Comparison of 2 assays for measuring serum total thyroxine concentration in dogs and cats

Abstract

Background No gold standard assay for serum total thyroxine (TT4) concentration in small animals exists. The Microgenics DRI TT4 (MTT4) assay is used by most reference laboratories. Hypothesis/Objectives IDEXX Catalyst Total T4 (CTT4) and Immulite 2000 TT4 (ITT4) results will agree with MTT4 results. Animals Residual small animal sera were randomized before reanalysis (dogs, CTT4 versus MTT4: n = 176, ITT4 versus MTT4: n = 74; cats, CTT4 versus MTT4: n = 319, ITT4 versus MTT4: n = 79). Methods Validation and method comparison study. Serum TT4 concentration was measured on all analyzers. Pairwise Pearson correlation, cumulative sum linearity test, regression, and Bland‐Altman method were performed. Results CTT4 versus MTT4 in dogs: constant bias (y‐intercept) was 0.10 μg/dL (95% confidence interval [CI], 0.05‐0.15), proportional bias (slope) was 0.86 μg/dL (95% CI, 0.83‐0.89); in cats, constant bias was 0.13 μg/dL (95% CI, 0.08‐0.20) and proportional bias was 1.01 μg/dL (95% CI, 0.98‐1.03), but the test for linearity failed. Bland‐Altman plots identified increasing disagreement with increasing serum TT4 concentrations. ITT4 versus MTT4 in dogs, constant bias was 0.14 μg/dL (95% CI, 0.04‐0.22) and 0.22 μg/dL (95% CI, 0.09‐0.33) for cats; proportional bias was 0.76 (95% CI, 0.72‐0.80) for dogs and 0.71 (95% CI, 0.69‐0.74) for cats. Conclusions and Clinical Importance Differences in CTT4 and MTT4 results affect interpretation at higher serum TT4 concentrations. The ITT4 proportional bias will underestimate serum TT4 concentrations in dogs and cats, compared to MTT4. Serial TT4 measurements should be done using the same assay.

Treatment failure in hyperthyroid cats after radioiodine (I‐131) injection

“…Because of the large retrospective nature of this study, it was not possible to provide details of measurement methods in every case. It is recommended that the same assay technique and laboratory are used when monitoring TT4 concentration and a recent study has suggested significant differences between results using different methods, particularly at higher hormone concentrations 19 . In addition, when TT4 concentration was reassessed, this was sometimes tested as part of investigations when the cat was presented for another illness, meaning that TT4 concentration might have been falsely low in some cases, due to nonthyroidal illness.…”

Autoimmune polyendocrine syndrome in a standard poodle with concurrent non‐endocrine immune‐mediated diseases

“…The initial assessment of thyroid status in this case was made based on a total T4 measurement using the IDEXX catalyst in‐house analyser, without the concurrent measurement of canine thyroid stimulating hormone (TSH). While this analyser is found to be accurate for serum total T4 (TT4) concentrations within the reference range 31 . it is conceivable that thyroid hormone autoantibodies, which have reported to be detected in 6.3% of cases diagnosed with hypothyroidism, could have resulted in a falsely normal TT4 result via assay interference 32 .…”

“…While this analyser is found to be accurate for serum total T4 (TT4) concentrations within the reference range. 31 it is conceivable that thyroid hormone autoantibodies, which have reported to be detected in 6.3% of cases diagnosed with hypothyroidism, could have resulted in a falsely normal TT4 result via assay interference. 32 A normal TSH would have increased confidence in the diagnosis of euthyroidism at that timepoint, while an elevated TSH may have suggested the patient was in the early stages of the disease, prompting further assessment, for instance measurement of free-T4.…”

Serum Total Thyroxine Evaluation in Critically Ill Feline Patients

“…Concentrations of tT4 in small animal serum samples were stable from 3 months to 2 years when stored at −20 °C [ 9 , 17 , 18 , 19 , 20 ], while human serum tT4 demonstrated a stability for almost 7 years at −80 °C [ 21 ]. Our serum samples were stored from a minimum of 3 months to a maximum of 4 years at −80 °C.…”