Breed, Coat Color and Hair Length as Risk Factors for Feline Hyperthyroidism

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Crossley V.J., Chang Y., Fowkes R.C., et al.

Conference Proceedings, (2016). American College of Veterinary Internal Medicine, Denver:

 

Hyperthyroidism commonly affects geriatric cats, but its aetiology is poorly understood. Previous studies have shown Siamese, Himalayan, and Burmese cats to have reduced risk of developing hyperthyroidism. These breeds have colorpoint coats as a result of temperature-sensitive mutations in the tyrosinase gene, which limit conversion of tyrosine to melanin pigment. Tyrosine is essential for synthesis of both melanin and thyroid hormones and, as such, coat pigmentation may impact relative tyrosine availability for thyroid hormone production. This study aimed to identify potential associations between coat phenotype and development of hyperthyroidism, by investigation of breed, coat color and hair length as risk factors for feline hyperthyroidism.

This retrospective study of cats aged ≥ 10 years, referred to a single UK veterinary teaching hospital (2006–2014), used electronic patient records to obtain owner-reported data regarding breed, coat color, age and sex, and clinical data to classify cats as hyperthyroid/euthyroid. Bayesian multivariable logistic regression was used to evaluate breed, age, and sex as risk factors for hyperthyroid status in all the cats (purebred and non-purebred). To avoid interactions with coat length or color in purebreds, the effect of coat color/pattern, color dilution, base pigment, white markings, and hair length were assessed only in non-purebred cats in a separate analysis. Variables with p < 0.2 in univariate analyses were evaluated in multivariable models, with final variables significant at p < 0.05. Risk factors are quantified as odds ratio ([OR] P-value).

Of 3934 cats included in the final analysis, 885 were hyperthyroid and 3049 euthyroid. Multivariable results showed Burmese (OR 0.01, p = 0.005), Persians (OR 0.20, p < 0.001), Siamese (OR 0.32, p = 0.006), Abyssinians (OR 0.04, p = 0.035), and British shorthairs (OR 0.52, p = 0.014) had reduced risk of hyperthyroidism compared to domestic shorthairs. Domestic longhairs (OR 1.34, p = 0.015) showed increased risk of hyperthyroidism compared to domestic shorthairs, but coat color, color dilution, base pigment, and white markings did not have a significant effect. Overall, females (OR 1.39, p < 0.001) had increased risk of hyperthyroidism compared to males, and, compared to 10 year olds, increased risk was found in cats aged 11–12 years (OR 1.45, p = 0.007), 13–14 years (1.83, p < 0.001), and 15– 17 years (OR 1.50, p = 0.004).

To authors’ knowledge this is the first study to report increased risk of hyperthyroidism in long haired, non-purebred, domestic cats. Consistent with a number of previous studies, increased risk of hyperthyroidism was found with increased age and female sex, and reduced risk in Burmese, Siamese, and Persian breeds. However, this study also newly identified two further purebreds, Abyssinian and British shorthair, at reduced risk of hyperthyroidism, neither of which were exclusively colorpoint. Coat coloration was not found to be associated with risk of hyperthyroidism in the analysis of domestic cats; however, reliance on secondary data may have resulted in misclassification errors and coat phenotype may be a poor surrogate marker for melanin concentrations. Further studies are required to evaluate tyrosine availability as a potential factor in the aetiology of feline hyperthyroidism, and to investigate the apparent protective effect in certain purebreds and associations between hyperthyroidism and hair length.