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 micrograms/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