Isolated Elevation of Free Triiodothyronine in Euthyroid Patients: A Diagnostic Pitfall

Objective Free triiodothyronine (FT3) is frequently ordered in the evaluation of thyroid dysfunction. Despite its theoretical appeal as the biologically active thyroid hormone, concerns persist regarding its analytical reliability and clinical utility. We aimed to describe a case series of clinically and biochemically euthyroid patients with isolated elevations in FT3 and to discuss the implications for clinical decision-making. Methods We conducted a retrospective descriptive case series of eighteen adult patients with elevated FT3 in the presence of normal thyrotropin (TSH), free thyroxine (FT4), and total triiodothyronine (TT3), and no clinical evidence of hyperthyroidism. Results Mean age was 50.3 ± 14.4 years. TSH (median 1.18 [0.74–2.68] mIU/mL), FT4 (0.88 ± 0.14 ng/dL), and TT3 (112 ± 18.5 ng/dL, n=12) were within reference ranges. FT3 was elevated in every case (median 3.95 [3.70–4.40] pg/mL; reference 1.5–3.5). None of the patients demonstrated clinical hyperthyroidism, and isolated FT3 elevation persisted on repeat testing while TSH, FT4, and TT3 remained within reference ranges. Conclusion Isolated FT3 elevation in euthyroid patients is most probably due to an analytical artifact that may mislead clinicians. FT3 should be interpreted cautiously and within the context of TSH and other thyroid hormone measurements. When assessment of triiodothyronine is required, TT3 may currently provide greater clinical reliability.