Breed, Hair Length, and Hyperthyroidism Risk in Cats

The causes of hyperthyroidism in cats are not well understood. Previous studies have indicated that some breeds (Burmese, Himalayan, Persian, and Siamese) have a lower risk for developing hyperthyroidism than other cats, possibly because of the gene mutation responsible for their colorpoint coats, say the authors of a study that examined breed and hair length in hyperthyroid cats. The study was published in the Journal of Veterinary Internal Medicine.

The amino acid tyrosine is a precursor of both melanin and thyroid hormone. A temperature-sensitive mutation in the tyrosinase gene in colorpoint cats prevents tyrosine from converting to melanin except at the extremities, which are cooler than the rest of the body. Preventing tyrosine from converting to melanin could leave more tyrosine available for thyroid hormone production, theoretically protecting against thyrotropin overproduction and the subsequent development of hyperthyroidism, suggest the authors.

Insufficient dietary tyrosine has been linked with loss of dark coloration in black cats. The investigators theorized that coat color and hair length could be related to hyperthyroidism risk if these factors also affect tyrosine availability.

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To evaluate the associations between coat phenotype and the risk of hyperthyroidism, the researchers retrospectively reviewed the records of 4705 cats at least 10 years old seen at the Royal Veterinary College in London between January 2006 and June 2014. About one-third of the cats were seen at the after-hours primary care emergency service. The rest had been referred by their primary care veterinarians.

Of the 4705 cats, 975 (20.7%) were classified as hyperthyroid, including cats referred for radioactive iodine treatment. (The prevalence of hyperthyroidism in the study population was probably higher than in the general UK cat population.) Most of the cats (83.0%) were not purebred; 94.4% were neutered.

Breed, sex, neuter status, age, and referral status were all significantly associated with hyperthyroidism risk. Breeds with a lower risk for hyperthyroidism than domestic shorthair cats were Burmese, Abyssinian, Tonkinese, Persian, Siamese, British shorthair, and purebred crosses. The risk for hyperthyroidism was higher in domestic longhair cats than in domestic shorthair cats.

Female cats had a higher risk than males, and neutered cats had a higher risk than intact cats in the overall population (this association was not significant when only nonpurebred cats were analyzed). Cats aged 11 to 17—but not those 18 years and older—were at higher risk for hyperthyroidism than were 10-year-old cats.

Among cats that were not purebred or purebred crosses, coat color was not significantly associated with hyperthyroidism. However, cats with long hair were at higher risk than cats with short hair.

“In addition to supporting previously published findings regarding breed, age, and sex, our study newly identified 3 additional breeds at decreased risk of hyperthyroidism [Tonkinese, Abyssinian, and British shorthair] and found evidence for increased risk of disease in longhaired cats,” write the authors. This epidemiologic study does not explain the mechanisms behind risk associations, they add.

The investigators had hypothesized that cats with light coats would be at lower risk for hyperthyroidism than those with dark coats. The results did not support this hypothesis. The authors note that melanin is not the only factor responsible for coat color in cats, adding that the associations between breed and hyperthyroid risk might not be related to pigmentation.

The authors suggest 2 potential explanations for the increased risk of hyperthyroidism in longhaired cats. Longhaired cats could have an increased tyrosine requirement for pigment production. Cats with long hair may also have higher exposure to thyroid hormone—disrupting environmental substances because of their increased hair surface area or grooming time.

The investigators conclude that further studies are needed to show whether the lower risk for hyperthyroidism in some breeds is related to pigmentation or to other factors.

Dr. Laurie Anne Walden received her doctorate in veterinary medicine from North Carolina State University. After an internship in small animal medicine and surgery at Auburn University, she returned to North Carolina, where she has been in small animal primary care practice for over 20 years. Dr. Walden is also a board-certified editor in the life sciences and owner of Walden Medical Writing, LLC. She works as a full-time freelance medical writer and editor and continues to see patients a few days each month.