Background In our hospital, physicians noticed high free T4 (fT4) concentrations without complete suppression (<0.01 mU/L) of thyroid stimulating hormone (TSH) in blood samples of patients at the outpatient clinic, which appeared to occur more often following the introduction of a new fT4 immunoassay. fT4 concentrations above the reference interval without complete suppression of TSH (=discordance) may be explained by analytical issues (not further explained here), the use of incorrect reference intervals, or patient-related factors (e.g. medication, population). The aim of the study was to establish the contribution of the possible factors involved.

Methods First, reference intervals of the current Cobas (Roche) and former Delfia (Perkin Elmer) fT4 assay were re-evaluated using blood samples of healthy volunteers. Then, TSH and fT4 requests and the frequency of discordant pairings of patients from Amsterdam UMC were analysed in retrospect. Finally, a literature search was performed to assess whether time of blood draw and time of ingestion of levothyroxine (L-T4) may contribute to higher fT4 concentrations in patients using L-T4.

Results The original reference intervals belonging to the Delfia (10-23 pmol/L) and Cobas (12-22 pmol/L) assay were confirmed. The Delfia (n=103, 3.2%) and Cobas cohort (n=145, 4.3%) showed comparable frequencies of discordance. Interestingly, 80% of the discordant results belonged to L-T4 users. Review of the literature showed that fT4 concentrations vary depending on time of blood draw and, therefore, time of L-T4 intake. Besides, fT3/fT4 ratios are different in L-T4 users vs healthy controls and indicate an adapted regulation of the thyroid axis in patients using L-T4.

Discussion and conclusion Increased fT4 concentrations without complete suppression of TSH could not be explained by incorrect reference intervals, but may be explained by L-T4 intake. Physicians and laboratory specialists should be aware of this phenomenon to avoid questioning the assay’s performance or, worse, adapting L-T4 dose in patients.