1) Thyroid hormone levels in preterm infants <30 weeks were much lower than in term infants, 2) the postnatal surge of thyroid hormones normally seen at 24 to 48 hours of age in term infants did not occur in our group of preterm infant, and 3) low FT3 and FT4 levels are associated with higher mortality and severity of lung disease.

The THORN trial was a multicenter, randomized, doubleblind, placebo-controlled clinical trial to test the hypothesis that administration of triiodothyronine (T 3 ) and hydrocortisone would decrease mortality and respiratory morbidity in preterm infants of less than 30 wk gestation. Two hundred fifty-three infants were randomized to receive either 6 g·kg Ϫ1 ·d Ϫ1 of T 3 with 1 mg·kg Ϫ1 ·d Ϫ1 of hydrocortisone or 5% dextrose (placebo) as a continuous i.v. infusion for 7 d. The dose was halved on d 5. Our first primary outcome was death or ventilator dependence at 1 wk, and the second was death or oxygen dependence at 2 wk. The overall mortality rate for both groups was 11.4%. Relative risk of death or ventilator dependence at 1 wk, treated versus placebo, was 0.87, p ϭ 0.2, and death or oxygen dependence at 2 wk, 1.00, p ϭ 0.9. We examined the relationship between free T 3 (FT 3 ) and free thyroxine (FT 4 ) levels in the first 7 d and the primary outcome death or ventilator dependence at 1 wk in all 253 babies. We found significant positive correlations of p ϭ 0.05 for FT 3 and p ϭ 0.002 for FT 4 . Thus the higher the FT 3 and FT 4 levels, the better the outcome. No beneficial effects of T 3 and hydrocortisone were shown. In this study, although FT 3 levels were doubled by the treatment infusion, FT 4 levels were significantly suppressed. The lack of any beneficial effect of T 3 in our study may be explained by suppression of A number of studies have demonstrated that premature babies have low levels of T 3 and T 4 (1-5) and that hypothyroxinemia has been associated with developmental problems (6 -8) and increased respiratory morbidity (9, 10). Consequently, several studies have examined thyroid hormone substitution in preterm infants, but most of these have involved small numbers of subjects. One large study of T 4 given to babies of less than 30 wk gestation assessed neurodevelopmental outcome and found no overall beneficial effect (11). However mental outcome in a subgroup of T 4 -treated infants less than 27 wk gestation was significantly better than in the placebo infants when tested at 2 y of age. Two studies examining respiratory factors as end points, one giving T 4 (12) and another T 3 (13), have not shown any clear-cut benefit.In spite of improvements in neonatal care during the past few decades (14), most tertiary level neonatal units have seen an increase in the incidence (15) of bronchopulmonary dysplasia (chronic lung disease) defined as an oxygen requirement at the age of 4 wk or beyond 36 wk conceptual age. Much of the increase can be attributed to the increasing survival of very premature babies born before 30 wk gestational age, which in turn raises the question of why these infants develop chronic lung disease when pulmonary surfactants are readily available.

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