The Role of Iodine for Thyroid Function in Lactating Women and Infants

Abstract

Iodine is a micronutrient needed for the production of thyroid hormones, which regulate metabolism, growth, and development. Iodine deficiency or excess may alter the thyroid hormone synthesis. The potential effects on infant development depend on the degree, timing, and duration of exposure. The iodine requirement is particularly high during infancy because of elevated thyroid hormone turnover. Breastfed infants rely on iodine provided by human milk, but the iodine concentration in breast milk is determined by the maternal iodine intake. Diets in many countries cannot provide sufficient iodine, and deficiency is prevented by iodine fortification of salt. However, the coverage of iodized salt varies between countries. Epidemiological data suggest large differences in the iodine intake in lactating women, infants, and toddlers worldwide, ranging from deficient to excessive intake. In this review, we provide an overview of the current knowledge and recent advances in the understanding of iodine nutrition and its association with thyroid function in lactating women, infants, and toddlers. We discuss risk factors for iodine malnutrition and the impact of targeted intervention strategies on these vulnerable population groups. We highlight the importance of appropriate definitions of optimal iodine nutrition and the need for more data assessing the risk of mild iodine deficiency for thyroid disorders during the first 2 years in life.

Keywords: deficiency; excess; infan; iodine; lactation.

Graphical Abstract

Figure 1.

Dietary reference intakes for iodine, risk of iodine malnutrition, and subsequent thyroid dysfunction in individuals at iodine intakes ranging from deficient to excessive. The AR is the daily iodine intake estimated to meet the requirements of half the healthy individuals in a specific population group. At this intake level the risk of inadequacy is 50% to an individual. The RDA is the average daily iodine intake level sufficient to meet the iodine requirement of nearly all (97%-98%) of healthy individuals. At this intake level the risk of iodine deficiency is only 2% to 3% to an individual. The AI is the intake level assumed to be adequate when there are insufficient data to define an AR. At intakes between the RDA and the UL, the risk of inadequacy and of excess is low. At intakes above the UL, the risk of adverse effects increases. The dashed line indicates the target population distribution of iodine intakes in iodine sufficiency. The population intake distribution should be adjusted for within-person variability using the estimated AR cutpoint method (99). In iodine sufficiency, the proportion of individuals with intakes less than AR and greater than UL is less than 2% to 3%, respectively. The lower part of the figure indicates the physiological adaptation to low and excessive iodine intakes. ¹Iodine deficiency may also cause transient hyperthyroidism in adults (60). Adapted in part from (100). AI, adequate intake; AR, average requirement; RDA, recommended daily allowance; UL, upper level.

Figure 2.

Schematic illustration of daily AR and iodine excretion pathways in lactating women and infants. Iodine is primarily excreted in urine, but in during lactation additionally also in breast milk. A small proportion of ingested iodine is excreted in feces and sweat, but the exact amounts are uncertain. A daily iodine intake of 72 µg/day has been proposed to meet the AR during the first 6 months of life (181). The AR in infants may be used to define the AR in lactating women as iodine excreted in breast milk must meet the dietary requirements of exclusively breastfed infants. By adding the infant AR (72 µg/day × 90% bioavailability) to the AR of nonlactating women (95 µg/day) (172), the AR for lactating women is estimated to approximately 175 µg/day. Iodine sufficiency is currently assessed by the population median UIC and BMIC, but may also be defined as less than 3% of individuals with intakes less than AR (after adjusting for intraindividual variability) (see Fig. 1). The UIC corresponding to the AR in infants is approximately 125 µg/L (181) and the subsequent population median UIC greater than 200 µg/L, higher than the median UIC threshold of 100 µg/L currently used to define adequate iodine nutrition in infants. A population median BMIC greater than 100 µg/L likely indicates adequate iodine intake in lactating women. ¹(181); ²Iodine stores in thyroid; ³(182); ⁴(172). AR, average requirement; BMIC, breast milk iodine concentration; UIC, urinary iodine concentration.

Figure 3.

Association between BMIC (blue) and maternal UIC (orange) and/or infant UIC (yellow) from cross-sectional studies conducted in lactating mothers and their breastfed infants. Data points show median BMIC, median maternal UIC, and median infant UIC obtained in mother-infant pairs (sample size, n = 52-739) (112, 145, 146, 166, 167, 203, 204, 211, 246-258). The dashed lines indicate current thresholds for the median BMIC and median UIC above which the iodine intake is considered adequate (21). The shaded areas indicate suggested optimal range for median BMIC and infant UIC based on current evidence presented in this review. The R² values were calculated based on the published data. ¹(21). BMIC, breast milk iodine concentration; UIC, urinary iodine concentration.