Endocrine Abstracts

JOINT2855

Introduction: Measurement of free thyroxine (fT4) is challenging because of its picomolar concentrations, intricate equilibrium with binding proteins, and the minute fraction of “free” relative to total hormone. Method related differences in fT4 measurement may be under-appreciated by clinicians and laboratories alike, which has implications for the clinical management of patients with thyroid disease. To capture these differences, we undertook a multi-method study comparing fT4 immunoassays on the platforms most widely used in Australia: Roche Cobas, Abbott Architect, Siemens Atellica, Beckman DxI and Ortho Vitros.

Methods: fT4 was measured in 94 serum samples spanning the clinical dynamic range, from severe hypothyroidism to euthyroid status and severe hyperthyroidism. For completeness, TSH and fT3 were also measured in parallel. Passing & Bablok linear regression was performed relative to the Roche assay which was chosen as an arbitrary gold standard.

Results: Passing & Bablok regression generated the following equations: Abbott fT4 = 0.5503*Roche fT4+3.944; Siemens fT4 = 0.945*Roche fT4+1.549; Beckman fT4 = 0.800*Roche fT4 -0.331 and Vitros fT4 =1.204*Roche fT4-1.685. Method related differences in fT4 results were subtle in the hypothyroid range, became more pronounced with increasing fT4, and were marked in the hyperthyroid range. There were also notable differences in the analytical range of each assay, with some (eg Siemens) capturing a more than two-fold wider linear range than others (eg Abbott). In comparison, method-related differences in TSH were minor and differences in fT3, though more conspicuous than TSH, were smaller than fT4 method-related differences.

Discussion: Method related differences in fT4 lead to conspicuous differences in the apparent biochemical severity of hyperthyroidism. In patients with severe hyperthyroidism, measured fT4 concentrations on the Siemens, Roche and Ortho platforms were between 2-3 times higher than the Abbott platform, which showed only modest fT4 levels and was conspicuously flat in the hyperthyroid range. Interestingly, the one-step fT4 immunoassays (Roche, Siemens and Ortho) were much more closely aligned and read significantly higher than the two-step assays (Abbott and Beckman). Our study points to the clinical value of standardising fT4 methods and highlights the risks of monitoring hyperthyroid patients across laboratories using different methods. Method related differences may masquerade as biochemical worsening or improvement of disease, even in the absence of true change, and may confound dosing of anti-thyroid medication and other clinical decision making.