[Reference values for blood levels of free thyroxine, free triiodothyronine and thyroxine-binding globulins in euthyroid children. Method: luminescence-enhanced enzyme immunoassay]

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, age-dependent thyroid hormone reference ranges for the new ADVIA Centaur immunoanalyzer are not available. Thyroid-stimulating 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 daysto 18 years from an inland (n=308) and a seaside city (n=152) using the Bayer ADVIA Centaur analyzer. Square root functions defining the upper and lower limit of the continuous age-dependent reference range and the median line for each thyroid hormone were estimated applying a parametric regression technique. Continuous age-dependent 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 FF3 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 sex-specific effects on age-adjusted hormone concentrations were found except for a slight residence-specific effect on age-adjusted FT4, which was probably due to assay imprecision. Due to the lack of international standardization, the assay-specific evaluation of reference ranges is very important. Continuous age-dependent reference ranges comply better with the biological reality and are more reliable than discontinuous reference ranges.

Establishment of reference ranges for thyrotropin, triiodothyronine, thyroxine and free thyroxine in neonates, infants, children and adolescents

Thyrotropin, triiodothyronine, thyroxine and free thyroxine were determined in the sera of 714 euthyreotic neonates, infants, children and adolescents (age range 5 days to 18 years), using the IMx from Abbott Laboratories. Thyrotropin, triiodothyronine and free thyroxine were determined with microparticle enzyme immunoassays (MEIA). Thyroxine was determined with a fluorescence polarization immunoassay (FPIA). The proband collective was divided into 9 age groups, and each age group into males and females. In accordance with the recommendations of the International Federation of Clinical Chemistry, the 95% scatter range was taken as the reference range. In the different age groups, the ranges of some hormones showed significant differences between males and females. When no significant difference existed between the sexes, the results for males and females were evaluated statistically as a single group. Only a few reference groups showed normal Gaussian distributions. Therefore, in addition to the 50th percentile, the 2.5th and 97.5th percentiles were also calculated for all reference groups. Minimal and maximal values were also determined. The U-test of Mann & Whitney was used to test for significant differences between individual reference groups, and groups showing no significant differences were combined. The corresponding references ranges were then calculated.