Hyperthyroidism is one of the most common diseases of domestic cats, with up to 10% of geriatric animals affected. This disease is characterized by increased levels of thyroxine (T4) due to hypersecretion of the hormone from nodules on the thyroid gland. This results in weight loss despite increased appetite, increased heart rate, hypertension, damage to the liver, kidneys, and other organs, and, if untreated, death. While many options exist to treat this disease, understanding its causes and risk factors may improve early diagnostics and prevention.
Previous studies and anecdotal reports have noted several demographic characteristics linked to hyperthyroidism. These factors have included female sex, longer hair, non-purebred status, and darker or non-pointed coat colors. It has been theorized that these factors may be associated with availability of the amino acid tyrosine (a precursor of both T4 and melanin), changes in exposure to goitrogens on longer hair, or other, unidentified factors.
This study was designed as a retrospective cross sectional study of medical records at a secondary care center in the United Kingdom. The purpose of the study was to investigate links between the incidence of hyperthyroidism and breed, coat color, coat length, age, sex, and neuter status. Only cats older than 10 years were evaluated.
Data from 4075 cats were analyzed, of whom 975 (20.7%) were hyperthyroid. This value is likely elevated due to the nature of the clinic as a referral center for I131 therapy. 83% of all cats were purebred, 48% were female, and 98% were neutered. Rates of hyperthyroidism increased from 10 through 17 years of age, then decreased from 17-18 years.
Data was analyzed by univariate statistical analysis, and factors with a P<0.2 were selected for inclusion in a multivariate model. Coat color and hair length were only modeled in non-purebred cats due to confounding effects of specific coat requirements in certain breeds.
This analysis found several factors that were tied to either protection or predisposition for hyperthyroidism. The study concluded that risk factors for hyperthyroidism include long hair, female sex, and non-purebred status. Burmese, Tonkinese, Siamese, Persian, British Shorthair, and Abyssinian cats had reduced rates of hyperthyroidism. The Burmese breed was the most protected. While several of the “low risk” breeds had either color pointing or dilute coat colors, coat color was not associated with risk of hyperthyroidism in non-purebred cats.
Neutered cats were more likely to be hyperthyroid in the general population, but not when confined to non-purebreds. This may be a real effect, but the authors mention that it may also have been confounded by the medical records system.
Some drawbacks to this study exist. Though the large number of cats needed to draw conclusions makes this a difficult proposition, a prospective study drawing from primary (rather than referral) practices would provide better insight into results. Retrospective evaluation of medical records means that errors in identification of breed, coat color, or sex may have occurred. Investigation of the underlying physiologic mechanisms of these risk factors will be needed in future studies to fully understand this disease. (MRK)