Serum thyroglobulin (Tg) levels were determined in preterm neonates. Very high values were found at birth. A significant negative correlation was observed between Tg levels and gestational age. A longitudinal study indicates that a very sharp decrease in Tg levels occurred after birth. Three weeks after birth, the values were close to the values of full-term neonates. These high hTg values could result either from an increased turnover of hTg or from a lower clearance rate of this iodoprotein.