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Transient Hypothyroxinemia in Preterm Infants: The Role of Cord Sera Thyroid Hormone Levels Adjusted for Prenatal and Intrapartum Factors

Community Health Sciences (F.L.R.W., C.B., S.A.O.) and Maternal and Child Health Sciences (G.J.M., R.H.), University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, Scotland, United Kingdom; and Department of Internal Medicine (H.v.T., T.J.V.), Erasmus University Medical Center, 3015 GE Rotterdam, The Netherlands

Address all correspondence and requests for reprints to: Prof. Robert Hume, Maternal and Child Health Sciences, University of Dundee, Ninewells Hospital and Medical School, Dundee DD1 9SY, Scotland, United Kingdom. E-mail: r.hume{at}dundee.ac.uk.

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 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.

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 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 overrecruitment 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.

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				F. L. R. Williams, S. A. Ogston, H. van Toor, T. J. Visser, R. Hume, and with collaboration from the Scottish Preterm Thyro Serum Thyroid Hormones in Preterm Infants: Associations with Postnatal Illnesses and Drug Usage J. Clin. Endocrinol. Metab., November 1, 2005; 90(11): 5954 - 5963. [Abstract] [Full Text] [PDF]

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