Sabatino B.R., Kirk C.A., Rohrbach B.W., et al.
Hyperthyroidism is common among older cats, but its pathogenesis remains poorly understood. Siamese and Himalayan cats have a reduced risk of hyperthyroidism than other cat breeds. Tyrosine is the amino acid precursor for thyroxine and melanin. Earlier studies reported tyrosine as a limiting amino acid in some cat foods resulting in poor coat melanin production in dark- coated cats. Because Siamese and Himalayan cats have unique tyrosine metabolism responsible for a pointed coat color, this study evaluated the relationship between tyrosine status and coat color in hyperthyroid cats. In addition, key cofactors in thyroid hormone metabolism (iodine and selenium) were evaluated as co- dependent variables. The objective of this study was to determine if tyrosine, phenylalanine, iodine, or selenium levels are altered in hyperthyroid cats compared to normal cats and if light or pointed coat color is protective with greater tyrosine availability due to lowered use for melanin production.Twenty-seven client-owned cats with (n=12) and without (n=15) hyperthyroidism were studied. Coat color (9 white or pointed; 18 dark), breed, and diet history were recorded. Whole blood was collected for CBC, serum chemistry, total thyroxine (TT4), serum iodine, serum selenium, and plasma amino acid determination in fasted cats. A mixed model ANOVA with cat and group included as class variables was used to evaluate the relationship between group study factors (significance = P < 0.05).
Chemistry and CBC values were similar among groups except for TT4 levels. Tyrosine, phenylalanine, iodine, and selenium lev- els were not significantly different among light or dark cats or cats with or without hyperthyroidism. Altered tyrosine metabo- lism associated with coat color does not explain the reduced risk of hyperthyroidism in pointed or light coat colored cats.