The State of U.S. Iodine Nutrition: How Can We Ensure Adequate Iodine for All?

T he report by Caldwell et al. (1) appearing in this issue of Thyroid has important implications for public health. It includes data from the most recent National Health and Nutrition Examination Surveys (NHANES) and pilot data from the National Children's Study (NCS). Together, these provide the largest U.S. sample of urinary iodine concentrations (UIC), a marker for population iodine status, in pregnant women. It is also the first U.S. study to report trimester-specific and region-specific UIC measures. The overall U.S. median UIC in 2009–2010 was 144 μg/L; while still in the optimal range, it has fallen from 164 μg/L in 2007–2008 and is the lowest value observed since NHANES began in the early 1970s. As in other recent NHANES analyses, UIC was lower in women of childbearing age than in other groups. The median UIC for U.S. pregnant women in NHANES 2005–2010 was 129 μg/L, well below the 150 μg/L threshold advocated by the International Council for the Control of Iodine Deficiency Disorders (ICCIDD) Global Network and the World Health Organization for pregnancy (2). Interestingly, median UIC values in pregnant women were low in the first and second trimesters, and normalized by the third trimester. More studies are needed to determine whether this is related to dietary changes, the addition of iodine-containing supplements relatively late in pregnancy, or other factors.

Adequate dietary iodine intake is particularly important in pregnancy, since it is required for normal fetal neurodevelopment. It has been recommended that pregnant women should ingest 220 μg iodine daily, higher than the 150 μg/d recommended for nonpregnant adults (3). A recent systematic review by Bougma and colleagues (4) concluded that iodine deficiency in utero or in early infancy results in a 6.9 to 10.2 point IQ decrement. A recent study in Tasmania examined educational outcomes in 9-year-old children in relation to their mothers' pregnancy UIC values (5) and found that children of women with UIC <150 μg/L had poorer spelling, grammar, and English literacy scores compared with those of women with UIC ≥150 μg/L. Similarly, the Avon Longitudinal Study of Parents and Children (ALSPAC) in the United Kingdom recently found that children of women with UIC values <150 μg/g creatinine in pregnancy were at increased risk for low verbal IQ, reading accuracy, and reading comprehension scores at age 9 (6).

Important sources of dietary iodine in the United States include iodized salt, dairy foods (due to use of iodophor cleansers by the dairy industry), and bread (due to the use of iodate as a bread conditioner by some commercial bakeries) (7). Iodized salt use in the United States has likely declined over the last several decades due to recommendations for reduced salt intake for blood pressure control and to the increasing use of noniodized salt in commercially processed foods. This may explain why salt consumption was not a significant predictor of UIC among women of childbearing age in NHANES (1). With the exception of iodized salt, to which iodide is added voluntarily as a public health measure, major sources of iodine in the U.S. diet are largely unregulated, unmonitored, and added for the convenience of food manufacturers rather than with a public health goal in mind. Because it is not required, most food sources of iodine are not listed on package labeling.

Can we identify which U.S. individuals are at risk for iodine deficiency? While UIC is an excellent marker for the dietary iodine status of populations, urinary iodine measurements cannot be used as a marker of an individuals' iodine status due to substantial day-to-day and diurnal variation in urinary iodine excretion (8). However, the data from Caldwell and colleagues (1) suggest that we may be able to identify some groups at particular risk. The NHANES data demonstrate that non-Hispanic blacks are at increased risk for low iodine intake compared with non-Hispanic white and Hispanic populations, possibly due, at least in part, to their lower reported dairy intake. Similarly, we reported low urinary iodine levels in Boston-area individuals with vegan, but not vegetarian, diets (9). In the NCS sample, median UIC values varied regionally, with low median UIC values seen at study sites in California, Pennsylvania, and Wisconsin. More studies are needed to determine the causes of these regional variations.

Given low median UIC values in pregnant U.S. women for the last several years; concerns about adverse effects on child development; and the absence of widely available, labeled, and regulated food sources of iodine in the United States, supplements have been recommended for vulnerable populations. The American Thyroid Association, Endocrine Society, and the Neurobehavioral Teratology Society have all recommended that U.S. women who are pregnant, lactating, or planning a pregnancy should ingest a daily supplement containing 150 μg of iodine in the form of potassium iodide (10 –12). Unfortunately, 49% of the types of prenatal multivitamins marketed in the United States do not contain any iodine (13), and data from NHANES (2001–2006) demonstrated that only 20% of U.S. pregnant women routinely took an iodine-containing supplement (14). Achieving optimal iodine supplementation for pregnant U.S. women is currently an urgent and important public heath goal. However, systematic dietary changes that would ensure adequate iodine intake across the general U.S. population could ultimately make such supplementation unnecessary.

How could a consistent source of iodine be safely added to the U.S. diet? Universal salt iodization has been the strategy most widely adopted around the world to ensure adequate iodine nutrition (15). In the 1940s, an attempt was made to mandate iodization of all U.S. table salt. However, this was thwarted by opposition from food processors and by those who argued that the law would unconstitutionally force individuals to take a medication (16). Since then, there has been no concerted effort to ensure iodine fortification of the U.S. diet. In some countries, such as Belgium, Denmark, and Australia, mandated use of iodized salt in bread baking has been used to achieve adequate population iodine nutrition (15). In Switzerland, iodized salt is used voluntarily in a wide variety of processed foods; with careful monitoring of population UIC values, this has ensured optimal and stable iodine nutrition (17,18). This approach would also likely be highly effective in the United States. In order to accomplish this, we will need coordinated efforts by food manufacturers, continued population-level iodine status assessments, and the monitoring of iodine content of common food sources by U.S. scientific and regulatory agencies. Although challenging, we believe that this is a potentially achievable goal.