052 - Reference Intervals in Endocrinology: the case of TSH
Autor(s): R.M. Dorizzi M. Nizzoli
Issue: RIMeL - IJLaM, Vol. 5, N. 3-S1, 2009 (MAF Servizi srl ed.)
Page(s): 52-61
Summary
The values of all the laboratory tests have no meaningwithout comparison to a reference that allows a judgmentand therefore an action. The commonest reference in thelaboratory are the reference intervals (RI) (individual referencevalues, group reference values and decision limits)that allow the interpretation of the tests results and themedical decision making. The case of TSH is particularlysignificant of the several considerations that must be takenin account in the definition of the RI and of the consequencesof the lack of coordination between laboratoryand clinics in the calculation of the limits and especially theupper one. The 2002 NACB guidelines recommended arevision of the TSH reference interval and stated that thetraditional upper limit (around 4-5 mU/L) does not allowto discriminate euthyroidism from mild hypothyroidism.Really, the Whickham survey, carried out between 1972and 1974, reported that TSH concentration higher than 2mU/L was associated to higher risk of hypothyroidism.This paper opened a wide debate about the opportunity2 RIMeL / IJLaM 2009; 5 (Suppl.)and the implication of lowering the upper limit down to2.5-3 mU/L. In the last decade the performance of theanalytical methods changed a lot with relevant consequenceson the measured concentration. Important trials, suchas Colorado Thyroid Disease Prevalence and NationalHealth and Nutrition Examination Survey (NHANES III),have been carried out using a method that has been suddenlyremoved from the market. A narrative review ofliterature shows that: 1) the comparability of TSH resultsof the methods marketed in Europe is low; 2) the upperTSH limit calculated using the methods available in Europeare between 3.7 and 4.5 mU/L; 3) the indirect methodscan be used for calculating the TSH reference intervals.According to our data, the TSH upper limit in our regionusing Centaur (Siemens Diagnostics) analyzer is 3.7 mU/Land using Modular (Roche) analyzer is 4.3 mU/L.
Key-words: Reference intervals, CLSI, endocrinology, TSH,Indirect methods.