Motor nerve conduction velocity in very preterm infants in relation to l-thyroxine supplementation

Background: Transient hypothyroxinemia is common in preterm infants and has been associated with neurodevelopmental dysfunction and slow nerve conduction velocity. It is still unknown whether l-thyroxine supplementation is required. During an l-thyroxine supplementation trial, motor nerve conduction velocity was measured to answer the question whether l-thyroxine supplementation improves motor nerve conduction velocity. Methods: Two hundred infants < 30 weeks' gestational age were enrolled in a randomized, double-blind, placebo-controlled l-thyroxine supplementation trial. l-Thyroxine (8 μg/kg birthweight per day) or a placebo was administered during the first 6 weeks of life. Motor nerve conduction velocity was measured in the ulnar and posterior tibial nerve shortly after birth, at 2 weeks, at 40 weeks, and at 66 weeks postmenstrual age. Results: At 2 weeks, the ulnar motor nerve conduction velocity had improved in the l-thyroxine group compared with the placebo group, although the difference was not statistically significant (difference between means: 0.8 msec; 95% CI: −0.13 to 1.80; p = 0 .06). Later on, no effect of l-thyroxine supplementation on motor nerve conduction velocity was found. Conclusion: This study shows that in infants < 30 weeks' gestational age l-thyroxine supplementation during the first 6 weeks of life does not clearly improve motor nerve conduction velocity. (J Pediatr 1998;132:64-9)