Endocrine Abstracts

Introduction: Free thyroxine (FT4) assays are available on different commercial platforms where manufacturers provide different reference ranges. The lower limit of normal varies considerably between 8 and 13.9 pmol/l depending on the FT4 assay being used. There is concern about the validity of these reference ranges in paediatric populations, especially for the Roche FT4 assay; here there is a tendency for FT4 results to be below age-related lower limits of 13.9 or 13.6 pmol/l. As a result, children with low FT4 levels with a normal TSH are at risk of being over- or under-investigated for ‘central’ hypothyroidism.

Methods: Roche Elecsys cobas and Abbott Architect FT4 immunoassays were used. Serum samples (n = 59) collected from paediatric patients found to have a normal TSH with a FT4 below the lower limit of normal on the Roche FT4 assay were analysed for FT4 using the Abbott assay (lower limit = 9 pmol/l). Medians were compared using Wilcoxon signed-rank test. A comparison of FT4 results using the two lower limits of normal for each assay was performed.

Results: The age range of the paediatric patients was from 9 months to 17 years (median age = 14 years). Whilst the median FT4 concentrations were statistically different (P < 0.001), they were clinically comparable with FT4 levels of 12.5 pmol/l and 11 pmol/l for the Roche and Abbott assays respectively. All 59 samples were found to have a FT4 within the reference range (≥9 pmol/l) for the Abbott assay.

Conclusions: Hypothyroxinaemia without a raised TSH can result from a number of disorders but most important to exclude is central hypothyroidism which may result from a lesion near the pituitary or hypothalamus. The decision to investigate is critically dependent on the FT4 results. We have shown that with two commonly used assays there is only a small variation in actual FT4 concentrations, but a major difference in manufacturers’ reference ranges. All patients in this cohort have euthyroid results with one assay (Abbott) and abnormal results with the other (Roche). This is an unsatisfactory situation and requires resolution.