Severe iodine deficiency results in impaired thyroid hormone synthesis and/or thyroid enlargement (goiter). Population effects of severe iodine deficiency, termed iodine deficiency disorders (IDDs), include endemic goiter, hypothyroidism, cretinism, decreased fertility rate, increased infant mortality, and mental retardation. (See Pathophysiology and Presentation.)[1]
Iodine is a chemical element. It is found in trace amounts in the human body, in which its only known function is in the synthesis of thyroid hormones. Iodine is obtained primarily through the diet but is also a component of some medications, such as radiology contrast agents, iodophor cleansers, and amiodarone. (See Etiology.)
Worldwide, the soil in large geographic areas is deficient in iodine. Twenty-nine percent of the world’s population, living in approximately 130 countries, is estimated to live in areas of deficiency (see the Table).[2] This occurs primarily in mountainous regions such as the Himalayas, the European Alps, and the Andes, where iodine has been washed away by glaciation and flooding. Iodine deficiency also occurs in lowland regions far from the oceans, such as central Africa and Eastern Europe. Persons who consume only locally produced foods in these areas are at risk for IDD. See the distribution of iodine deficiency in the image below. (See Epidemiology.)
| Iodine Deficiency | None | Mild | Moderate | Severe |
| Median urine iodine, mcg/L | >100 | 50-99 | 20-49 | < 20 |
| Goiter prevalence | < 5% | 5-20% | 20-30% | >30% |
| Neonatal thyroid stimulating hormone (TSH), >5 IU/mL whole blood | < 3% | 3-20% | 20-40% | >40% |
| Cretinism | 0 | 0 | + | + |
Normal dietary iodine intake is 100-150 mcg/day. The US Institute of Medicine’s (IOM’s) recommended dietary allowance (RDA) of iodine is as follows:
Adults and adolescents - 150 mcg/day
Pregnant women - 220 mcg/day
Lactating women - 290 mcg/day
Children aged 1-11 years - 90-120 mcg/day
Infants - Adequate intake is 110-130 mcg/day
WHO’s recommendations are similar, although the organization recommends 200 mcg/day for pregnant or lactating women and 50-90 mcg/day for infants younger than 1 year.
Sources of dietary iodine
In areas where iodine is not added to the water supply or food products meant for humans or domesticated animals, the primary sources of dietary iodine are saltwater fish, seaweed, and trace amounts in grains. The upper limit of safe daily iodine intake is 1100 mcg/day for adults; it is lower for children.[3, 4, 5, 6]
In the United States, iodine has been voluntarily supplemented in table salt (70 mcg/g). Salt was selected as the medium for iodine supplementation because intake is uniform across all socioeconomic strata and across seasons of the year, supplementation is achieved using simple technology, and the program is inexpensive. The estimated annual cost of iodine supplementation of salt in the United States is $0.04 per person. (See Treatment and Medication.)
Other major sources of dietary iodine in the United States are egg yolks, milk, and milk products because of iodine supplementation in chicken feed, the treatment of milk cows and cattle with supplemental dietary iodine to prevent hoof rot and increase fertility, and the use of iodophor cleaners by the dairy industry.[7]
Changes in US iodine intake
In the early 1900s, the Great Lakes, Appalachian, and northwestern regions of the United States were endemic regions for IDD, but since the iodization of salt and other foods in the 1920s, dietary iodine levels generally have been adequate. However, sustaining these iodization programs has become a concern.
Data collected in the United States by National Health and Nutrition Examination Survey I (NHANES I) for the years 1971-1974 showed that the median urinary iodine level was 320 mcg/L, reflecting adequate dietary iodine intake.[8] However, by the time of NHANES III (1988-1994), the median urinary iodine value had fallen to 145 mcg/L.
The reduction in US dietary iodine intake since the 1970s has likely been the result of the removal of iodate conditioners in store-bought breads, widely publicized recommendations for reduced salt and egg intake for blood pressure and cholesterol control, the increasing use of noniodized salt in manufactured or premade convenience foods, decreased iodine supplementation of cattle feed, poor education about the medical necessity of using iodized salt, and reduction in the number of meals made at home.[8, 9, 10]
The NHANES surveys of 2001-2002, 2005-2006, and 2007-2008 showed that US dietary iodine intake has stabilized.[9, 10] Although the most recent NHANES survey reveals adequate iodine intake in the general US population, certain groups have an insufficient intake of iodine, such as pregnant women, who were found to have a median urinary iodine concentration of 125 mcg/L).[11]
Population-based assessment and treatment
In population-based assessments, iodine sufficiency can be determined based on the results of a spot urine test for iodine and creatinine.[9] Supplementation can be achieved by using iodized salt in cooking or a once-daily multiple vitamin containing sodium iodide. (See Workup, Treatment, and Medication.)[3, 12]
Pediatric considerations
Iodine stores within the thyroid increase with age in pediatric patients. Therefore, infants and young children tend to have higher131 I uptake than do adults. Additionally, newborns and young infants are much more severely affected by iodine deficiency than are adults and are more likely to become overtly hypothyroid. (See Presentation.)
Prenatal considerations
Women with severe iodine deficiency are more likely to experience infertility, and pregnancy in this group is more likely to result in miscarriage or congenital anomalies. Thyroid hormones are essential for fetal brain growth and development, and severe maternal iodine deficiency may lead to mental and growth retardation or cretinism in offspring. Even in areas of borderline iodine intake, as many as 10% of women may develop goiter during pregnancy. (See Presentation.)
Patient education
The public must understand the importance of using iodized salt, especially in the United States, where iodization of salt is not mandated by law. Several areas of the world, including the United States, Australia, and the Netherlands, where iodine deficiency was eradicated by voluntary methods, have later shown a significant decrease in iodine intake.
For patient education information, see the Thyroid and Metabolism Center and the Pregnancy Center, as well as Thyroid Problems and Miscarriage.
