Introduction: Thyroxine is essential for growth and brain development in neonates. Early detection of hypothyroidism by measurement of free T4 (fT4) is therefore paramount. Moreover, the initial monitoring of Thyrax suppletion requires accurate fT4 measurements.
Methods: In the Maasstad Hospital fT4 has been measured with the Immulite 2500 and from 2010 onward with the Dimension VISTA. Both instruments show good agreement when tested with adult samples. Samples of neonates are measured in our hospital and in addition are sent to the university hospital of Rotterdam for measurement with the Vitros ECi. We compared the Vitros fT4 results with the measurements in our hospital.
Results: In all patients, fT4 results were lower than measured by the Vitros. We found an age dependent, very variable difference (range 0.428.5 pmol/l) between methods. In neonates 25 days of age the difference was larger than in cord blood and decreased with age. The differences between patient results obtained with VISTA/Immulite and Vitros could not be explained by the results from external quality control surveys. Neither could they be explained by variations in TBG concentration: TBG is not increased in young neonates. Illustrative is a difference of 14 pmol/l in fT4 results, found in an 8 days old, TBG deficient neonate.
Conclusion: Although an explanation of our observation is still missing, it is important for pediatricians to be aware of the fact that neonatal fT4 results can be very method dependent. In our hospital, this could potentially lead to overtreatment with T4. The phenomenon is largely unknown in our country.