Prevalence of and risk factors for feline hyperthyroidism in South Africa

Abstract

Feline hyperthyroidism is a metabolic disease of middle-aged to older cats that has shown a marked increase in its world-wide prevalence within the last three decades. This disorder is now recognised as the most common feline endocrinopathy in many geographical locations, with diabetes mellitus coming a close second. Epidemiological studies performed to date have also suggested a geographic variation in the prevalence of the disease. This seemingly apparent variation may, in fact, reflect differences in dietary, environmental or genetic factors. Although clinical features of feline hyperthyroidism as well as its pathological lesions are well described, the exact pathogenesis of the disease still remains obscure and despite a plethora of epidemiological studies, clear risk factors for the disease have not been identified. Further information on worldwide prevalence of the disease and possible causative factors would increase our understanding of the aetiology of this disease and help identify any preventive measures. As far as the author is aware, prevalence studies have not yet been performed in South Africa, a geographic area in which hyperthyroidism in cats has relatively recently been observed and reported and the incidence of which anecdotally appears to be on the increase. The purpose of this study was to determine the prevalence of feline hyperthyroidism in South Africa and to identify any potential risk factors associated with the disease in this geographic location. This analytical cross-sectional study was conducted in cats that were presented for either geriatric check-ups and routine vaccinations or various illnesses at five general veterinary practices throughout South Africa between February 2014 and June 2015. Cats were included in the study if they were nine years of age or older and had resided solely in South Africa. Cats were excluded from the study if their demeanour precluded the collection of a blood sample, or if they had been treated with drugs that could potentially affect tT4 and TSH concentrations. At the time of blood sampling a questionnaire was completed regarding the health status of the cat, vaccination history, internal and external parasite control, diet and environment. Consent for blood collection was obtained from the owners and ethical approval for the study was also applied for and granted by the Animal Ethics Committee of the University of Pretoria. Serum tT4 and cTSH concentrations were determined in all cats by use of a chemiluminescent competitive immunoassay (Immulite ® 1000 Canine total T4, Siemens Medical Solutions Diagnostics) and a chemiluminescent immunometric assay (Immulite ® 1000 Canine TSH, Siemens Medical Solutions Diagnostics) respectively. The reference interval for tT4 was 14-50 nmol/L and for cTSH 0- 0.07ng/ml (laboratories historical reference intervals). Serum fT4 concentrations were measured in cats with a serum tT4 concentration between 30-50nmol/L and a serum cTSH concentration less than 0.03ng/ml using equilibrium dialysis (Antech Diagnostics Inc ® Free T4 by Equilibrium Dialysis) with a reference interval of 10- 50pmol/L (laboratories historical reference intervals). All cats with a serum tT4 concentration greater than 50nmol/L or cats with a serum tT4 concentration between 30-50 nmol/L, a serum cTSH concentration less than 0.03ng/ml and a serum fT4 concentration greater than 50pmol/L were classified as hyperthyroid. Prevalence of hyperthyroidism, with exact binomial 95% confidence intervals (CIs), was calculated for all cats combined, for cats classified as healthy (no clinically significant disease identified) and for those classified as sick. Prevalence was compared between healthy and sick cats using a two-tailed Fisher's exact test. Univariable associations between potential risk factors and hyperthyroidism were assessed using a two-tailed Fisher's exact test. Thereafter, all predictors were entered into a multiple logistic regression model to estimate their effect on the risk of hyperthyroidism. Associations between clinical signs and hyperthyroidism were assessed on a univariable level using a two-tailed Fisher's exact test. The study population consisted of a total of 302 cats. The most common breeds represented included domestic shorthair (DSH) (n=201), domestic longhair (DLH) (n=29), Siamese and Siamese crosses (n=26), Persian and Persian crosses (n=10), Burmese (n=10), Balinese (n=6) and Maine Coon and Maine Coon crosses (n=5). There were 161 females and 141 males, 265 of which were sterilised. The median age of the cats was 12 years (range 9-24 years).