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Post-natal thyroid function in low birth weight infants; a longitudinal assessment of free thyroxine and thyroid
Insights
Extremely low birth weight (ELBW) infants show significant declines in thyroxine (T4) and free T4 (FT4) in the first two weeks of life. Measuring FT4 may aid in assessing thyroid function in these vulnerable neonates.
Area of Science:
- Neonatal endocrinology
- Pediatric thyroid disorders
- Premature infant care
Background:
- Limited data exists on serum free thyroxine (FT4) and thyroid hormone binding globulin (TBG) in the immediate postnatal period for preterm infants.
- Thyroid hormone levels are critical for neonatal development and adaptation.
Purpose of the Study:
- To longitudinally evaluate serum FT4, TBG, total thyroxine (T4), and thyroid stimulating hormone (TSH) in preterm infants of varying birth weights.
- To investigate the changes in these thyroid hormones during the early postnatal period.
Main Methods:
- Serum samples were collected from infants at birth (cord blood), 2 days, 1 week, and 2 weeks of age.
- Infants were categorized into four groups based on birth weight: extremely low birth weight (ELBW, ≤1000 g), low birth weight (1001–1350 g), moderate low birth weight (1351–2499 g), and full-term.
- Free T4 concentrations were measured using a radioimmunoassay (RIA) kit.
Main Results:
- Infants with ELBW exhibited precipitous declines in total T4 and, to a lesser extent, FT4 concentrations at 1 and 2 weeks postnatally.
- These significant postnatal decreases in T4 and FT4 in ELBW neonates have not been previously reported.
- The observed changes may be linked to metabolic or nutritional challenges associated with extreme prematurity.
Conclusions:
- Serum FT4 measurement is a valuable adjunct for assessing ELBW infants with low T4 values, particularly between 1 to 2 weeks of age.
- FT4 assessment could potentially serve as a primary screening tool for hypothyroidism, pending the development of a suitable filter paper-based assay.
Abstract:
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.