ZRT Laboratory

from Office of Dietary Supplements
National Institutes of Health
Click here for the original article.

Iodine is an essential component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Thyroid hormones regulate many important biochemical reactions, including protein synthesis and enzymatic activity, and are critical determinants of metabolic activity [1,2]. They are also required for proper skeletal and central nervous system development in fetuses and infants [1].

Iodine status of general U.S. population

Iodine status is typically assessed using urinary iodine measurements. Urinary iodine reflects dietary iodine intake directly because people excrete more than 90% of dietary iodine in the urine [4]. Spot urine iodine measurements are a useful indicator of iodine status within populations [19,20]. However, 24-hour urinary iodine or multiple spot urine measurements are more accurate for individuals [4]. For a population of school-aged children or non-pregnant adults to be iodine sufficient, median urinary iodine concentrations should be greater than 100 mcg/L and no more than 20% of the population should have values lower than 50 mcg/L [3].

Urinary iodine measurements from NHANES have been used since 1971 to monitor the iodine status of the U.S. population [21]. Since the inception of the NHANES monitoring program, urinary iodine measurements have shown that the U.S. population is iodine sufficient. However, urinary iodine levels decreased by more than 50% between 1971–1974 and 1988–1994 [2,22]. Much of this decline was due to decreased levels of iodine in milk due to the reduced use of iodine-containing feed supplements and iodophor sanitizing agents in the dairy industry [23], as well as the reduced use of iodate dough conditioners by commercial bakers. The use of erythrosine, an iodine-containing food dye commonly used in fruit-flavored breakfast cereals, also decreased during this time [23], though it is unclear to what extent this change actually affected urinary iodine levels since the bioavailability of iodine from erythrosine has been found to be low [24]. This sharp decline in urinary iodine levels caused some concern during the late 1990s that the iodine sufficiency of the U.S. population could be at risk if this trend continued [22].

More recent NHANES measurements indicate that urinary iodine levels have stabilized in the U.S. population. The median urinary iodine level in NHANES 2003–2004 participants aged 6 years or older was 160 mcg/L, and 11.3 ± 1.8% had a urinary iodine level lower than 50 mg/L [21]. Among women of reproductive age, the median urinary iodine level was 139 mcg/L, and 15.1 ± 3.2% had levels lower than 50 mcg/L. Women who are restricting their dietary salt intake however, have been found to have lower urinary iodine concentrations and might be more likely to be iodine deficient than women who aren't restricting salt intake [25].

Iodine Status of U.S. Pregnant Women

According to the WHO, a median urinary iodine concentration of 150–249 mcg/L indicates adequate iodine nutrition during pregnancy, while values less than 150 mcg/L are considered insufficient [3,7]. An analysis of NHANES 2003–2004 data showed that pregnant U.S. women had a median urinary iodine concentration of 181 mcg/L [21], while NHANES 2001–2006 data indicate a somewhat lower value of 153 mcg/L [26].

These data suggest that most pregnant women in the United States are iodine sufficient but that some might have marginal iodine status. Overall, these findings affirm that the general U.S. population has adequate iodine intake but support the need for continued iodine monitoring, particularly among such subgroups as pregnant women and children, who are most susceptible to iodine deficiency disorders.

Iodine Deficiency

Iodine deficiency has multiple adverse effects on growth and development, and is the most common cause of preventable mental retardation in the world [28]. Iodine deficiency disorders result from inadequate thyroid hormone production secondary to insufficient iodine [5]. During pregnancy and early infancy, iodine deficiency can cause irreversible effects.

If a person's iodine intake falls below approximately 10–20 mcg/day, hypothyroidism occurs [1], a condition that is frequently accompanied by goiter. In infants and children, less severe iodine deficiency can also cause neurodevelopmental deficits such as somewhat lower-than-average intelligence as measured by IQ [1,29]. Mild to moderate maternal iodine deficiency has also been associated with an increased risk for attention deficit hyperactivity disorder in children [30]. In adults, mild-to-moderate iodine deficiency can cause goiter as well as impaired mental function and work productivity secondary to hypothyroidism. Chronic iodine deficiency may be associated with an increased risk of the follicular form of thyroid cancer [31].

Groups at Risk of Iodine Inadequacy

• People living in regions with iodine-deficient soils
• People with marginal iodine status who eat foods containing goitrogens
• People who do not use iodized salt

 For reference, see the original article.