Reference Percentiles and Changes over Time for Total Thyroxine in Preterm Infants: A Retrospective Cohort Study

Area of Science:

• Neonatal endocrinology
• Public health screening programs

Background:

• Hypothyroxinemia in preterm infants complicates congenital hypothyroidism screening.
• Limited data exists on total thyroxine (tT4) reference ranges for preterm neonates.
• Accurate screening requires population-specific reference values.

Purpose of the Study:

• Establish reference percentiles for tT4 in dried blood spots of Mexican preterm infants.
• Analyze tT4 concentration changes over time relative to gestational age.
• Evaluate the impact of these findings on tT4-based newborn screening protocols.

Main Methods:

• Retrospective cohort study including 1561 preterm infants.
• Measurement of tT4 concentrations in dried blood spots.
• Analysis of tT4 levels at different time points and gestational ages.

Main Results:

• Established 10th percentile tT4 values for gestational ages 24-27, 28-30, 31-34, and 35-36 weeks.
• Observed increasing tT4 concentrations in infants <30 weeks, stable levels in 31-34 weeks, and decreasing levels in 35-36 weeks.
• tT4 levels changed significantly over the first few weeks of life and with corrected gestational age.

Conclusions:

• Preterm infants exhibit distinct tT4 concentration patterns.
• Current screening cutoff values for congenital hypothyroidism may need adjustment for preterm populations.
• Lower tT4 cutoff values are suggested for preterm infants to reduce false positives.