Biological variation of total thyroxine (T4), free T4 and thyroid-stimulating hormone in 11 clinically healthy cats

Abstract

Objectives The aim of this study was to investigate the biological variation of total thyroxine (T4), free T4 (fT4) and thyroid-stimulating hormone (TSH) in 11 clinically healthy cats aged between 3 and 15 years old, in Sydney, Australia. Methods Blood was collected weekly for up to 6 weeks and serum T4, fT4 and TSH concentrations were analysed using canine-specific reagents. Restricted maximum likelihood was used to estimate within-subject, between-subject and analytical variance components, which were recorded in terms of the related coefficients of variation. The index of individuality and reference change values were then calculated for each analyte. Results T4 and TSH had intermediate individuality, indicating both subject-based and population-based reference intervals (RIs) could be used, with the knowledge that population-based RIs are suboptimally sensitive. fT4 had high individuality, indicating subject-based RIs are more appropriate than population-based RIs. Conclusions and relevance This study has demonstrated that subject-based RIs could be more sensitive than population-based RIs for the diagnosis of thyroid dysfunction in cats.

Biological variation of serum thyrotropin and thyroid hormones concentrations determined at 8‐week intervals for 1 year in clinically healthy cats

“…The CV G for free T 4 varies substantially between studies and was less in the current year-long study (7.1%) compared to the previous 6-week-long studies by us using the same methodology (13.4%) 10 and others using a different methodology (31.8%). 11 CV G for total T 3 was less than our previous short-term study. 10 Potential explanations in differences in CV G between studies include differences in biological variability with sampling interval and overall duration of the study, differences in homogeneity between study populations of cats, trends towards development of subclinical thyroid disease or disease affecting thyroid hormones, different assay methodologies used for total and free T 4 , and sampling at different geographical locations during different seasons.…”

“…These studies observed greater interindividual variation relative to intraindividual variation with intermediate to high IoIs. [9][10][11] Collectively, these shortterm studies of biological variation suggested that it is important for clinicians to consider using RCVs to appreciate important biological changes in serial measurement of TSH and thyroid hormones, especially in cats with clinical concern for emerging thyroid disease.…”

“…24 All cats in this study had been gonadectomized, and therefore none were experiencing seasonal changes in gonadal hormones or reproductive states that can account for some of the seasonal variation in thyroid hormones due to the changing metabolic demands on breeding animals. It is worth noting that all three studies evaluating thyroid hormones over shorter sampling intervals and frequency were performed at different geographical locations and seasons, including Fall in North America, 10 Winter in Northern Europe, 9 and Autumn-Spring in Australia 11 and multiyear studies to more robustly test for circannual patterns in thyroid hormones, may provide insight to differences in biological variability measured, especially in the case of free T 4 .…”

“…[4][5][6] Analytes with high intraindividual and low interindividual varia- Previous studies have shown that concentrations of serum total thyroxine (T 4 ), free T 4 , and TSH, evaluated weekly over relatively short periods of time (5-6 weeks) in cats, had greater interindividual than intraindividual biological variation. [9][10][11] Previous studies of biological variation in dogs and people have shown that differences in the intraindividual and interindividual variability of some analytes may, in part, be influenced by the length of time over which an analyte is assessed. 12,13 Some investigators have suggested that if the frequency of serial analyte measurement is long enough to detect variation around the homeostatic setpoint, then the frequency of measurement and time span of the study should not have a significant impact on measured biological variation of a given analyte.…”

Biological variation of biochemical analytes determined at 8‐week intervals for 1 year in clinically healthy cats

“…Considerable data exist describing biological variation of biochemical analytes has been extensively described in people, 2,26,27 but less so in veterinary species, including cats. [7][8][9][10]16,28 This study builds upon previous work that evaluated biological variation weekly or with variable frequency over 42-100 days 7-10 by determining the biological variation of 19 biochemical analytes in clinically healthy cats sampled every 8 weeks over 1 year.…”