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Background: The aim of this study was to determine the reference intervals (RIs) for thyroid stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3) and FT3/FT4 ratio using indirect methods. Methods: We analyzed 1256 results TSH, FT4 and FT3 collected from a laboratory information system between 2017 and 2021. All measurements were performed on a Siemens ADVIA Centaur XP analyzer using the chemiluminescent immunoassay. We calculated the values of the 2.5th and 97.5th percentiles as recommended by the IFCC (CLSI C28-A3). Results: The RIs derived for TSH, FT4, FT3 and FT3/FT4 ratio were 0.34–4.10 mIU/L, 11.3–20.6 pmol/L, 3.5–6.32 pmol/L and 0.21–0.47, respectively. We found a significant difference between calculated RIs for the TSH and FT4 and those recommended by the manufacturer. Also, FT3 values were significantly higher in the group younger than 30 years relative to the fourth decade (5.26 vs. 5.02, p=0.005), the fifth decade (5.26 vs. 4.94, p=0.001), the sixth decade (5.26 vs. 4.87, p<0.001), the seventh decade (5.26 vs. 4.79, p<0.001) and the group older than 70 years old (5.26 vs. 4.55, p<0.001). Likewise, we found for TSH values and FT3/FT4 ratio a significant difference (p <0.001) between different age groups. Conclusions: The establishing RIs for the population of the Republic of Srpska were significantly differed from the recommended RIs by the manufacturer for TSH and FT4. Our results encourage other laboratories to develop their own RIs for thyroid parameters by applying CLSI recommendations., Uvod: Cilj ove studije bio je da se odrede referentni intervali (RI) tireotropnog hormona (TSH), slobodnog tiroksina (FT4), slobodnog trijodotironina (FT3) i odnosa FT3/FT4 indirektnom metodom procene referentnih intervala. Metode: Analizirali smo 1256 dobijenih vrednosti TSH, FT4 i FT3 u periodu između 2017. i 2021. godine. Rezultate smo uzeli iz laboratorijskog informacionog sistema. Sva merenja su izvedena na Siemens ADVIA Centaur XP analizatoru pomo}u hemiluminiscentnih imunohemijskih testova. Izračunali smo vrednosti 2,5-og i 97,5-og percentila prema preporuci IFCC-a (CLSI C28-A3). Rezultati: Procenjeni RI za TSH, FT4, FT3 i odnos FT3/FT4 bili su 0,34-4,10 mIU/L; 11,3-20,6 pmol/L; 3,5-6,32 pmol/L i 0,21-0,47. Utvrdili smo značajnu razliku između izračunatih RI za TSH i FT4 i onih koje preporučuje proizvođač. Takođe, vrednosti FT3 bile su značajno ve}e u grupi mlađoj od 30 godina u odnosu na četvrtu deceniju (5,26 vs. 5,02; p = 0,005), petu deceniju (5,26 vs. 4,94; p = 0,001), šestu deceniju (5,26 vs. 4,87; p<0,001), sedmu deceniju (5,26 vs. 4,79; p<0,001) i grupu stariju od 70 godina (5,26 vs. 4,55; p<0,001). Isto tako, za vrednosti TSH i odnos FT3/FT4 pronašli smo značajnu razliku (p <0,001) između različitih dobnih grupa. Zaključak: Procenjene vrednosti referentnih intervala za TSH i FT4 za stanovništvo Republike Srpske značajno su se razlikovale od preporučenih RI od strane proizvođača. Naši rezultati podstiču druge laboratorije da izrade sopstvene RI za parametre štitne žlezde primenom CLSI preporuka.

Background: The aim of this study was to determine the reference values for thyrotropin (TSH), thyroid hormones (total and free thyroxine, T4 and fT4; total and free triiodothyronine, T3 and fT3), thyroglobulin (Tg) and thyroid antibodies (thyroid peroxidase, TPOAb and thyroglobulin antibody, TgAb) in the population of the Republic of Srpska. Methods: A total of 250 euthyroid subjects were enrolled in this study. A direct method for choosing reference subjects was used to establish reference intervals. The hormones and thyroid antibodies were measured by an electrochemiluminescence immunoassay method (ECLIA, Roche Diagnostics, Mannheim, Germany). We calculated the re f erence intervals by MedCalc, version 12.1.4.0 (MedCalc software, Belgium) as recommended by the IFCC (CLSI C28-A3). Results: Using guidelines recommended by the National Academy of Clinical Biochemistry (NACB) and based on standard statistical approaches, the reference intervals derived for TSH, fT4, T4, fT3, T3 were 0.75-5.32 mIU/L, 12.29-20.03 pmol/L, 73.49-126,30 nmol/L, 4.11-6.32 pmol/L, 1.15-2.32 nmol/L and for Tg, TPOAb, TgAb were 3.63-26.00 mu g/L, lt 18.02 mIU/L, lt 98.00 mIU/L, respe ctively. We found a significant difference (p lt 0.05) in TSH and fT3 values between different age groups as well as in T4, fT4 and fT3 values between ge nder groups. Conclusions: The established reference values for the population of the Republic of Srpska were significantly different from the values recommended by the manufacturer of reagents (Roche Diagnostics). Our results showed that a laboratory needs to establish its own reference values in order to set up a proper diagnosis, as well as to treat patients successfully.