<qa>  What is the biologically active substance not measured in our previous study?Transient hypothyroxinemia, as demonstrated by low T4 and free T4 levels with normal levels of TSH, in preterm infants has been associated with increased severity of illness and adverse outcomes. Effects of thyroid function in term infants is less well studied. Previous research in the Special Care Nursery at Christiana Hospital has indicated that transient hypothyroxinemia in intubated term infants was associated with increased severity of illness and the need for more intensive rescue therapies. However, free T4, the biologically active substance was not measured in our previous study. 