Comparative Analysis of Elevated Serum Total and Free Triiodothyronine Results Obtained by Immunoassays

BACKGROUND: Measurement of triiodothyronine (T3) levels is important for the laboratory diagnosis and management of certain cases of thyrotoxicosis and for identifying conditions involving altered peripheral conversion of thyroxine (T4) to T3. However, whether total T3 (TT3) or free T3 (FT3) should be measured remains unclear. METHODS: In this study, we performed a retrospective analysis of the results from 605 blood samples from 465 patients, in which thyroid-stimulating hormone (TSH), free T4 (FT4), TT3, and FT3 levels were measured in parallel. All the parameters were quantified using a widely used, fully automated electrochemiluminescence immunoassay platform, and reference intervals were based on those provided by the diagnostic test manufacturer. RESULTS: Despite a strong correlation between the TT3 and FT3 values (T3T ≤ 200 ng/dL: b = 0.0020, 95% confidence interval [CI]: 0.0019-0.0021, r = 0.841; T3T > 200 ng/dL: b = 0.0054, 95% CI: 0.0044-0.0064, r = 0.855), among 41 patients with elevated T3 values, 13 showed elevation of either TT3 or FT3, with a mild discordance between these values. Analysis of these discordant cases revealed a predominant pattern in which the FT3 levels better reflect clinical features and TSH and FT4 results than TT3 levels. CONCLUSIONS: The TT3 and FT3 results were generally well correlated, with elevated FT3 demonstrating performance superior to that of elevated TT3. Future studies should validate these findings across diverse populations with specific reference intervals and multiple immunoassay platforms.