Risk-factor analysis for feline hyperthyroidism in the metropolitan area of Porto Alegre : a pilot case-control study

Background: Hyperthyroidism is the most common endocrinopathy in elderly cats and its prevalence varies greatly geographically. Since it was first described in 1979, it has shown a continuously increasing incidence. This fact has given rise to several hypothesis, and studies about the effects of endocrine disruptors on human and animal health are in full development. Aging, bisphenol-A and other endocrine disruptors present in the dust, were already documented as risk factors for feline hyperthyroidism. The aim of this study was to perform a risk analysis on the habits and lifestyle variables of domestic cats and their owners, and its possible relationship with feline hyperthyroidism. Materials, Methods and Results: Retrospective case-control epidemiological study. A 30-questions questionnaire was applied to owners of 28 hyperthyroid cats as well as to owners of 55 euthyroid cats aging more than eight-years (n:m) and living in the metropolitan area of Porto Alegre – RS. Criteria for hyperthyroidism diagnosis was assumed as compatible clinical presentation confirmed by elevated serum thyroxine concentration. Euthyroid control cats were selected by convenience from hospital medical records based on the absence of hyperthyroidism diagnosis and age. Univariate odds ratio (OR) and respective 95% confidence interval (95%CI) analysis was performed for each of the 22 variables surveyed. Exact Fischer´s test was performed to determine P-value, which was considered significant if P < 0.05. Student´s unpaired t-test was applied for age comparison between groups. Mean age of hyperthyroid cats was 13.2 ± 2.7 years (range: 7-18), while euthyroid cats mean age was 11.6 ± 2.4 years (range: 8-16) (P < 0.01). Higher exposure to canned foods was considered as a risk factor for hyperthyroidism in this population (OR = 2.87; 95% CI = 1.1 - 7.5; P = 0.032) as well as aging more than 12-years-old (OR = 3.14; 95% CI = 1.10 – 8.97; P = 0.048). Likewise, weekly or monthly bathing frequency represented a strong risk factor in those hyperthyroid cats studied (OR = 7.57; 95% CI = 1.41 - 40.55; P = 0.013). Regarding other items surveyed, such as the use of plastic accessories, contact with domestic dust, use of endoparasiticides, ectoparasiticides and vaccines, it was not possible to identify any association of these variables as risk or protective factors. Discussion: Even with a modest sample size, these results were in accordance with previous studies that analyzed hundreds of cats showing that advanced age, as well as the consumption of canned foods, are risk factors for the development of the disease. This study also raised the possibility of a risk factor related to frequent bathing. The explanation would be due to the possible presence of endocrine disruptors in cosmetic products used for bathing. This hypothesis needs further studies, since previous recommendations on hyperthyroidism preventive management suggests that bathing could have a protective effect since it cleans off the dust particles present in the fur. However, evidences that triclosan and parabens present in cosmetic products may act as thyroid endocrine disruptors have been recently raised. Further studies would be required to determine the effects of such inputs in felines’ health, since the etiopathogenesis of hyperthyroidism is uncertain and surely multifactorial. These genetic, environmental, and nutritional factors; should be analyzed together, as far as possible. Moreover, these findings are helpful to create preventive strategies against feline hyperthyroidism and corroborate with data previous published in epidemiological studies in other countries.