Thyroxine administration to infants of less than 30 weeks gestational age decreases plasma tri-iodothyronine concentrations

Abstract

Objective: To investigate the effect on thyroid hormone metabolism of the administration of thyroxine to very preterm infants. Design and methods: Two hundred infants of less than 30 weeks gestation were enrolled into a randomized, double-blind, placebo-controlled trial. Thyroxine (T 4 ) (at a fixed daily dose of 8 mg/kg birthweight) or placebo was started 12-24 h after birth and discontinued 6 weeks later. Plasma concentrations of T 4 , tri-idothyronine (T 3 ), reverse T 3 (rT 3 ), TSH, and thyroxine-binding globulin were measured weekly during trial medication and 2 weeks thereafter. Results: The T 4 and the placebo group each comprised 100 infants. Antenatal, perinatal, and postnatal clinical characteristics were comparable in both groups. T 4 and rT 3 were significantly increased in the T 4 group. TSH concentrations were depressed in the T 4 group and T 3 was significantly decreased, probably as a result of TSH depression. The T 4 /T 3 and T 4 /rT 3 ratios differed significantly between the two study groups. Conclusions: Daily T 4 administration during the first 6 weeks after birth to infants of less than 30 weeks gestation prevents hypothyroxinemia, but decreases plasma T 3 concentrations. Our finding possibly implies that very preterm infants should receive supplements of both T 4 and T 3 .

An explanatory randomised placebo controlled trial of levothyroxine supplementation for babies born <28 weeks’ gestation: results of the TIPIT trial

“…After discontinuation of supplemental LT4, FT4 concentrations were significantly lower in babies who had received supplementation than those given placebo, suggesting an ability to respond to withdrawal of the external supply of hormone at least by 32 weeks’ CGA. Babies in the placebo group had increasing plasma FT4 concentrations between day 28 and week 36, in accordance with the natural history of this condition described previously [28]. …”

Phase 1 Trial of 4 Thyroid Hormone Regimens for Transient Hypothyroxinemia in Neonates of &lt;28 Weeks' Gestation

“…A reasonable conclusion is that 8 μ g/kg per day is overtreatment and that 4 μ g/kg per day can produce a biochemically euthyroid state with less frequent suppression of thyrotropin. Moreover, T 3 production was not suppressed by using this lower dose regimen compared with previous reports 57. The current trial becomes the first to succeed in raising thyroid hormone levels to achieve the predefined threshold values of biochemical euthyroidism without suppressing thyrotropin values to <0.4 mIU/L in the majority of infants.…”

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

“…However, in our trial T 4 supplementation seemed to be associated with an improvement in mental developmental outcome in the subgroup of infants of less than 27 weeks gestation (13). Besides increasing plasma T 4 and free T 4 levels, T 4 supplementation resulted in a suppression of TSH levels, whereas plasma T 3 levels did not increase (15), but, on the contrary, decreased (16). It is believed that the low plasma T 3 level in premature infants is related to low activity of the type I deiodinase (DI), which is responsible for the endogenous production of T 3 by an outer ring deiodination of T 4 to T 3 (17).…”