The aim of this study is to investigate differences in electroencephalography (EEG) evolution between preterm infants with and without transient hypothyroxinemia of prematurity (THOP) in order to find differences in the interburst interval and the background pattern and in the maturation of the sleep-wake cycle.

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.

In a subgroup of these preterm patients (n=42) sequential EEGs, recorded during their stay at the neonatal intensive care unit, are available. These EEGs will be analyzed in a fully automatic way to assess functional EEG- brain maturation.

In this way, the investigators want to investigate whether deviations of normal preterm EEG-brain maturation can be discerned in preterm neonates with THOP and without THOP.

In preterm infants with GA < 32 weeks, developmental follow up data are available at the corrected age of 9 months and 2 years (Follow up Convention). The investigators will use these results and link them to the EEG findings and THOP data.

Inclusion Criteria:

• gestational age < 28 weeks
• serial EEG recordings available
• thyroid function test on the first day of life and at the end of the first week of life available

Exclusion Criteria:

• Presence of congenital abnormalities

Preterm infants who had EEG recordings in the framework of the Resilience study and gestational age < 28 weeks.