The THORN trial was a multicenter, randomized, double-blind, 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 micro g.kg(-1).d(-1) of T(3) with 1 mg.kg(-1).d(-1) 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 = 0.2, and death or oxygen dependence at 2 wk, 1.00, p = 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 = 0.05 for FT(3) and p = 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 FT(4) in the treatment group.