Since it was first documented in 1979, hyperthyroidism has become increasingly more common in cats. It is now the most commonly
diagnosed endocrinopathy in cats. This may be a true increase in disease incidence, or it may be attributable to other factors,
such as the longer life spans of cats or better detection by veterinarians.
Hyperthyroidism is characterized by increased serum concentrations of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). The increases in serum thyroid hormone concentrations are usually the result of benign adenomatous hyperplasia of the thyroid
glands. In less than 2% of the cases, the hyperthyroidism is caused by thyroid adenocarcinoma. Ectopic thyroid tissue may
be present as well, which may have implications for the type of treatment chosen. No specific etiologies have been identified,
but environmental and genetic factors have been studied to help explain the increase in the incidence of hyperthyroidism over
the past few decades.
ENVIRONMENTAL AND DIETARY FACTORS
In recent years, three large studies have examined possible environmental or dietary factors in the pathogenesis of hyperthyroidism.
One of the studies included 100 cats with hyperthyroidism and 163 control cats.1 The cats' medical records were reviewed, and the owners completed questionnaires. Data collected included demographic variables,
environmental exposures, and diet (including the preferred flavors of canned cat food). In this study, housing (indoor vs.
outdoor), exposure to fertilizers and herbicides, regular use of flea products, and having a smoker in the house were not
associated with an increased risk of hyperthyroidism. The results suggested that cats that preferred fish or liver and giblets
flavors of canned cat food may have a significantly increased risk of hyperthyroidism.
In a second study, owners of 379 hyperthyroid and 351 control cats were asked about their cats' potential risk factors, including
breed, demographic factors, medical history, indoor environment, exposure to topical and en vi ron mental chemicals, and diet.2 The researchers then evaluated the association between these potential risk factors and the outcome of disease. The cats
of Siamese and Himalayan breeds (which are genetically related) had a reduced risk of developing hyperthyroidism. Cats that
used cat litter were at greater risk of developing hyperthyroidism than cats that did not. Topical ectoparasiticides increased
the cats' risk of developing hyperthyroidism. Cats that ate commercial canned food had about a twofold increase in the risk
of disease, compared with cats that did not eat canned food. When these four variables (breed, cat litter use, canned cat
food consumption, and topical ectoparasiticide use) were further analyzed, the researchers found a persistent protective effect
of breed (Siamese or Himalayan). In addition, they found a twofold to threefold increase in the risk of developing hyperthyroidism
among cats that ate mostly canned cat food and a threefold increase in risk among cats using cat litter. Use of commercial
flea products did not retain a strong association when analyzed further. This study suggests that more research into dietary
and other environmental factors (such as cat litter) is warranted.
In a recent retrospective study, the medical records of 169,576 cats seen at nine veterinary teaching hospitals from 1978
to 1997 revealed 3,570 diagnoses of feline hyperthyroidism.3 This information was compared to a case-controlled study population of 109 cats with hyperthyroidism and 173 normal cats
assessed at one of the nine veterinary teaching hospitals between 1998 and 2000. The researchers found that there was a significant
increase in the age-specific hospital prevalence of cats with hyperthyroidism over the 20-year period. Overall, two factors
were associated with a greater risk of developing hyperthyroidism: each additional year of age and the consumption of pop-top
canned food at various points in a cat's life. Female cats had a greater risk if they ate food from pop-top cans or from a
combination of pop-top and regular cans. Male cats had a greater risk if they ate food from pop-top cans and as they aged.
This research suggests that the increasing prevalence of feline hyperthyroidism is not due to the aging of the cat population
alone, but that canned foods may be an important factor as well.3 However, further research into this area is warranted because up to 25% of cats that develop hyperthyroidism have never eaten
canned foods.4 Likely, the cause of hyperthyroidism is multifactorial, with genetic factors also playing an important role.