Iodine Status and Thyroid Function in Lactating Women and Infants – a Survey in the Zagreb Area, Croatia

Lactating women (LW) and infants have high dietary iodine requirements and are at risk of iodine deficiency. The aim of the study was to assess iodine status and thyroid function in LW and their breastfed infants in Zagreb, Croatia. The study included 133 LW and breastfed infant pairs. Urinary iodine concentration (UIC) and thyroid function parameters were measured in all subjects. In LW, breast milk iodine concentration (BMIC) was measured and iodine and salt rich food frequency questionnaire data were collected. Results of analysis indicated that 99.2% of the LW used iodized salt in household and 20.4% used iodine-containing vitamin and mineral supplements. Median (IQR) UIC was 75 µg/L (19.0-180.5 µg/L) in LW and 234 µg/L (151.0-367.5 µg/L) in infants, whereas BMIC was 121 µg/kg (87.8-170.8 µg/kg). Multivariate regression analysis revealed BMIC to be a significant predictor of infant UIC (p<0.001). Positive correlation was recorded between LW and infant thyroid function. This was the first study in Croatia demonstrating BMIC to be a reliable biomarker of iodine status during lactation and predicting iodine intake in breastfed infants. The study confirmed that mandatory salt iodization in Croatia ensured sufficient dietary iodine for LW and optimal iodine intake for breastfed infants via breast milk.