Growth-restricted premature infants are at increased risk for low thyroxine

Objective: To evaluate, in extremely premature infants, the relationship be tween growth restriction and early total thyroxine levels, and to determine how maternal, prenatal, perinatal and neonatal variables influence the rel ationship. Study design: 719 infants born at four medical centers in Massac husetts, New York and New Jersey between 1991 and 1993 were studied. Entry criteria included: gestational age 23-30 weeks, birth weight 500-1500 g, an d a serum thyroxine level obtained in the first week of life. Infants born to mothers with a history of thyroid disease were excluded. Birth weight an d total thyroxine level are expressed as z-scores (standard deviation units ) to adjust for their relationship to gestational age. Results: In linear r egression analysis, there was a 0.18 decrease in the total thyroxine z-scor e for each 1.0 (1 standard deviation unit) decrease in birth weight z-score (p=0.0001). Adjustment for multiple potential maternal, prenatal, perinata l and neonatal confounders failed to identify a factor or factors that coul d account for the observed association. Conclusions: The early total thyrox ine level in extremely preterm infants was significantly associated with bi rth weight z-score. This relationship persisted even after adjustment for m aternal, prenatal, perinatal and neonatal confounders suggesting antenatal influences. Of clinical importance, growth-restricted infants are at increa sed risk for early hypothyroxinemia and, possibly, to its related morbiditi es. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.