Studies estimate that marked hypothyroxinemia (thyroxine [T4] <6.5μg/dl) is present in 25% of all prematures and 50% of babies born at <30 weeks gestation. The effects of hypothyroxinemia on neonatal status and developmental outcome in VLBW infants are unknown. We hypothesized that the T4 obtained as part of the routine newborn screen would correlate with developmental outcome scores in VLBW infants. We retrospectively collected the T4 screen for 185 infants with BW <1250 gm admitted to the NICU from 1/1/88 to 12/31/89. The mean T4 was 5.9±3.4 μg/dl; the mean BW was 980±180 gm. Correlation analyses were completed to evaluate the relationship of screening T4 with neonatal risk factors and with the Bayley Mental Developmental Index (MDI) and Psychomotor Developmental Index (PDI) scores at 7, 12 and 24 months. Pearson Correlation Coefficients:Table ↑ Screening T4 correlated with ↑ Apgar scores, ↑ birth weight, ↑ gestational age and inversely with days hospitalized. There was no relationship with days of mechanical ventilation or oxygen. Screening T4 correlated with the 7 month MDI, but did not show a significant correlation with 7 month PDI, nor with developmental outcome scores at 12 or 24 months. We conclude that T4 screen correlates with parameters of neonatal illness severity in VLBW infants. These preliminary data suggest that although screening T4 may impact on Bayley MDI at 7 months, the effects of T4 level on developmental outcome diminish by 12 to 24 months of age. In summary, neonatal hypothyroxinemia is a measure of illness severity in VLBW infants and appears to affect developmental performance at 7 months of age.

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