Short-term changes in maternal and neonatal urinary iodine excretion

Investigation of maternal urinary iodine (UI) excretion in the immediate antenatal and early postpartum periods showed a precipitous fall in median values from 93 microg/L antenatally to 36 microg/L at delivery subsequently rising to 49 microg/L and 63 microg/L at days 3 and 10 postpartum respectively. The fate of ingested iodine not appearing in the maternal urine is unknown but measurement of UI in babies born to nursing mothers suggested transfer from the mother with median neonatal values of 117 and 159 microg/L being recorded at days 3 and 10. While maternal UI seemed to relatively unaffected by breast feeding, median UI from breast feeding babies (148 microg/L) was significantly greater than in those bottle feeding (50 microg/L). This was also reflected by the finding that no breast feeding baby had a UI values < 50 microg/L in comparison to 50% of bottle feeders. The depressed values in mothers and relatively high values in their infants could present a false picture and suggest the need to defer any investigations of iodine status at this time. The findings do however suggest a need for further investigations aimed at determining the fate of iodine ingested perinatally and its possible physiological significance in maintaining thyroid status in the mother and neonate.