Changes in plasma thyroid hormone levels after a single dose of triiodothyronine in premature infants of less than 30 weeks gestational age

Objective: Evaluation of thyroid hormone response to a single administratio n of triiodothyronine (T-3) early postnatally to premature infants of <30 w eeks gestational age. Design: A prospective clinical trial with historical control. Methods: Ten infants born <28 weeks gestational age and ten infants born be tween 28 and 30 weeks gestational age were given 0.5 mug/kg T-3 intravenous ly at 12 h alter birth. The infants <28 weeks gestational age were also tre ated with thyroxine (T-4; 8 <mu>g/kg, once daily) during the first 6 weeks of life, Premature infants from a previous trial served as a matched histor ical control group. Analysis of variance for repeated measurements was perf ormed. Results: For the infants 28-30 weeks gestational age mean plasma T-3 concen trations were significantly higher in the T-3-treated group (P = 0.027) for at least 2 weeks, whereas mean plasma levels of T-4 free T-4 and TSH were comparable. For the infants <28 weeks gestational age plasma T-3 levels wer e also significantly different after correction for gestational age (P = 0. 0002), with either comparable or higher values in the T3-treated infants up to 56 days after injection of T-3. Mean plasma free T-4 levels were lower during the first 3 days and higher or comparable thereafter (P = 0.0014), a nd TSH suppression was more evident in the T-3-treated infants (P = 0.003). Conclusion: A single administration of T-3 to premature infants <30 weeks g estational age early postnatally results in a sustained increase of plasma T-3 levels during the first weeks of life, In infants of 28-30 weeks gestat ional age this occurs without change in plasma free T-4 levels, whereas in infants <28 weeks gestational age a transient decrease of plasma free T-4 w as present. The increase in plasma T-3 is possibly caused by a T-3-induced increase of type I deiodinase activity.