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Feline Hyperthyroidism

Do you have an older cat that is losing weight, yet has a ravenous appetite? Is your cat vomiting on a regular basis? Then your cat could be suffering from Feline hyperthyroidism. Hyperthyroid disorders may also increase energy levels and cause irritability. It is the second most common endocrine disorder in cats.

Feline hyperthyroidism is usually caused by a benign growth of the thyroid gland, called a bilateral adenoma. Malignant adenocarcinoma only occurs in 3 to 5 % of these cases. These tumor result in excessive levels of thyroid hormone, thyroxine (T4) and triiodothyronine (T3) being produced. Thyroid hormones control the body’s basal metabolic rate of an individual.

Clinical signs of hyperthyroidism may vary from mild to severe. Most commonly these cats will have lost weight, be hyperactive and polyphagic (have an increased appetite and be constantly hungry). The cat may have intermittent vomiting and diarrhea. Cardiac arrhythmias and murmurs are common. The cat may have a rapid heart rate also known as tachycardia. These cats are prone to the development of hypertrophic cardiomyopathy and congestive heart failure. The most common age for the development of hyperthyroidism is 13 years of age. Less than 5% of the cases of hyperthyroidism occur in cats less than 10 years of age and most are house cats or primarily inside only cats. The disease occurs less in the Siamese cat and mixes thereof, such as the Himalayan.

Approximately 10% of cats with the disease will develop apathetic hyperthyroidism in which the characteristic signs of hyperactivity and increased appetite are replaced by depression and inappetance. The weight loss may be more exaggerated in these cats.

One physical examination the thyroid gland may feel enlarged on palpation. The cat will typically have tachycardia and be hypertensive (high blood pressure).

Diagnosis may usually be made through the measurement of thyroid hormones via a blood test. The serum T4 levels are typically high in most cats with hyperthyroidism. Approximately 5 to 10% of cats with hyperthyroidism will have normal levels of T4. Cats not exhibiting high T4 levels may have concurrent nonthyroidal illness which causes suppression of total T4 levels. These cats will however have a high free T4 levels in conjunction with characteristic clinical signs of disease.

Environment and lifestyle may play a role in the development of Feline Hyperthyroidism. Several references have recently indicated a possible link among cats that consume a diet of mostly of canned food. It has been shown that cats fed a diet of primarily canned food have a threefold increase in the development of these thyroid tumors. It has been speculated that the aluminum cans with pop-top cans are lined with a substance called bisphenol-A-diglycidyl ether (BADGE) or isoflavones, which is transferred into any food containing oils or fats. Researchers have also found that the BADGE level found in canned pet foods containing whitefish, salmon and other seafood flavors tends to be higher than dry or non-seafood canned items. These levels have been found to be 12 times higher than the level found in dry food diets and some cats appear to be receiving more than 100 times the dietary BADGE exposure than an American adult. To date this link has not been confirmed and could possible be the result of an aging cat population.

There is a strong correlation with cats using odor-control kitty litter, especially those with enhanced absorbency, with a greater risk for the development of hyperthyroidism.

In the August 2007 issue of Environmental Science and Technology, researchers are proposing a link to the ether from flame retardants, widely found in household dust, with the development of hyperthyroidism in cats. Flame retardants contain polybrominated diphenyl ethers or PBDEs. Researchers at Indiana University and the University of Georgia are currently investigating findings of elevated levels of environmental PBDE contamination. These compounds are also found in the plastic casings of computers and televisions, as well as furniture cushions, mattresses and carpet padding.

Hyperthyroidism in cats began appearing widely approximately 30 years ago which correlates with the same time frame that flame retardants with PBDEs were introduced into household products and is around the same time frame feline hyperthyroidism was first diagnosed. Indoor cats ingest large amounts of PBDE-tainted dust due to their meticulous grooming habits and prolonged contact with furniture. PBDE’s mimic thyroid hormones which some researchers propose may result in an overactive thyroid.

PBDE’s use has been banned and discontinued in many areas. Although their use is being discontinued in many areas, it is important to identify the specific compounds causing the endocrine disorders, to ensure that a replacement flame retardant does not have the same health consequences.

