The influence of gestational stage on urinary iodine excretion in pregnancy.

INTRODUCTION: Median urinary iodine concentration (UIC) is the most commonly used indicator of population iodine nutrition. However, its validity as an indicator of dietary intake relies on a stable relationship between dietary iodine intake and urinary excretion. Physiological alterations in normal pregnancy, such as increased glomerular filtration rate, potentially invalidate UIC as an assessment tool in pregnancy. OBJECTIVE: The objective of the study was to document the impact of advancing gestation on UIC in normal pregnancy and determine whether the current reference intervals for general population iodine monitoring are appropriate for use in the context of pregnancy. DESIGN: Tasmania has a well-described history of mild iodine deficiency (school-age median UIC of 84 microg/liter). We assessed UIC in 759 urine samples from 431 women attending the Antenatal Clinic at the Royal Hobart Hospital, Tasmania's primary teaching hospital. MAIN OUTCOME: The overall median UIC during pregnancy was 75 microg/liter (95% confidence interval 70.03-79.97 microg/liter) at a median gestation of 19.4 wk. Stratification by gestation, however, revealed a dynamic relationship between ioduria and gestation. Median UIC was elevated in early pregnancy and subsequently declined with advancing gestation. CONCLUSION: In this mildly iodine-deficient population, current reference intervals for UIC overestimated the adequacy of iodine nutrition during the first and early second trimester of pregnancy. Gestation-specific UIC reference intervals are required to classify iodine nutrition during pregnancy. This is particularly important in populations with borderline iodine deficiency.

A case for universal salt iodisation to correct iodine deficiency in pregnancy: another salutary lesson from Tasmania.

OBJECTIVE: To assess the impact of iodine fortification of bread on the iodine status of pregnant women, and to determine if studies of iodine levels in school-age children were indicative of women's gestational iodine status. DESIGN: Urinary iodine surveys of pregnant Tasmanian women before and after bread was fortified with iodine in October 2001. PARTICIPANTS AND SETTING: 285 women attending the Royal Hobart Hospital (RHH) antenatal clinic from 1 October 2000 to 30 September 2001 and 517 women attending the RHH antenatal clinic or primary health care centres in 2003-2006. MAIN OUTCOME MEASURES: Median urinary iodine concentration (UIC) for comparison against the World Health Organization recommendation of of 150-249 microg/L for pregnant women. RESULTS: Before supplementation, the median UIC of the 285 women attending the RHH antenatal clinic was 76 microg/L. After supplementation, median UICs were 81 microg/L for 288 women attending primary health care centres and 86 microg/L for 229 women attending the RHH antenatal clinic. Differences in mean UIC were not significant for either the antenatal clinic group (P=0.237) or the primary health care group (P=0.809) compared with the pre-supplementation group. CONCLUSIONS: Iodine deficiency in pregnancy persists despite being corrected in Tasmanian children. Successful iodine supplementation must target reproductive-age and pregnant women and be substantiated by ongoing monitoring during pregnancy and lactation. A robust national program for correcting iodine deficiency is urgently needed. Mandatory universal salt iodisation has international endorsement, and should be considered the preferred strategy for eliminating iodine deficiency in Australia.