Thyroid function in very preterm newborns: Possible implications

Thyroid hormones are essential for brain maturation. Very preterm infants, who are at risk of neurodevelopmental disabilities also have low thyroxine (T-4) and free thyroxine (FT4) values in the first weeks after birth. This transient hypothyroxinemia may in part be causal to the neurodevelopmental problems. We have carried out a randomized, double-blind, placebo-controlle d trial with T-4 in 200 infants less than 30 weeks gestation. T-4 (or place bo) was given in fixed dose of 8 mu g/kg birth weight per day during the fi rst 6 weeks after birth. It resulted in a significant increase of T-4, FT4, and reverse triiodothyronine (rT(3)). Thyrotropin (TSH) secretion was supp ressed, and, probably as a result of TSH suppression, triodothyronine (T-3) levels were decreased in the T-4 group. Mortality was 14% in the T-4 group and 21% in the placebo group (NS). No effect was found on morbidity. Heart : rate was significantly higher in T-4-treated infants less than 28 weeks g estation, but not in T-4-treated infants 28 weeks or more, who had the high est FT4 levels. In the study groups as a whole, no clear effect of T-4 admi nistration was found on neurodevelopmental outcome. However, there was a st rong trend toward improvement of adverse outcome, defined as death or abnor mal developmental outcome at 2 years of age. In addition, mental outcome in a subgroup of T-4-treated infants less than 27 weeks' gestation was signif icantly better than in placebo infants of the same age group. In conclusion this trial does not clearly have conclusive results. New trials of thyroid hormone treatment should be carried out in preterm infants, in order to in vestigate whether indeed T-4 supplementation is required in preterm infants less than 27 or 28 weeks gestation. Addition of T-3 to the treatment sched ule needs to be considered.