Thyroid guidelines - are thyroid-stimulating hormone assays fit for purpose?

Most thyroid-stimulating hormone (TSH) assays now have the sensitivity required by thyroid guidelines and allow the reliable identification of patients with both overt and subclinical hyperthyroidism. Clinical guidelines usually quote decision limits for TSH, but often ignore the issue of whether variability in bias between assays should be considered when such decision limits are implemented. Clinicians and laboratories should appreciate that these decision limits arise largely from historical data that used TSH assays with poorly defined bias. It is thus unlikely that laboratories will be able to apply an appropriate method-related bias adjustment to these TSH cut-offs. Clinicians should appreciate that TSH decision limits should thus be regarded as typical target figures rather than an absolute cut-off and thus can be applied with some degree of flexibility. There is currently insufficient evidence to justify a significant lowering of the upper reference limit for TSH, but fine-tuning of current reference ranges is required since there appears to be no association between the ranking of the assay bias in the UK National External Quality Assessment Service scheme and the manufacturers' quoted reference ranges. There is room for further improvement in TSH assays and this can best be achieved if manufacturers, laboratories and clinicians work together to produce TSH assays and reference ranges that show closer agreement between methods. Until this is achieved, future studies that examine the relationship of TSH with symptoms and treatment should ensure that sufficient information is included in the publication to allow the method related bias of the TSH assay to be clearly described.