CPD ACCREDITED ARTICLE
Feline
Hyperthyroidism
Dr Joanne McLean BSc ( Vet Biol ) BVSc ( Hons ) Internal Medicine Resident : Bryanston Veterinary Hospital and Specialist Centre PO Box 67092 , Bryanston , 2021 Email : joanne @ fuseprop . co . za
Feline hyperthyroidism is a multisystemic disorder arising from excess production of the active thyroid hormones ( triiodothyronine [ T3 ] and / or thyroxine [ T4 ]) from an abnormally functioning thyroid gland . Approximately 97- 99 % of cases result from benign nodular hyperplasia , adenomatous hyperplasia or adenoma of the thyroid gland .
The autonomous secretion of T4 and T3 produces a negative feedback effect on the pituitary gland , suppressing the release of thyroid-stimulating hormone ( TSH ) such that any normal thyroid tissue atrophies . In 70 % to 75 % of cats with hyperthyroidism , both thyroid glands are affected . Only 1 % - 3 % of cases are caused by malignant thyroid carcinoma . Hyperthyroidism is seen mainly in middle-aged to older cats , with the age at presentation ranging from 4 to 23 years ( mean of 13 years ). Only 5 % of hyperthyroid cats are younger than 10 years at time of diagnosis . There is no sex or breed predisposition , although Siamese and Himalayan ( colour-point Persian ) cats seem to be under-represented in some studies .
Incidence and Risk Factors The worldwide incidence of feline hyperthyroidism has steadily increased since the early 1980s and it is now one of the more frequently diagnosed disorders in small animal practice . Despite an increase in incidence however , the exact cause of the disease is still
unknown . A number of theories have been proposed involving factors related to diet ( possibly including the presence of goitrogens , eating canned cat food , iodine content , or frequent food changes ), environmental causes ( possibly associated with cat litter , environmental toxins , pollution , or exposure to allergens ), a genetic mutation , abnormal immune responses , or altered hormonal responses .
Several substantial epidemiological studies from the USA , UK , Hong Kong and New Zealand have revealed some contradictory results , but also some common risk factors .
These include an increased risk in indoor cats , female cats , cats in multi-cat households , cats with dental disease ( independent of age ), use of topical flea preparations and pesticides , use of cat litter ( not linked to increased risk in indoor cats ), consumption of certain flavours of canned foods ( fish or liver and giblet flavour ) and increased risk in non-pure bed cats .
Issue 04 | AUGUST 2016 | 13