Serum free thyroxine concentration is not reduced in premature infants with respiratory distress syndrome.

OBJECTIVE: We used improved methods of assay to determine whether pituitary-thyroid function is altered in premature infants with respiratory distress syndrome (RDS) during the first week of postnatal life. METHODS: Serum free thyroxine (T4) was measured by direct equilibrium dialysis, total thyroxine (TT4) by radioimmunoassay, and thyrotropin by a sensitive immunometric assay in 90 premature infants (45 healthy control subjects and 45 with RDS) during their first week of life after 25 to 30 weeks of gestation. Infants in the RDS group received exogenous surfactant therapy. RESULTS: Free T4 and thyrotropin concentrations of infants were not significantly different between RDS and control groups. As expected, infants with RDS had significantly lower serum total T4 concentrations compared with control infants (p < 0.001). This difference was present even after stratification for gestational age (25- to 27-week group, p = 0.012; 28- to 30-week group, p = 0.002). Lower total T4 concentrations were attributable to lower T4 binding to serum proteins among infants with RDS compared with control subjects, especially in the 25- to 27-week gestation group (p = 0.0075). CONCLUSION: These data indicate that pituitary-thyroid function is not altered in premature infants with RDS. The low total T4 state in these premature infants is attributable solely to reduced serum T4 binding, as is often seen in acute nonthyroidal illnesses.

Reference ranges for newer thyroid function tests in premature infants.

OBJECTIVE: To establish reference ranges for recently developed assays of thyroid function in premature infants. METHODS: We measured serum free thyroxine (T4) by direct equilibrium dialysis and serum thyrotropin by a sensitive immunometric method in 104 preterm infants (25 to 36 weeks of gestational age) during the first week of life. RESULTS: The free T4 level correlated positively with gestational age (p < 0.0001; r2 = 0.09) and differed significantly between adjacent gestational age groups (p < 0.05). Free T4 concentrations (mean +/- SD) for the 25- to 27-, 28-to 30-, 31- to 33-, and 34- to 36-week groups were 18.0 +/- 5.2, 25.7 +/- 9.0, 30.9 +/- 9.0, 36.0 +/- 10.3 pmol/L (1.4 +/- 0.4, 2.0 +/- 0.7, 2.8 +/- 0.8 ng/dl), respectively. Two reference ranges for free T4 were determined, one for 25 to 30 weeks (6.4 to 42.5 pmol/L (0.5 to 3.3 ng/dl) and one for 31 to 36 weeks (16.7 to 60.5 pmol/L (1.3 to 4.7 ng/dl)). The logarithm of the value for thyrotropin correlated positively with gestational age (p < 0.001; r2 = 0.08); one reference range of 0.5 to 29 mU/L was determined for thyrotropin. CONCLUSION: This study extends information on thyroid function of preterm infants and establishes reference ranges for this population.

Head position affects intracranial pressure in newborn infants.

We studied the effects of six different head positions on intracranial pressure and cerebral blood flow velocity in six infants with a recent history of asphyxia and eight without. ICP was measured noninvasively using a transfontanel pressure transducer, and CBF was assessed using the continuous-wave Doppler method. We found that ICP was lowest with the head elevated and in the midline (P less than 0.01), and that ICP was higher in all infants in the dependent position (P less than 0.001). This increase was significantly greater in those who had had an episode of asphyxia during the 48 to 72 hours prior to the study (P less than 0.02). Therefore, we recommend a head elevation of 30 degrees in the midline in any infant with increased ICP or at high risk for cerebral injury, and caution against the use of the dependent position in these infants.