Post-natal thyroid function in low birth weight infants; a longitudinal assessment of free thyroxine and thyroid hormone binding globulin.

Because the concentrations of serum free thyroxine (FT4) and thyroid hormone binding globulin (TBG) have not been fully evaluated in preterm infants at the immediate post-natal period, we studied the longitudinal changes of serum FT4 and TBG, along with thyroxine (T4) and thyroid stimulating hormone (TSH), at birth (cord blood), 2 days, 1 week and 2 weeks of age in 7 infants with birth body weight less than or equal to 1000 g, 7 infants with body weight 1001 to 1350 g, 11 infants with body weight 1351 to 2499 g, and 11 full-term infants. Free T4 concentrations were measured by Corning Medical radioimmunoassay (RIA) kit. The infants with extremely low birth weight (ELBW) (body weight less than or equal to 1000 g) showed precipitous declines of total T4 and, to a lesser extent, of FT4 concentrations at 1 and 2 weeks of age. These post-natal T4 and FT4 decreases in ELBW neonates have not previously been reported. The clinical significance of this finding remains, speculative, but it may be due to metabolic or nutritional problems related to extreme prematurity itself. This study suggests that measurement of FT4 is a useful adjunct to the assessment of ELBW infants with very low T4 values, if done between 1 to 2 weeks of age, and could be used as a primary hypothyroid screening tool instead of T4 measurements, provided that an FT4 assay is developed that uses the elute of blood spotted on filter paper.