Preterm newborns have low serum thyroxine (T4) levels compared with late-gestational fetuses. Low thyroid hormone levels are associated with increased severity of neonatal illness and neurodevelopmental dysfunction. We assessed the endocrine and clinical effects of increasing serum T4 levels in preterm newborns with a gestational age <31 wk. Forty newborns were randomized in a double blind protocol: 20 infants received a daily dose of 20 microg/kg L-T4 for 2 wk, whereas 20 control infants received saline. Serum concentrations of T4, triiodothyronine (T3), reverse T3 (rT3), thyroglobulin (TG), and TSH were measured weekly as well as serum levels of GH, prolactin, and IGF-I. After 2 wk, a TSH-releasing hormone (TRH) test was performed. Neonatal illness and outcome was evaluated by noting heart rate, oxygen requirement, duration of ventilation, development of chronic lung disease, oral fluid intake, and weight gain; a Bayley score was done at the corrected age of 7 mo. L-T4 administration induced a marked increase in serum T4 without apparent change in T3 levels, whereas the postnatal decline in serum rT3 was more gradual. L-T4 treatment was associated with a decrease in serum TG and TSH levels. TRH injection induced a definite rise in serum TSH and T3 in controls, but not in L-T4 treated newborns. Neither L-T4 treatment, nor TRH administration appeared to alter circulating levels of prolactin, GH, or IGF-I. In contrast to the pronounced endocrine effects, no clinical effects of L-T4 administration were detected.