Feline Hyperthyroidism: Thyroid Disorders in Cats

Filed Under: Cats, Diseases

Feline hyperthyroidism is a disease characterized by weight loss, increased appetite, higher energy levels and possible irritability. It is the most common endocrine disorder in cats.

Feline hyperthyroidism is usually caused by a benign growth of the thyroid gland, called a bilateral adenoma. Malignant adenocarcinomas only occurs in 3 to 5% of these cases. These tumors result in excessive levels of the thyroid hormones thyroxine (T4) and triiodothyronine (T3) being produced. Thyroid hormones control the 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). They may also 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, and less than 5% of the cases of hyperthyroidism occur in cats less than 10 years of age. The disease appears to occur 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.

Upon physical examination the thyroid gland may feel enlarged when palpated. The cat will typically have tachycardia and be hypertensive.

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, but 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 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 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 lids 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 levels found in canned pet foods containing whitefish, salmon, and other seafood flavors tend to be higher than dry or non-seafood canned items. These levels have been found to be twelve 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 possibly be the result of an aging cat population.

Researchers have also shown a 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, they are proposing a link between the ether in flame retardants widely found in household dust with the occurance of hyperthyroidism. Flame retardants contain polybrominated diphenyl ethers or PBDEs. Researchers at Indiana University and the University of Georgia are currently investigating the consequences 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 that 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. Even though 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.

The American Society for the Prevention of Cruelty to Animals in New York recommends the following to limit your pets exposure to PBDE’s:

  • Use a vacuum with a HEPA filter and change the filter regularly.
  • Use an air conditioner with HEPA filter and change the filter regularly.
  • Cover tears in upholstery that expose polyurethane foam, especially if the foam is crumbling.
  • 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 three ways. Thyroidectomy is the surgical removal of the tumor. The pet will be normal within two 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 four 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. It is reasonably priced but 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 recur when the medication is discontinued. Thyroid hormone levels are typically measured at three to six 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 where 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 and the isolation of the pet following treatment.

References:

“Feline Hyperthyroidism Link?” Pet Age. October 2007.p 66.

“Study Links Cat Disease to Flame Retardants”. Pet Product News International. October 2007. p. 28.

Schultz, Krista. “EPA Probes Suspected Link between Household Chemical, Feline Hyperthyroidism”. DVM. October 2007. p. 1S.

Gartrell, Carla, DVM, DACVIM. “Feline Hyperthyroidism”. Great Smoky Veterinary Conference Notes. 2007.

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.

Topics: canned food, cats, flame retardant, hyperthyroidism, litter, thyroid

Symptoms: increased appetite, increased energy, weight loss

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