Objective: This paper re-evaluates the requirements for iodine during pregnancy, lactation and the neonatal period, and formulates original proposals for the median concentrations of urinary iodine (UI) that indicate optimal iodine nutrition during these three critical periods of life. This paper also discusses the measurements that are used to explore thyroid functions during the same periods.

Design: An extensive and critical review of the literature on thyroid physiopathology during the perinatal period.

Setting: Human studies conducted in various regions throughout the world.

Subjects: Pregnant women, lactating women, and newborns.

Results: The following proposals are made after extensive review of the literature: the requirement for iodine by the mother during pregnancy is 250-300 μg day-1; during lactation the requirement is 225-350 μg day-1; and during the neonatal period the requirement of the infant is 90 μg day-1. The median UI that indicates an optimal iodine nutrition during these three periods should be in the range of 150-230 μg day-1. These figures are higher than recommended to date by the international agencies.

Conclusions: Pregnant women and young infants, but especially the second group, are more sensitive to the effects of an iodine deficiency (ID) than the general population because their serum thyroid-stimulating hormone (TSH) and thyroxine are increased and decreased, respectively, for degrees of ID that do not seem to affect thyroid function in the general population. Systematic neonatal thyroid screening using primary TSH could be the most sensitive indicator to monitor the process of ID control.