Serum Thyroid Hormone Levels in Preterm Infants Born before 33 Weeks of Gestation and Association of Transient Hypothyroxinemia with Postnatal Characteristics

Background: Fetal thyroid function and the hypothalamopituitary-thyroid axis continue to mature throughout pregnancy. Therefore, thyroid hormone levels of premature infants differ from those of mature ones. Our primary objective was to evaluate the reference values of serum thyroid hormones in preterm infants born before 33 wk gestation. The second objective was to define a cut-off value for transient hypothyroxinemia of prematurity (THOP) according to gestational age and association of THOP with postnatal characteristics in these infants. Subjects and methods: We recruited a cohort of 200 infants (26-32 wk gestation) admitted to neonatal intensive care units (NICU) between March 2008 and February 2009. We assessed serum levels of thyroid hormones and thyrotropin (TSH), at 1(st), 2(nd), and 3(rd)-4(th) wk of life. Thyroid-binding globulin (TBG), thyroglobulin (T-g), and urinary iodine values were also measured at the 1(st) wk of life. The infants were divided into two groups according to gestational ages; group 1: 26-29 wk and group 2: 30-32 wk. Association of THOP with postnatal characteristics of these infants were evaluated. Results: TT4 and TT3 values steadily increased from 1(st) wk to 3(rd)-4(th) wk while FT4 value did not significantly changed. FT3 value slightly decreased from 1(st) wk to 2(nd) wk and not significantly changed after this period. TSH value steadily decreased from 1(st) wk to 3(rd)-4(th) wk. For all postnatal ages, TT4, TT3, and FT3 values were lower in the lower gestational age group, while there was no significant difference for FT4 and TSH values between two gestational age groups. THOP rate decreased from first wk (24.0%, n=47) to 3(rd)-4(th) (14.0%, n=24) in all infants. When adjusted for age, THOP was associated with need for mechanical ventilation (P=0.03, OR:0.65, CI 95% 0.4-0.9) and for having respiratory distress syndrome (RDS) (P=0.02, OR:0.61 C195%; 0.4-0.9). Longer hospital stay (P=0.006, OR:0.96, CI 95% 0.94-0.99) was also found to be associated with THOP. Conclusions: In preterm infants below 30 wk, thyroid hormones were lower and urinary iodine values were higher compared to infants with older gestational age. THOP at the first wk of life may convey important prognostic information about neonatal morbidity and length of hospitalization stay.