A step to limit your pet’s exposure to PBDE’s the American Society for the Prevention of Cruelty to Animals in New York recommends the following

  1. Use a vacuum with a HEPA filter and change the filter regularly.
  2. Use an air conditioner with HEPA filter and change the filter regularly.
  3. Cover tears in upholstery that expose polyurethane foam, especially if the foam is crumbling.
  4. Cover mattresses with tightly woven allergen barriers to reduce dust.

Regardless of the inciting factor for Feline Hyperthyroidism, the condition is treated in one of four ways. Thyroidectomy is the surgical removal of the tumor. The pet will be normal within 2 days following surgery. Surgery does carry a significant morbidity and mortality rate. The risk of surgical intervention may be decreased through the use of Methimazole or Tapazole® for 4 weeks prior to surgery which helps to reduce the risk of an anesthetic related death.

The second way to treat hyperthyroidism is through the use of Methimazole itself.
Methimazole is an antithyroid drug and is selective in its action, is reasonably priced but it is often necessary to continue its usage on a daily basis. The dose is adjusted to maintain circulating thyroid hormone levels within the normal range. Symptoms may reoccur when the medication is discontinued. Thyroid hormone levels are typically measured at 3 to 6 month intervals to adjust the dosage of methimazole therapy.

The third way to treat hyperthyroidism is through the use of radioactive Iodine (radioisotope 131). Iodine is selectively taken up by the thyroid gland were it actively destroys the hyperactive tissue and spares the normal tissue within the thyroid. That is why for many, it is the treatment of choice. The problem with this treatment is radiation safety, expense and the isolation of the pet following treatment.

Iodine is a necessary building block for thyroid hormones. Hill’s Pet Nutrition has developed a new diet for hyperthyroid cats named y/d that restricts this essential building block and thereby lowers the level of thyroid hormones available in the body. Typical diets contain from 1.5 parts per million (ppm) to 99 ppm of iodine, while y/d lowers the iodine content of the food to 0.1 to 0.3 ppm. This diet alone has been shown to decrease the severity of clinical signs seen with hyperthyroid disease in as little as three weeks, with most cats becoming euthryoid (normal thyroid hormone levels) within eight weeks. For y/d to be effective it must be used as the only source of nutrition for these pets.

Regardless of the treatment used for hyperthyroidism, the response to treatment should be monitored by periodic blood tests for total T4 concentration.

Following treatment for hyperthyroidism 15% of cats will be found to be azotemic or be in kidney failure, regardless of the treatment employed. Kidney failure results from increased levels of thyroid hormone which in turn increases cardiac output and renal blood flow. The increased blood flow to the kidneys will artificially lower the values of biochemistry tests used to evaluate kidney function. The resulting kidney failure will become evident 2-4 weeks following a return to normal thyroid hormone levels. Most of these patients will do well, but should be treated as would any cat suffering from kidney failure.

References:

  • Dodds, Jean. “Myths about Thyroid Disorders, Vaccines.” Veterinary Practice News. June 2016. P. 6 and 50.
  • “Feline Hyperthyroidism Link?” Pet Age. October 2007.p 66.
  • Gartrell, Carla, DVM, DACVIM. “Feline Hyperthyroidism”. Great Smoky Veterinary Conference Notes. 2007.
  • Heflin, Marissa. “Nutrition for Feline Hyperthyroidism?” Veterinary Practice News. September 2011. P. 24.
  • Heflin, Marissa. “Managing Feline Hyperthyroidism with Nutrition.” Veterinary Practice News. October 2011. P. 26.
  • “Hyperthyroidism and Kidney Failure – a Concern? Blue Pearl Companion. Winter 2016. P. 3.
  • Kahn, Cynthia Editor. The Merck Veterinary Manual. 9th Edition. p. 466-467.
  • “Packaging may Contribute to Feline Hyperthyroidism”. Veterinary Practice News. November 2007 p. 16.
  • Schultz, Krista. “EPA Probes Suspected Link between Household Chemical, Feline Hyperthyroidism”. DVM. October 2007. p. 1S.
  • “Study Links Cat Disease to Flame Retardants”. Pet Product News International. October 2007. p. 28.
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