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.
