Abstract. The performances of a radioimmunoassay method, a chemiluminescent immunoassay method, and a chemiluminescent-enzyme immunoassay method were evaluated for the analysis of cortisol and total thyroxine in blood samples obtained from dogs, cats, horses, and humans (reference samples). The analysis of cortisol in human and animal samples exhibited good precision, linearity, and recovery. The 3 methods gave comparable values for the ACTH-induced increase and the dexamethasone-induced decrease in cortisol concentrations in animal samples. The recoveries of total thyroxine from human samples, analyzed by the 3 methods, were comparable. However, the basal total thyroxine concentrations determined by the chemiluminescent immunoassay method were 30-40% lower than those determined by the radioimmunoassay and the chemiluminescent-enzyme immunoassay methods in animal samples. In both human and animal samples, the plot of thyroxine values obtained by the radioimmunoassay method against those obtained by the chemiluminescent immunoassay method or the chemiluminescent-enzyme immunoassay method was linear. However, although the slope of the radioimmunoassay versus chemiluminescent-enzyme immunoassay curve was close to unity, the slope of the radioimmunoassay versus chemiluminescent immunoassay curve was 0.6. This result suggests that, compared with the radioimmunoassay method, the chemiluminescent immunoassay method underestimated thyroxine values in animal samples but not in human samples. Although all 3 methods yielded comparable changes in thyroxine concentrations in response to thyroid stimulating hormone, they did not yield comparable thyroxine concentrations in response to T 3 suppression in dogs and cats.The measurement of hormones in plasma or serum is commonly made by radioimmunoassay (RIA) using 125 I or 3 H-labeled hormones. RIA is a simple, rapid, and sensitive method for the measurements of thyroxine (T 4 ) and cortisol in both human and animal samples. The use of radioactive isotopes, however, requires appropriate licensing, storage facility, safety devices, and record keeping that increase the operating cost. Recently, several nonradioactive immunoassay methods such as a chemiluminescent immunoassay (CIA) a and a chemiluminescent-enzyme-immunoassay (CEIA) b methods have been developed for the analysis of hormones in blood samples. The detection limits reported for the nonradioactive methods have been reported to be comparable to or better than those for RIA methods.2 The nonradioactive methods, however, have been validated only for human samples, and veterinary use of these assays has not yet been established.The diagnosis of thyroid and adrenocortical disor- Received for publication May 24, 1996. 261 ders in animals requires appropriate analytical tests of pituitary, thyroid, and adrenal cortex. The plasma or serum concentrations of hormones 8,11 are widely used to diagnose thyroid and adrenocortical diseases in animals. An abnormality in the regulation of the thyroid gland by the pituitary can be d...
