Free thyroxine levels in hospitalized newborns: depressed levels in critical, nonthyroidal illness

Area of Science:

• Neonatal endocrinology
• Pediatric thyroid disorders

Background:

• Thyroid hormone levels, specifically thyroxine (T4) and triiodothyronine (T3), are typically lower in preterm infants compared to term infants.
• The clinical significance of these lower levels and the distinction between hypothyroidism, prematurity, and illness remain unclear, impacting treatment decisions.

Purpose of the Study:

• To investigate and compare thyroid hormone levels (T4, T3, free thyroxine (FT4), and thyroid-stimulating hormone (TSH)) in preterm and term infants.
• To determine the factors influencing these hormone levels, differentiating between prematurity, illness, and true hypothyroidism.

Main Methods:

• Weekly measurements of T4, T3, FT4, and TSH were conducted in 75 infants with gestational ages ranging from 25 to 42 weeks.
• Hormone levels were analyzed in relation to gestational age, clinical status (illness), and time post-birth.

Main Results:

• Thyroxine and triiodothyronine levels were consistently lower in preterm infants.
• Free thyroxine levels showed minor differences between term and preterm infants and remained stable over time.
• Infant maturity was the strongest predictor of T4 and T3 levels, while nonthyroidal illness was most correlated with FT4 levels.
• All surviving infants were euthyroid at discharge.

Conclusions:

• Hypothyroxinemia in preterm infants is likely a physiological adaptation related to reduced protein-bound thyroxine and nonthyroidal illness, rather than true hypothyroidism.
• Free thyroxine measurements are crucial for assessing thyroid status in neonates, distinguishing between illness-related changes and pathological hypothyroidism.