Feline hyperthyroidism in Hong Kong : prevalence and risk factors

Abstract

Feline hyperthyroidism is an important disorder in middle-aged and older cats. The cause and pathogenesis of the disease is still unknown and there are few published incidence rates or prevalence estimates. A descriptive cross-sectional study was conducted to determine the prevalence of and potential risk factors for feline hyperthyroidism in Hong Kong. Serum thyroxine (T4) was measured in 305 cats 10 years and older that presented at various veterinary clinics in Hong Kong between June 2006 and August 2007. The veterinarians taking the samples completed a questionnaire regarding the health of each cat. Each owner completed a questionnaire regarding vaccination history, internal and external parasite control, diet and the environment of their cat. Serum total T4 concentration was determined by use of a commercially available radioimmunoassay kit (Coat-a-count®, DPC®). For total T4 the feline reference interval was 12.8-50.0 nmol/L (1.0-3.9 ug/dL). All cats with a serum total T4 concentration of greater than 50.0 nmol/L were classified as hyperthyroid. Alanine aminotransferase (ALT) and alkaline phosphatase (ALP) activities were measured in all the samples. The prevalence of feline hyperthyroidism in Hong Kong was estimated at 3.93% (95% CI : 2.05-6.77) and there was no significant difference in prevalence between healthy (3.16%) and sick (4.37%) cats. This demonstrates that although this disease is present in Hong Kong, the prevalence is lower than the reported prevalence in other parts of the world. Risk factors that were examined included age, sex, breed, number of cats in household, vaccinations, parasite control, indoor environment, type of diet and type of water. Risk factors for hyperthyroidism identified by multivariate analysis were age and breed. Affected cats were more likely to be older (>15 years) and domestic shorthair cats were less likely to be diagnosed with hyperthyroidism than the other breeds combined. There was no statistically significant relationship between sex, vaccinations, parasite control or indoor environment and the development of hyperthyroidism. There was also no statistically significant relationship between the consumption of a canned food diet by the cats and hyperthyroidism. There were no characteristic clinical features amongst the cats that were hyperthyroid and only one cat exhibited the typical clinical syndrome of ravenous appetite with severe weight loss. The lack of distinctive clinical signs could be due to the presence of a mild or early form of the disease, but can also be due to an atypical form of the disease. This study showed that the disease needs to be considered if any of the following factors are present in an older cat : polyphagia, diarrhoea, and a significant raise in ALT and ALP activities. This study concluded that the prevalence of hyperthyroidism in cats in Hong Kong is less than in most other parts of the world, despite the presence of previously identified risk factors. Comparative epidemiological studies will be necessary to compare the presence of possible risk factors between feline populations in Hong Kong and elsewhere.