Transient hypothyroxinemia in preterm infants: the role of cord sera thyroid hormone levels adjusted for prenatal and intrapartum factors.
BACKGROUND
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 T4 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, T4-binding globulin, and TSH, and to provide an appropriate definition of transient hypothyroxinemia.


METHODS
The study design is a cohort study (n = 620) in 11 Scottish neonatal intensive care units.


METHODS
Infants were delivered at 23- to 42-wk gestation and recruited between January 1998 and September 2001.


RESULTS
Using -2 SD of adjusted T4 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 T4 supplementation. We suggest the use of serum T4 levels on postnatal d 7 that are below -1 SD of adjusted cord T4 levels of equivalent gestational age. This criterion avoids over-recruitment of the more mature infants in whom universal T4 supplementation is detrimental to neurodevelopmental outcome, but still allows selection of the least mature entrants on whom universal T4 supplementation is beneficial.
Williams
Fiona L R
FL
Community Health Sciences, University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, Scotland, United Kingdom.
Mires
Gary J
GJ
Barnett
Carol
C
Ogston
Simon A
SA
van Toor
Hans
H
Visser
Theo J
TJ
Hume
Robert
R
D002624:Chemistry, Clinical / Q000592:standards; D015331:Cohort Studies; D005312:Fetal Blood; D005865:Gestational Age; D006130:Growth Disorders / Q000097:blood; D006801:Humans; D007037:Hypothyroidism / Q000097:blood* / Q000175:diagnosis*; D007231:Infant, Newborn; D007234:Infant, Premature / Q000097:blood*; D012016:Reference Values; D013963:Thyroid Hormones / Q000097:blood*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
D013963:Thyroid Hormones; D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine