Abstract
Experience in neonatal screening for congenital hypothyroidism has made evident a large overlap between eu- and hypothyroid T4-levels.Concomitant lowering of TBG can explain abnormally low T4-levels only to a certain extent. In order to evaluate the significance of low T4-values, T4/TBG ratio was compared with direct radioimmunological measurements of FT4 in 52 term and 70 preterm infants who had to be retested because of borderline values in our TSH/T4-screening-program.
RESULTS: T4 (x:5,6-12,O μg/dl),TBG(x:18,3-27,3mg/1)and T4/TBG ratio(x:3,2-4,6) increase with gestational age. FT4(x: 2,1-2,4ng/dl) is not significantly different in term and preterm infants.Preterm infants with abnormally low T4(<4 μg/dl)show FT4-values within the normal range--this in contrast to a decrease of T4/TBG ratio.
CONCLUSION: Lowering of T4 and T4/TBG ratio does not indicate a lack in free T4. The contradiction between lowered T4/TBG ratio and normal free T4 may be interpreted as a reduced T4-binding affinity of TBG due to immaturity or severe illness.
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Klett, M., Heinrich, U., Mayer, P. et al. Measurement of Free Thyroxine (FT4) in the Newborn: A reliable test to confirm euthyroisism in neonates with low T4-values. Pediatr Res 13, 1190 (1979). https://doi.org/10.1203/00006450-197910000-00058
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DOI: https://doi.org/10.1203/00006450-197910000-00058