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.