Evaluating iodine deficiency in pregnant women and young infants-complex physiology with a risk of misinterpretation.

OBJECTIVE To review methods for evaluating iodine deficiency in pregnant women and young infants and to discuss factors to be considered in the interpretation of their results. DESIGN Review of the literature regarding the various methods available for assessing iodine status. SETTING Population surveys and research studies. SUBJECTS Pregnant women and young infants. RESULTS Several factors to consider when assessing iodine status in pregnant women and young infants include: 1) the urinary iodine (UI) concentration (microg l-1) is not interchangeable with 24 h UI excretion (microg per 24 h); 2) the concentration of iodine in a spot or casual urine sample cannot be used to diagnose iodine deficiency in an individual; 3) a moderate fall in the concentration of serum free T4 during pregnancy is not a sign of maternal iodine deficiency; 4) an increase in the concentration of serum thyroglobulin (Tg) during pregnancy is not a sign of maternal iodine deficiency; 5) a higher concentration of TSH and Tg in cord blood than in maternal blood is not a sign of iodine deficiency in the mother or neonate; and 6) thyroid function in a full-term foetus, a neonate or a small child is not more sensitive to a mild iodine deficiency than in the mother. CONCLUSIONS If the iodine status of pregnant women and small children is not to be misjudged, the above six factors need to be taken into account.