Thyroid function in the sick very low-birth-weight infants.

This study was undertaken to determine postnatal changes in thyroid function in very low-birth-weight (VLBW) infants. A total of one-hundred VLBW infants participated. Serial examination of serum levels of thyroxine (T4), free T4, triiodothyronine (T3), and thyroid-stimulating hormone (TSH) was performed in the neonatal period. A total of eighty-nine infants survived to discharge, while eleven died during hospitalization. Transient hypothyroxinemia was found in forty-one (46.1 percent) of the survivors. One of the surviving infants had primary hypothyroidism. His data was excluded from the analysis. In the other eighty-eight surviving infants, TSH levels were within normal limits throughout the six-week study period. T4 and free T4 values decreased after the first day of life, reaching a nadir at one week of age, followed by progressive increases. The mean cord blood T3 level was very low; the serum T3 value increased progressively in the postnatal period. We found a correlation between low T4 and free T4 values and mortality and neonatal illness. Hypothyroxinemia was associated with critical illness. In conclusion, the postnatal changes in thyroid function in VLBW infants were characteristic, with transient hypothyroxinemia being common in these infants. Further investigation of the relationship between thyroid function, death, neonatal illness, and developmental outcome is warranted.