Hyperthyroidism is the most common endocrine disorder in the cat. Since being first recognized in 1977, the incidence has
increased steadily. This is, no doubt partly due to greater awareness and early screening, but certainly also due to a real
increase in occurrence of this disease. The etiology and pathogenesis are not certain, but epidemiological surveys have shown an increased incidence of this disease is seen in cats:
- fed a majority of tinned food in their diet,
- living strictly indoors, using litter,
- having a reported exposure to lawn herbicides, fertilizers and pesticides,
- Have been regularly treated with flea sprays or powders, and who aren't Siamese (one study only, breed predilection has not
confirmed in any other study).
- exposure to flame retardants
The disease has been reported in North America, Europe, Australia and New Zealand, but less frequently, or not at all, in
other parts of the world. These risk factors implicate environmental, nutritional and genetic factors. It is also logical
to assume, however, that cats who are well cared for and thus live longer, will be exposed to litter and canned foods.
Some of the goitrogens that are being studied include iodine and phyhalates (common in cat foods), resorcinol, polyphenols
and PCBs, all of which may also be in diets, especially those containing fish, or in the environment. These hydrocarbons
need to be metabolized in the liver, by glucuronidation, a process compromised in the cat, making this species more prone
to toxicities. Goitrogen exposure may be sporadic, rather than ongoing, as sporadic exposure has been shown to induce thyroid
hyperplasia in experimental models. Other theories consider that nodular goiter development may be a normal age-related condition.
Regardless of the cause(s), the condition of hyperthyroidism, is a multi-systemic disorder caused by excessive concentrations
of circulating thyroglobulins, T4 and T3, produced most commonly by benign, hyperplastic adenomatous glands, but rarely by
malignant, adenocarcinomatous glands. 97-99% of hyperplastic glands are benign and adenomatous. Approximately 70% of cases
have bilateral disease, a fact that is critical when considering surgical therapy and favours pre-surgery technetium scanning.
Most recently, attention has been directed towards brominated-flame retardants. Dr. Janice Dye presented an abstract at ACVIM
2007 in which she states: "Coincident with global introduction of BFRs into household consumer products nearly 30 years ago,
hyperthyroidism in cats has increased considerably. The etiopathogenesis of feline hyperthyroidism remains unknown. We hypothesized
that increasing exposure of pet cats to BFRs such as the polybrominated diphenyl ethers (PBDEs) has, in some manner, contributed
to the abrupt increase in and now common occurrence of feline hyperthyroidism...Our finding that pet cats in the U.S. have
high PBDE serum levels is in good accord with the most consistently identified risk factor for development of FH, namely indoor
living. We further propose that house cats—because of their meticulous grooming behavior—not only come in direct contact with
these consumer products, they readily ingest any volatilized PBDE-like material or PBDE-laden dust that deposits on their
fur. Future studies are needed to elucidate if and how PBDE bioaccumulation of this magnitude in cats can disrupt maintenance
of their thyroid-endocrine-axis." (Dye JA, Venier M, Ward CR, et al. Brominated-Flame Retardants (BFRs) in Cats—Possible Linkage
to Feline Hyperthyroidism?)
While hyperthyroidism is a disease of middle aged to old cats (4-22 years old), it has been reported in cats aging from 8
months - 22 years of age. There is no breed or sex predilection.