Optimization of the thyroid panel for diagnostic purposes: Thyrotropin cut-off values for the reflex addition of free thyroxine

Background-aim Measurement of serum thyrotropin is currently the recommended test for the screening of thyroid dysfunction, while serum free thyroxine is kept as a reflex test. In our laboratory, the strategy followed in adult individuals from Primary Care includes a ‘safety margin’ for requests with a thyrotropin ≤1.0 or ≥4.0 mIU/L (normal: 0.35–4.95 mIU/L). Our aim was to optimize the thyrotropin cut-off values for the addition of free thyroxine and, based on these cut-offs, to retrospectively analyze avoidable free thyroxine measurements and possible adverse clinical consequences. Methods Retrospective observational study performed in a tertiary care hospital between 2013 and 2018. We considered all laboratory requests for screening of thyroid dysfunction (TD) in adult patients from Primary Care. Requests from patients with a previous diagnosis of thyroid disease or pregnant women were excluded. Different receiver operating characteristic (ROC) curves were performed and the obtained thyrotropin cut-off values were compared. Economic savings were assessed considering the current cost of free thyroxin assays in our laboratory. Results From a total of 554,529 TD protocols included, 119,504 requests had free thyroxine added. From the ROC curve that enables ≥95% of abnormal free thyroxine results to be detected, the thyrotropin values obtained were ≥4.58 mIU/L and ≤0.94 mIU/L. These thyrotropin cut-off values would lead to a saving of 22.7% of annual free thyroxine measurements without adverse clinical consequences. Discussion Setting optimized thyrotropin cutoffs for reflex testing of free thyroxine would reduce the need for this test. Clinical laboratories need to offer not only true results, but also become the cornerstone in the optimization of resources.