Iodine Status and Reproductive Health in Northern Norway: Assessing iodine status, dietary intake and clinical practices in maternal and infant health – The MISA 2 and the FiT Futures 3 study

Background: Iodine is an essential trace element required for normal thyroid hormone production. Adequate iodine intake is particularly important during the reproductive years because iodine requirements increase during pregnancy and lactation. Maternal iodine intake furthermore determines the iodine supply to the breastfed infant. Recent studies from Norway and other European countries have reported insufficient iodine status among women of reproductive age, indicating that mild-to-moderate iodine deficiency remains a public health concern. Healthcare professionals are well positioned to provide iodine-related dietary guidance.

Aim: To evaluate iodine status among women of reproductive age at different stages, including breastfeeding mother-infant dyads, and to investigate healthcare professionals’ clinical practices related to iodine nutrition.

Methods: This thesis comprises three papers based on data from the MISA 2 study (2017–) and the Fit Futures 3 study (2021-2022), including women of reproductive age in Northern Norway. A cross-sectional study assessed iodine status among non-pregnant and pregnant women by measuring urinary iodine concentration (UIC), dietary iodine intake, and determinants of UICs. A longitudinal study with repeated measurements at 8 weeks and 4 months postpartum assessed iodine status in breastfeeding mother–infant dyads using maternal and infant UIC, breastmilk iodine concentration (BMIC), and dietary iodine intake. A cross-sectional survey investigated iodine-related knowledge and clinical practices related to iodine among midwives and public health nurses.

Results: Insufficient iodine status was observed among women at different reproductive stages in Northern Norway. Estimated dietary iodine intake was below the recommended intake levels, and UIC indicated insufficient iodine status across all groups of women in this thesis. Iodine supplement use was consistently associated with higher UIC and emerged as the strongest dietary determinant of iodine status across the study populations. The lowest intake of iodine-rich foods and the lowest use of iodine-containing supplements were observed among non-pregnant women. Among breastfeeding mother–infant dyads, changes in BMIC were significantly associated with changes in infant UIC. Infant iodine status remained adequate according to current WHO criteria. Furthermore, by both midwives and public health nurses, a lack of knowledge about iodine was observed and they placed a greater emphasis on other key nutrients and recommendations than iodine in their clinical practice.

Conclusion: The findings demonstrate a suboptimal iodine status among women of reproductive age in Northern Norway together with low iodine-related knowledge and limited clinical practice among healthcare professionals. The findings emphasize the importance of strengthened preventive strategies, particularly before conception.