Context. Transient hypothyroxinaemia is common in infants <30 weeks gestation and is associated with neurodevelopmental deficits. Reductions in T4 and T3 levels with TSH unchanged are the key features of severe illness using surrogate indices of overall severity of illness but these do not inform the impact of individual disease conditions or drug usage.

Objective. To investigate the contribution of postnatal factors to the variations in serum levels of iodothyronines, TBG and TSH

Design. An infant cohort study (23-34 wk gestation; n = 780) between January 1998 and September 2001.

Setting. Eleven level III Scottish neonatal intensive care units.

Patients. Cohorts of infants delivered at 23-34 weeks gestation.

Main Outcome. Serum levels of iodothyronines, TBG and TSH at 7, 14, and 28 days adjusted for the potentially significant postnatal influences (n = 31).

Results. Serum levels of TSH, FT4, T3, T4 are variably but significantly associated with bacteraemia, endotracheal bacterial cultures, persistent ductus arteriosus, necrotising enterocolitis, cerebral ultrasonography changes, oxygen dependence at 28 days and the use of aminophylline, caffeine, dexamethasone, diamorphine and dopamine.

Conclusions. There are many more associations of postnatal factors with transient hypothyroxinaemia than had previously been considered in preterm infants. Alternative strategies should be considered for correction of hypothyroxinaemia rather than sole reliance on the direct therapy of hormone replacement. A more oblique preventative approach may be necessary through reduction in the incidence or severity of individual illness(es). Similarly alternatives to those drugs which interfere with the hypothalamic-pituitary-thyroid axis should be evaluated (e.g. other inotropics instead of dopamine).