In September 2023, Amy Riek, MD and fellow Washington University School of Medicine colleagues Yanchun Lin, PhD; Ann Gronowski, PhD; and Christopher Farnsworth, PhD of the department of Pathology and Immunology, had their journal article titled “Limited Utility of Free Triiodothyronine Testing” published in The Journal of Applied Laboratory Medicine.
When thyroid stimulating hormone (TSH) is suppressed and free thyroxine (fT4) is normal or decreased, this raises concern for isolated T3 thyrotoxicosis, which can be identified with free triiodothyronine (fT3) testing. However, standards for fT3 reflex testing have not yet been determined, and many fT3 levels do not add useful clinical information.
In their research, the authors intend to identify a TSH cutoff which would maximize the utility of measuring fT3 when fT4 is normal or decreased.
Dr. Riek and colleagues state that “T3 thyrotoxicosis is a relatively rare diagnosis and fT3 measurement has limited utility in the vast majority of patients.” Furthermore, they suggest that an improvement to clinical utility and reduction of unneeded testing could be provided by a fT3 reflex for patients with TSH <0.01 μIU/mL with concurrently normal or low fT4.
Yanchun Lin, Amy E Riek, Ann M Gronowski, Christopher W Farnsworth, Limited Utility of Free Triiodothyronine Testing, The Journal of Applied Laboratory Medicine, Volume 8, Issue 5, September 2023, Pages 847–855, https://doi.org/10.1093/jalm/jfad032