This paper summarises the study protocol for the randomised controlled trial of iodine supplementation in preterm infants. Iodine is essential for the synthesis of thyroxine, and thyroxine is essential for normal brain development in utero and for the first 2-3 years of life. The recommended iodine intake in parenteral nutrition regimens is 1 μg/kg/day and commercially available parenteral solutions for infants reflect these recommendations. In the absence of other iodine sources, infants are vulnerable to negative iodine balance and insufficiency. As many preterm infants are fed parenterally for prolonged periods with solutions which have been shown to be iodine-deficient, the I2S2 Trial was designed to establish whether iodine supplementation of preterm infants benefits neurodevelopment.