Variation in iodine handling during normal pregnancy

Reports in the literature are divided on changes in thyroid volume and urinary iodine excretion (UI) during normal pregnancy. Reports from Ireland, an area of modest dietary iodine intake (median UI 70 microg/L) showed an increase in UI that rose to a median value of 135 microg/L in the first trimester (T1) and continued at 124 microg/L in the second (T2) and 122 microg/L in the third trimester (T3). In parallel with the increase in UI, mean ultrasound measured thyroid volume increased by a maximum of 47% over nonpregnant values in the third trimester (T3). Although these findings were consistent with studies in Cardiff, UK (median UI 73 microg/L), which also showed a pregnancy-associated rise in UI excretion (maximum 176 microg/L) accompanied by a 30% increase in median thyroid volume, they differed from findings in Sri Lanka (median UI 146 microg/L), a country in which a successful program of salt iodination has recently been implemented, which showed no significant changes in UI excretion (T3 maximum 154 microg/L) but did show a modest (20%) maximum increase in median thyroid volume at T3. Prospective studies on Irish subjects showed that median UI fell precipitously to nonpregnant control values (76 microg/L) at delivery. In addition, UI in neonates sampled at 3-days postdelivery showed that excretion was greater in breast-fed than in bottle fed infants. Differences in reported UI excretion patterns during pregnancy may may reflect the existence of a threshold above which increased renal clearance results in increased iodine loss but that is masked at higher iodine intakes. Assuming constant dietary iodine intake during pregnancy, any increased urine loss will inevitably lead to negative iodine balance and thyroid depletion. In these circumstances, increased thyroid volume may in part be a compensatory mechanism to allow for greater iodine storage.