BACKGROUND AND OBJECTIVES:

Preterm infants commonly have transient hypothyroxinemia of prematurity after birth, which has been associated with deficits in general intellectual functioning, memory, attention, and academic achievement. However, research has predominantly focused on thyroxine levels in the first 2 weeks of life and outcomes are limited to the preschool period. Our objective was to evaluate the relationships between free thyroxine (fT4) levels over the first 6 weeks after very preterm (VPT) birth with cognitive functioning and brain development at age 7 years.

METHODS:

A total of 83 infants born VPT (<30 weeks’ gestation) had fT4 concentrations measured postnatally and 2- and 6-week area under the curve (AUC) summary measures were calculated. Follow-up at age 7 years included a neuropsychological assessment and brain MRI. Univariable and multivariable regression modeling was used where AUC for fT4 was the main predictor of neurodevelopmental outcome at age 7 years.

RESULTS:

Multivariable modeling revealed that higher, not lower, postnatal fT4 levels (2-week AUC) were associated with poorer cognitive performances at age 7 years on tasks of verbal learning (P = .02), verbal memory (P = .03), and simple reaction time (P < .001). A similar pattern of results was found when the 6-week AUC was examined. No significant associations between postnatal fT4 levels and brain volumes at age 7 years were identified.

CONCLUSIONS:

Results are contradictory to previous observations and suggest that after adjustment for confounders, higher postnatal fT4 levels in VPT infants, rather than lower levels, may be a marker of adverse neuropsychological development in childhood.

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