Subclinical hypothyroidism: a 'laboratory-induced' condition?

OBJECTIVE In current literature and guidelines, there is a tendency to define absolute TSH concentrations at which patient follow-up or even pharmaceutical intervention should be initiated. As TSH concentrations depend on the analytical method/platform used for TSH quantification, absolute cut-off values may pose threats for uniform clinical decision-making. In this study we therefore set out to clarify to what extent the method/platform and the reference values applied for TSH influence the clinical interpretation of thyroid parameters. DESIGN AND METHODS We retrospectively analyzed anonymous TSH results from the Dutch external quality assessment program (EQAS) in relation to reference values advised by different manufacturers. We also examined TSH/free thyroxin (fT4) reference ranges and prevalence of thyroid pathology among different Dutch laboratories, including four cases in which a switch in the measuring platform was made. RESULTS Our data show that interpretation of thyroid parameters is not only influenced by between-method/platform variation, but is also substantially affected by the variation in TSH/fT4 reference intervals applied in individual laboratories. Additionally, we show that the transition to a novel analytical method/platform can result in a shift in the prevalence of thyroid pathology, especially for subclinical hypothyroidism. CONCLUSIONS Subclinical hypothyroidism can be a 'laboratory-induced' condition. This is an undesirable situation in regard to the clinical implications such a diagnosis can have for patients.