no side effects. Not only does the radioactive iodine only localize in thyroid tissue, it only destroys hyperfunctional thyroid tissue (wherever it is). Normal thyroid cells are suppressed through negative feedback loops, and as such do not concentrate any of the iodine. These spared normal thyroid cells then 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 and are euthyroid. Because 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 4-5 days, while the animal is excreting the radioactivity. The typical hyperthyroid cat treated with radioactive iodine will gain significant weight during the short hospitalization period, have a good appetite and do well. Continued weight gain over the next 2-3 weeks is expected and averages ¾ pound (or about 15-20% of body weight).


Malignant thyroid adenocarcinomas are rarely diagnosed in cats, occurring in less than 1-2% of all hyperthyroid cats. Cats with malignant thyroid tumors will have identical clinical signs as do cats with benign hyperthyroidism. In addition, T3 and T4 levels do not correlate with malignancy; and therefore cannot be used to differentiate malignant versus benign tumors. Although excisional thyroidectomy followed by histopathologic analysis is the confirmatory method for malignancy; imaging of the thyroid glands is often highly predictive of malignancy. On a thyroid scan, malignant adenocarcinomas will often times be found in abnormal locations and have very abnormal shapes, sizes and focal areas of increased or decreased uptake (“hot” or “cold” spots). When this is seen, surgical removal of the questionable thyroid tissue is performed, followed by histopathologic confirmation. Once confirmed, “high dose” radioactive iodine therapy is typically recommended for possible metastatic disease. Long term prognosis in cats with malignant thyroid tumors is dependent on the size, degree of invasiveness, and metastatic potential of the primary tumor, but can range from weeks to over 3 years (average of about 1 year).

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 manifestations of the disease are significant weight loss, loss of the normal hair coat luster and patchy hair loss, normal to increased appetite, irritability and/or restlessness. It is important to remember that up to 30% of hyperthyroid cats will actually be anorexic (the so called “apathetic” hyperthyroid cat). Many cats will have palpably enlarged thyroid glands, however many cats with very large glands will have glands that cannot be easily palpated. This is caused by caudal displacement of the glands due to the effects of gravity, as they are commonly located in the thoracic inlet or thorax. Although most cats exhibit many of the clinical signs, it is rare for any individual cat to present with all of the clinical signs. Many cats will also demonstrate intermittent vomiting and diarrhea as well as increased water consumption and urination.


While most cats with hyperthyroidism are diagnosed based on clinical signs and increased T3 and T4 values, 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 (99m Tc). 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. When properly performed, there is little to no chance of a misdiagnosis. 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 adenomas versus a malignant adenocarcinoma.

Hyperthyroidism is a clinical syndrome that exists when there is overproduction of circulating thyroid hormone. Feline hyperthyroidism is the single most common endocrine disorder of cats, which show are the only non-human species in which spontaneous thyrotoxicosis develops. It has been reported that up to 31% of all cats have thyroid adenomas on post-mortem examination. Some veterinarians believe that the true incidence of this disease has increased recently. Others believe that veterinarians realize the prevalence of the disease and are thus diagnosing it more often. The cause of hyperthroidism in cats is unknown, and it is the subject of heated ongoing research. Heredity, environmental concerns, and diet have all been suggested as possible etiologies, however nothing has been proven. The hypermetabolic state of chronically increased levels of circulating thyroid hormone results in deleterious effects on multiple organ systems. Eighty-seven percent (87%) of hyperthyroid cats will have some degree of hypertrophic cardiomyopathy, although it is rare for a cat to present with overt heart failure. Most cats will have increased heart rates (>220 bpm) and a significant proportion will have a gallop rhythm due to turbulent blood flow in the heart. Thromboembolic disease (“saddle thrombi”) is also associated with the cardiovascular disease. The majority of cats will have abnormalities on routine chemistry panels reflecting damage to the liver, kidneys and muscle. Ninety-eight percent (98%) of all affected cats have benign disease (functional thyroid adenomatous hyperplasia), and as a result have a good to excellent prognosis. Of the cats with benign disease, 2/3 of them will have bilateral disease (both thyroid glands 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. Only a very small percentage (<2%) of all hyperthyroid cats have a malignant tumor, with mixed, compact and follicular adenocarcinomas being the most common. The few cats that have a thyroid adenocarcinoma carry a poorer long-term prognosis.

View a presentation about Millie and the importance of the diagnostic thyroid scan