Reference intervals for preterm thyroid function during the fifth to seventh day of life

Area of Science:

• Neonatal Medicine
• Endocrinology
• Clinical Chemistry

Background:

• Lack of established reference intervals for serum free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) in preterm neonates (5-7 days old).
• Retrospective study utilizing a chemiluminescence immunoassay system.

Purpose of the Study:

• To establish reference intervals for serum FT3, FT4, and TSH in preterm neonates.
• To stratify these intervals by gestational age (GA) and sex.
• To provide diagnostic criteria for early detection of thyroid diseases in preterm infants.

Main Methods:

• Retrospective analysis of data from 2040 preterm neonates (GA 26-35 weeks) admitted to the NICU (2014-2019).
• Serum FT3, FT4, and TSH values were analyzed and stratified by GA (26-28, 29-32, 33-35 weeks) and sex.
• Statistical comparisons were performed to identify significant differences.

Main Results:

• Established reference intervals: FT3 (1.65–5.21 pmol/L), FT4 (8.64–25.41 pmol/L), TSH (0.406–12.468 mIU/L) for preterm neonates (GA 26-35 weeks).
• Significant differences in serum FT3 and FT4 levels were observed with increasing GA (P < 0.01).
• Serum FT3 levels were lower in males than females, particularly in the 29-32 weeks GA group; no significant sex-based differences were found for FT4 or TSH.

Conclusions:

• Established reference intervals provide crucial early diagnostic criteria for thyroid diseases in preterm neonates (GA 26-35 weeks).
• These intervals help prevent unnecessary retesting and interventions.
• FT4 reference intervals can guide thyroid hormone replacement therapy in congenital hypothyroidism.