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: To document the impact of advancing gestation on UIC in normal pregnancy, and to determine if 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 84mcg/L). 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 mcg/L (95% confidence interval 70.03–79.97 mcg/L) at a median gestation of 19.4 weeks. 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.