Biological variation of thyroid hormone in cats and application to diagnosis of feline thyroid disease

This thesis provides a comprehensive review of the current literature regarding hyperthyroidism in cats, including epidemiology, endocrinology, pathogenesis, clinical manifestation, current testing methods and biological variation of thyroid hormones. The study explores the utility of reagents used to determine thyroid hormone concentrations in feline patients, sources of variation that can affect the diagnostic utility of these analytes, and whether improvements can be made to the current diagnostic approach. Biological variation of thyroid hormones is a key component explored, and the study assesses intra- and inter-individual biological variation of thyroxine (T4), free thyroxine (fT4) and thyroid stimulating hormone (TSH) concentrations in clinically healthy cats to assess whether subject-based reference intervals are a more sensitive method of diagnosing and monitoring feline thyroid disease compared to the traditionally used population-based reference intervals. Blood was collected weekly for up to six weeks and restricted maximum likelihood (REML) was used to estimate within-subject, between-subject and analytical variance components, which were recorded in terms of the related coefficients of variation (CVI, CVG, and CVA) respectively. The index of individuality and reference change values were then calculated for each analyte. Thyroxine and TSH had intermediate individuality, indicating both subject-based and population-based reference intervals could be used, with the knowledge that population-based reference intervals are sub-optimally sensitive. Free T4 had high individuality, indicating subject-based reference intervals are more appropriate than population-based reference intervals. This study has demonstrated that subject-based reference intervals may be more sensitive than population-based reference intervals for the diagnosis of thyroid disease in cats