Context: Transient hypothyroxinemia is common in infants less than 30 wk gestation and is associated with neurodevelopmental deficits; it has no consensus definition. We previously suggested that appropriate ranges for postnatal serum T-4 values are at least cord levels corrected to an equivalent gestational age if the fetuses had remained in utero.
Objective: The study objective is to investigate the contribution of prenatal and intrapartum factors (n = 27) to the variations in cord levels of iodothyronines, T-4-binding globulin, and TSH, and to provide an appropriate definition of transient hypothyroxinemia.
Design: The study design is a cohort study (n = 620) in 11 Scottish neonatal intensive care units.
Patients: Infants were delivered at 23- to 42-wk gestation and recruited between January 1998 and September 2001.
Results: Using -2 SD of adjusted T-4 cord levels applied to postnatal d-7 values of equivalent gestational age, 14% of the 23- to 27-wk group, 1% of the 28- to 30-wk group, and 3% of the 31- to 34-wk group are hypothyroxinemic; using -1 SD, the respective figures are 41, 23, and 12%.
Conclusions: In the absence of neurodevelopmental follow-up studies to quantify transient hypothyroxinemia, a pragmatic criterion is necessary for selection of extreme preterm infants into clinical trials of T-4 supplementation. We suggest the use of serum T-4 levels on postnatal d 7 that are below -1 SD of adjusted cord T-4 levels of equivalent gestational age. This criterion avoids overrecruitment of the more mature infants in whom universal T-4 supplementation is detrimental to neurodevelopmental outcome, but still allows selection of the least mature entrants on whom universal T-4 supplementation is beneficial.