The measurement, definition, aetiology and clinical consequences of neonatal transient hypothyroxinaemia.

This review focuses on neonatal transient hypothyroxinaemia, a condition characterized by temporary postnatal reductions in concentrations of Total T4 or Free T4, with normal or low concentrations of thyroid stimulating hormone (TSH). There is neither an agreed quantitative definition, nor an agreed mode of measurement for the condition. Transient hypothyroxinaemia is not routinely monitored yet it is thought to affect about 50% of preterm infants; it was thought to be without long-term sequelae but observational studies indicate that neurodevelopment may be compromised. The aetiology of transient hypothyroxinaemia is complex. There are significant contributions from the withdrawal of maternal-placental thyroxine transfer, hypothalamic-pituitary-thyroid immaturity, developmental constraints on the synthesis and peripheral metabolism of iodothyronines and iodine deficiency. It is not possible to distinguish clinically, or from laboratory measurements, whether transient hypothyroxinaemia is an independent condition or simply a consequence of non-thyroidal illness and/or drug usage. An answer to this question is important because studies of thyroid hormone replacement have been instigated, with mixed results. Until the aetiology of transient hypothyroxinaemia is better understood it would seem prudent not to routinely supplement preterm infants with thyroid hormones. Iodine deficiency, non-thyroidal illness and drug usage are the most modifiable risk factors for transient hypothyroxinaemia and are the clear choices for attempts at reducing its incidence. We suggest that transient hypothyroxinaemia in preterm infants is defined as a normal or low TSH concentration in conjunction with a concentration of Total T4, that is ≤10th percentile of cord Total T4 of the equivalent gestational age had the infant remained in utero.