Third Year Internal Medicine Resident Purdue University West Lafayette, IN, United States
Background: Free thyroxine (fT4) measured by chemiluminescent immunoassay (fT4CLIA) and equilibrium dialysis (fT4ED) in known euthyroid dogs with nonthyroidal illness (NTI) needs investigation.
Objectives: 1) To determine the agreement between fT4CLIA and fT4ED in serum samples from known euthyroid dogs with NTI, 2) to evaluate the diagnostic sensitivity (correctly classify a dog as euthyroid) of each fT4 assay when TT4 concentration is normal and low, and 3) to evaluate concordant and discordant fT4CLIA and fT4ED results. Animals: Thirty-eight euthyroid dogs with NTI yielding 116 serum samples.
Methods: Method comparison study. In 116 serum samples, serum fT4 concentration was measured by fT4CLIA and fT4ED, and agreement was assessed by a Bland-Altman plot. In 38 dogs, the diagnostic sensitivity of each fT4 assay was calculated as the proportion of dogs having a normal serum fT4 concentration when the serum TT4 concentration was normal and low.
Results: Assay comparison showed agreement, with no significant systematic or proportional bias (Figure 1). The diagnostic sensitivities of the fT4CLIA and fT4ED were 94% (95% confidence interval (CI), 69.8-99.8%) and 100% (95% CI, 79.4-100%), respectively, when the serum TT4 concentration was normal, and 45% (95% CI, 24.4-67.8%) and 41% (95% CI, 20.7-63.6%), respectively, when the serum TT4 concentration was low. The fT4CLIA and fT4ED were concordant in 77% of dogs (Figure 1). Conclusions and Clinical Importance: In dogs with NTI having a low serum TT4 concentration, fT4CLIA and fT4ED should be interpreted cautiously as they are likely to be low and incorrectly classify a dog as hypothyroid.
Assay comparison showed agreement, with no significant systematic or proportional bias. The diagnostic sensitivities of the fT4CLIA and fT4ED were 94% (95% confidence interval (CI), 69.8-99.8%) and 100% (95% CI, 79.4-100%), respectively, when the TT4 concentration was normal, and 45% (95% CI, 24.4-67.8%) and 41% (95% CI, 20.7-63.6%), respectively, when the TT4 concentration was low. The fT4CLIA and fT4ED were concordant in 77% of dogs.
