Feline hyperthyroidism is an important disorder in middle-aged and older cats. The cause and pathogenesis is still unknown and there are few published incidence rates or prevalence estimates.
A descriptive, cross-sectional study was undertaken to determine the prevalence of and potential risk factors for feline hyperthyroidism in Hong Kong. Serum total thyroxine (T4) concentrations were measured in 305 aged cats that presented at various veterinary clinics in Hong Kong between June 2006 and August 2007. Data was collected about the health of the cats as well as their vaccination history, internal and external parasite control, diet and environment. Serum T4 concentration was determined by use of a commercially available radioimmunoassay kit (Coat-a-count®, DPC®). For T4 the reference interval was 12.8-50 nmol/L (1.0-3.9ug/dL). All cats with serum T4 concentrations greater than 50 nmol/L were classified as hyperthyroid. Serum alanine aminotransferase (ALT) and alkaline phosphatase (ALP) activities were measured in all cases. Prevalence of hyperthyroidism with exact binomial 95% confidence intervals was calculated for all cats combined, for cats classified as healthy and for cats classified as sick. Univariable associations between potential risk factors, clinical signs, raised ALT and raised ALP activities and hyperthyroidism were assessed using a two-tailed Fisher's exact test. A multiple logistic regression model was used to estimate the effect of the risk factors on the development of hyperthyroidism. The fit of the final logistic regression model for risk factors was assessed using the Hosmer-Lemeshow goodness-of-fit test.
The prevalence of hyperthyroidism in Hong Kong was 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 (P = 0.76). There was no statistically significant relationship between sex, vaccination status, parasite control, indoor environment or the consumption of canned food and the development of hyperthyroidism. Domestic shorthair cats were less likely to be diagnosed with hyperthyroidism and the two older age groups of cats (15-19 years old and >20 years old), were more likely to be diagnosed with hyperthyroidism (OR = 2.77; 95% CI = 0.82-9.4 and 11.88; 95% CI = 1.06-133.7, respectively).. There were no characteristic clinical features amongst the cats that had hyperthyroidism. The presence of polyphagia, diarrhea, palpable thyroid nodule and raised ALT and ALP activities was significantly associated with hyperthyroidism.
This study concluded that the prevalence in Hong Kong is less than in most other parts of the world, despite the presence of previously identified risk factors.