Why Iodized Salt Is Important for You and Your Thyroid

Elizabeth Pearce, MD

Does your table salt contain added iodine? If not, maybe you should switch. For almost 100 years, iodized salt has been the mainstay of iodine deficiency disorders (IDD) prevention efforts in the much of the world.

Over the past 20 years, there has been a decline in the US population's consumption of iodine owing to an emphasis on low-sodium diets, an increased reliance on processed foods, and greater use of sea or kosher salts, which predominantly don't contain iodine. This shift has led to an increase in iodine deficiency, particularly in vulnerable populations.

Iodine is needed for thyroid hormone synthesis; lack of iodine can cause adverse consequences known as IDD, such as goiter and hypothyroidism.

How much iodine is needed? The US Food and Drug Administration recommends a daily intake of 150 µg for adults and children aged 13 years or older.

With the lack of a validated biomarker for assessing individual status, recommendations for iodine intake are targeted on the basis of regional population. Median urinary iodine concentrations can be used to assess the iodine status of populations. Because of substantial day-to-day and even hour-to-hour variability of urinary iodine levels in individuals, neither spot nor 24-hour urine iodine measurements can be used to diagnose iodine deficiency in patients.

Iodine Deficiency and Iodized Salt

Many areas of the world have been iodine deficient throughout most of human history. Three decades ago, 113 countries had documented iodine deficiency. Since then, the implementation of salt iodization programs has resulted in a remarkable decrease in IDD prevalence.

Historically, salt has been used to fortify food with iodine because adding iodine to salt is inexpensive, salt is widely consumed, and most people ingest similar levels of salt from day to day.

If populations alter salt consumption, such as in response to concerns about cardiovascular risk, salt iodine content can be adjusted to keep iodine intakes stable.

It has been estimated that over the past 25 years, salt iodization has prevented IDD in 20.5 million newborns annually, with resulting improvements in cognitive development and future earnings translating into a global economic benefit of nearly $33 billion.

This represents a truly incredible and largely unsung public health accomplishment.

Work to prevent IDD is ongoing, with 21 countries still considered iodine deficient, and mild to moderate iodine deficiency persisting in pregnancy even in many countries which are considered iodine sufficient overall.

Iodized Salt and Other Sources of Iodine

A century ago, much of the northern United States was known as the "goiter belt." The epicenter of the goiter belt was in the Great Lakes region, where goiter from iodine deficiency was present in up to 70% of schoolchildren.

Salt iodization was introduced in the United States in 1924. By the 1940s, the United States had become iodine sufficient. However, iodine intake in the United States fell by half between the early 1970s and the 1990s, and in the past decade mild iodine deficiency has recurred in pregnant women (Hollowell et al; Perrine et al).

Salt iodization has never been mandated in the United States. About three quarters of the salt ingested in the United States comes from commercially processed foods, and food processors don't typically use iodine salt in their products. Only about 10% of salt in the population's diet is thought to derive from household salt added during cooking or at the table.

Recent estimates suggest that only about half of the salt sold for household use in this country is iodized (most sea salt and kosher salt is not). Identifying food iodine content can be challenging for consumers as it is not listed on package labels.

In the United States, milk and other dairy foods are a major source of dietary iodine. Cows' milk contains iodine owing to the addition of iodine to cattle feed and use of iodophor cleansers by the dairy industry.

On average, a serving of cow's milk contains 85 µg iodine (in contrast to dairy substitutes such as almond and soy milks, which contain negligible iodine amounts). Vegans and others who do not regularly consume dairy foods are at increased risk for iodine deficiency.

Other good nutritional sources of iodine in the United States include seafood, eggs, and some commercially baked breads (those that are made using iodate dough conditioners).

Iodine Deficiency in Pregnancy

The most severe effects of iodine deficiency occur when iodine intakes are too low in pregnancy and in early life. Severe iodine deficiency in pregnant women increases risks for miscarriage, stillbirth, birth defects, and perinatal and infant mortality. Because of the importance of thyroid hormone for normal brain development, even mild to moderate iodine deficiency in pregnant women and in early life has been linked to decreased child IQ.

In normal pregnancy, maternal thyroid hormone production increases by about 50%, requiring increased iodine. Some iodine is also transferred to the fetus in the second half of gestation, when the fetal thyroid starts to function. In addition, iodine losses in the urine are increased in pregnancy.

Because of the reemergence of mild iodine deficiency in pregnant women in the United States, and concerns that even mild iodine deficiency may have adverse effects on fetal brain development, the American Thyroid Association and the American Academy of Pediatrics recommend that women who are planning pregnancy, pregnant, or lactating should take a daily supplement containing 150 µg iodine (Alexander et al; Council on Environmental Health et al).

However, over 40% of the best-selling prenatal multivitamins in the United States contain no iodine. Prenatal multivitamin formulations marketed as over-the-counter are more likely to contain iodine than prescription products. The National Institutes of Health maintain a searchable dietary supplement label database.

Lactating women have high iodine requirements because iodine is actively secreted into breast milk, where it serves as a critically important nutritional source for the breastfed infant. Iodine requirements are increased to 220-250 µg/d in pregnancy and to 250-290 µg/d during lactation.

Iodine is an essential nutrient that is particularly important in pregnancy.

In the decades since the elimination of the US goiter belt, medical providers and the general public have developed a lack of awareness about the devastating effects of iodine deficiency.

As we approach 100 years since the introduction of salt iodization in this country, it's a good time to focus on ensuring that the US population continues to have optimal iodine nutrition.

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