Thyroid function in the preterm infant: A longitudinal assessment

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Serum T4, T3, rT3, TSH, and charcoal index-T3 (a measure of TBG) tests were performed on samples collected from cord blood and at 24, 48, and 72 hours, and at 1, 2, and 3 weeks of age in 35 preterm infants. Appropriate-for-gestational age preterm infants (N=13) served as control subjects and had an increase during the first few days of life in mean TSH, T4, and T3 values, although the magnitude of the changes was less than that reported for full-term infants. T3 and T4 values between 1 and 3 weeks of age remained lower than the relatively “hyperthyroid” levels existent in term infants. Results from infants with hyaline membrane disease (N=15) demonstrated depressions of TSH, T4, and T3 values immediately after birth with T3 values remaining significantly lower than those of AGA control subjects at 1, 2, and 7 days of age. Seven small-for-gestational age infants had increases in mean TSH, T4, and T3 values which were similar to those of control infants during the first few days of life. However, T3 values were significantly lower at 1 week, and T3 and T4 values were diminished (P<0.05) at 2 and 3 weeks of age in SGA infants when compared to the AGA group. Reverse T3, elevated in the cord blood of all infants, fell gradually toward normal term infant levels by 3 weeks. Indirect analysis of TBG failed to demonstrate any differences within or between the patient groups studied. We conclude that normal preterm infants have a pattern of thyroid function qualitatively similar but quantitatively different from that of term infants. The pattern itself is altered by the presence of hyaline membrane disease or intrauterine growth retardation.

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    Supported by National Institutes of Health Grant AM 14039 (Arthritis and Metabolism).

    Presented in part at the Forty-seventh Annual Meeting of the Society for Pediatric Research, San Francisco, CA, April 28, 1977.

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