Transient Hypothyroxinemia of Prematurity: Electrophysiological Changes in the Preterm Infants' Brain, a Retrospective Study

1. Definition of THOP The determination of thyroid hormones (TH) are assay-specific and related to the infants' gestational age (GA) and moment of determination. Immediately after birth, there will be a surge of TH, subsequently followed by a decrease to basal levels. Therefore, it is difficult to obtain reference values.

The investigators plan to set out specific reference values for the preterm patient population, based on the TH laboratory results of cord blood, performed in the clinical laboratory of UZ Leuven and available over the last 4 years. The results will be linked to the gestational age. Dependent whether the data distribution is normal or not, the investigators are planning to use standard deviations or percentiles to classify patients. 2. EEG findings In this retrospective study, quantitative EEG- sleep behavior at (near) term age (GA 36-44 weeks) in preterm infants born <28 weeks GA, will be analyzed (n = 87).

EEGs were taken in the framework of the Resilience study and hereby, parental informed consent was already obtained.

TH function is assessed in preterm infants ≤ 34 weeks as part of the clinical care protocol. No additional blood samples were taken.

Quantitative EEG measures will be compared between the preterm infants with THOP (circulating thyroxine levels< P10) and without THOP. Logisitic regression will be performed to determine the effect of thyroid function as well as other clinical and demographic variables, on functional brain development at term equivalent age. These results will also be linked to long-term neurodevelopment outcome.