L-THYROXINE TREATMENT OF PRETERM NEWBORNS - CLINICAL AND ENDOCRINE EFFECTS

Preterm newborns have low serum thyroxine (T-4) levels compared with l ate-gestational fetuses. Low thyroid hormone levels are associated wit h increased severity of neonatal illness and neurodevelopmental dysfun ction. We assessed the endocrine and clinical effects of increasing se rum T-4 levels in preterm newborns with a gestational age <31 wk. Fort y newborns were randomized in a double blind protocol: 20 infants rece ived a daily dose of 20 mu g/kg L-T-4 for 2 wk, whereas 20 control inf ants received saline. Serum concentrations of T-4, triiodothyronine (T -3), reverse T-3 (rT(3)), thyroglobulin (TG), and TSH were measured we ekly 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, durat ion of ventilation, development of chronic lung disease, oral fluid in take, and weight gain; a Bayley score was done at the corrected age of 7 mo. L-T-4 administration induced a marked increase in serum T-4 wit hout apparent change in T-3 levels, whereas the postnatal decline in s erum rT(3) was more gradual. L-T-4 treatment was associated with a dec rease in serum TG and TSH levels. TRH injection induced a definite ris e in serum TSH and T-3 in controls, but not in L-T4 treated newborns. Neither L-T-4 treatment, nor TRH administration appeared to alter circ ulating levels of prolactin, GH, or IGF-I. In contrast to the pronounc ed endocrine effects, no clinical effects of L-T-4 administration were detected.