To assess thyroid function in very low birth weight infants, serial free thyroxine (free T4) determinations were made between age one week and 12 weeks using two radioimmunoassay (RIA) kits, Amersham and Clinical Assays, in 18 infants of 28-weeks gestation or less. TSH was normal in all infants. Both assays showed initial free T4 concentrations well below values in full term infants. The most severely ill babies had free T4 values which were significantly lower than the least ill babies in the first four weeks of life by the Amersham method only. By 6 weeks of age free T4 values increased to the range seen in normal term infants at 1–4 months of age. The increase in free T4 may be a result of maturation of an immature hypothalamic-pituitary-thyroid axis or resolution of the sick euthyroid syndrome. The Amersham kit showed significantly lower free T4 concentrations compared to the clinical assays kit up to age 6 weeks. Caution must be used in interpreting free T4 values in VLBW infants in the first month of life in light of the progression of free T4 into the normal range by 6 weeks of life and differences found between the two free T4 RIA kits.
ASJC Scopus subject areas
- Pediatrics, Perinatology, and Child Health
- Endocrinology, Diabetes and Metabolism