Solving the dilemma: what to do as a simple clinician
Serum TSH is universally considered the best laboratory test to evaluate thyroid function. Current TSH reference ranges are 0.35.0 mU/l, but a narrower range (0.42.5) has been recently advocated, which better defines normal thyroid function. The question is still debated and matter of controversy, since it has been argued that with the narrower range, an additional 10% of the general population could be diagnosed as mild or subclinically hypothyroid, although most of them do not have thyroid disease or do not require thyroid hormone therapy. How to establish a TSH reference range, the health outcomes of borderline-normal serum TSH and the meta-analysis of studies on subclinical thyroid diseases are discussed in other lectures of this Symposium. This presentation will be focused on some practical points useful in the current clinical practice. i) The within-individual variation of serum TSH is lower than between-individual variation, implying that any given euthyroid individual will have a narrower TSH range than that of the general population. ii) Any serum TSH value must be correctly interpreted in the peculiar clinical, laboratory and instrumental context including exclusion of coexistent non-thyroidal illness or technical artifacts. iii) When employed to check the adequacy of therapy in hypo- and hyperthyroid conditions, the target value of serum should be within a narrower range (0.52.0), with the possible exception of elderly patients. iv) Although still not currently available, the clinician should be aware that different reference ranges are found in particular conditions (elderly, iodine deficiency, pregnancy) and clinical decisions on when and how to treat should be taken accordingly.
In summary, although TSH is a test associated with a good specificity, sensitivity and predictive value in the evaluation of thyroid function, physician must not consider only the laboratory result, but should employ all of the other skills of medical practice, always remembering the Hippocrates aphorism: Vita brevis, ars longa, occasio praeceps, experimentum periculosum, iudicium difficile.