Continuous age-dependent reference ranges for thyroid hormones in neonates, infants, children and adolescents established using the ADVIA® Centaur™ analyzer

In neonates, infants, children and adolescents serum concentrations of the thyroid hormones show a clear age dependency. Currently, agedependent thyroid hormone reference ranges for the new ADVIA(R) Centaur immunoanalyzer are not available. Thyroidstimulating hormone (TSH), free thyroxine (FT4), total thyroxine (T4), free triiodothyronine (FT3) and total triiodothyronine (T3) were determined in the sera of 460 healthy children aged 0 days to 18 years from an inland (n=308) and a seaside city (n=152) using the Bayer ADVIA(R) Centaur(TM) analyzer. Square root functions defining the upper and lower limit of the continuous agedependent reference range and the median line for each thyroid hormone were estimated applying parametric regression technique. Continuous agedependent reference ranges show a better fit to the obtained data than discontinuous reference ranges. The median TSH and FT4 concentrations decrease within the first year of life and are nearly constant until 18 years of age. The median T4 decreases within the entire age interval, whereas the median FT3 is constant. The median T3 increases within the first year of life and decreases afterwards. The calculated continuous reference ranges for T4, T3 and partly FT4 are consistent with published reference ranges. However, the reference ranges obtained for FT3 and partly TSH differed considerably, compared to published reference ranges. No significant residence or sexspecific effects on ageadjusted hormone concentrations were found except for a slight residencespecific effect on ageadjusted FT4, which was probably due to assay imprecision. Due to the lack of international standardization, the assayspecific evaluation of reference ranges very important. Continuous agedependent reference ranges comply better with the biological reality and are more reliable than discontinuous reference ranges.