Transient hypcthyroxinemia is frequent in preterm infants and is caused by a combination of factors as immaturity of hypothalamo-pituitary–thyroid system and immaturity of thyroid hormone metabolism. Thyroid hormone is important in maturation of the brain. Low neonatal thyroxine(T4) concentrations are related to neurodevelopmental impairment. Studies have shown that (T4) supplementation is mainly beneficial in preterm infants less than 27 to 29 weeks gestational age.