Have you ever wondered why salt is iodized? Most of us take the fortification of this everyday condiment for granted, yet the iodization of table salt represents one of the most surprisingly consequential — and little-known — U.S. public health initiatives of the 20th century.
In the early 1900s, iodine deficiency was rampant in much of the United States. Soil, particularly in the Great Lakes region, the Pacific Northwest, and the Appalachian Mountains, was low in the mineral iodine, and because most food was grown and produced locally, that meant people’s diets were coming up short. This led to widespread cases of goiter, which involves a pronounced swelling of the thyroid gland, and these regions became known as the “goiter belt.”
Meanwhile, scientists began to connect the dots between iodine and thyroid health. In 1922, University of Michigan professor David Cowie, MD, proposed fortifying table salt with iodine to prevent goiter. Cowie and colleagues lobbied salt companies, and in 1924 Morton Salt began adding iodine. By 1935, the incidence of goiter had fallen by as much as 90 percent in Michigan, and the remaining affected regions soon saw similar results.
We’ve learned a lot since then, including about the vital role iodine plays in human health beyond goiter prevention. Yet there’s still more to learn. Researchers continue to explore exactly how much iodine is optimal, for whom, and in what circumstances.
What Is Iodine?
Iodine is an essential micronutrient — “essential” in that it’s required for your body to function, but your body doesn’t make it, so you can get it only through your diet. Iodine supports virtually every aspect of health, primarily by helping create thyroid hormones. Iodine also has antioxidant, anti-infection, and potential anticancer properties.
A trace mineral, iodine is found in seawater as well as in certain rock formations and soil. Erosion and flooding have depleted soil iodine in many areas of the world, which has led to widespread iodine deficiencies and helps explain why iodine levels in food are so inconsistent (more on this later).
If you look at the periodic table of elements, you’ll see iodine is a halogen, a category that also includes bromine, chlorine, and fluorine. Halogens and halogen-based compounds, called halides, are often used in commercial cleaning products, as well as in pharmaceuticals and food additives. Research has found that exposure to some of these halogen-containing compounds can block the absorption of iodine in the thyroid. However, experts disagree about how much everyday exposure may threaten thyroid health — and what to do about it.
“Iodine is unusual in a lot of ways,” says Alan Christianson, NMD, author of The Thyroid Reset Diet. “It’s a high-energy atom. It’s further down on the periodic table than anything else we use nutritionally, so it’s chemically reactive. It has the capacity to trigger a great number of chemical reactions in the body.”
How Does Iodine Support the Thyroid?
The thyroid is a small, butterfly-shaped gland at the front of your neck that plays a key role in the endocrine system, your body’s hormone-regulation network. Its two lobes contain tiny sacs, called follicles, that store thyroid hormones. These hormones affect metabolism, heart rate, breathing, digestion, body temperature, skin and bone health, fertility, and brain development.
“The thyroid gland, acting through the hormones produced, has an effect on every body system,” explains April Lind, MD, a board-certified internal-medicine, pediatrics, and functional-medicine physician.
When the body needs thyroid hormones, the brain’s hypothalamus sends a signal to the nearby pituitary gland to release thyroid-stimulating hormone. This tells the thyroid to make the hormones thyroxine (T4) and triiodothyronine (T3).
Adequate iodine levels are mission-critical to this hormone-synthesis process, as well as to the conversion of T4 to the more active T3 in organs and tissues throughout the body.
“Without consistent dietary iodine, your thyroid will under- or overwork to make thyroid hormones,” writes Malini Ghoshal, RPh, MS, in The Iodine Balancing Handbook. “Both of these situations will cause multiple iodine-related health problems in the long term if they’re not corrected.”
Thyroid-hormone imbalances can affect energy, weight regulation, metabolism, and blood pressure.
How Much Iodine Do I Need?
The recommended daily allowance (RDA) of iodine for adults who are not pregnant or breastfeeding is a mere 150 micrograms. The upper intake level — the maximum daily amount considered safe — is 1,100 mcg, or just over 1 milligram.
“It’s been called the Goldilocks mineral,” says Christianson. “For most nutrients, you have a much bigger buffer zone than that.”
The iodine you consume in food and drink is absorbed in your gut and circulated through your blood. If your iodine intake is sufficient, only 10 percent of circulating iodine is brought into your thyroid; the rest is excreted in your urine within 24 to 48 hours.
In healthy adults, the thyroid concentrates and stores enough iodine to meet your body’s requirements for two to three months. It sends out a tiny amount of iodine per day, about 80 mcg, to make thyroid hormones.
If you routinely fall short on iodine, your thyroid gradually stops producing enough thyroid hormone to support your health.
I’m Pregnant: Now How Much Iodine Do I Need?
If you’re pregnant or nursing, you’ll need more iodine to support the health and development of your child. Iodine deficiency in pregnancy can result in maternal and fetal hypothyroidism, as well as in miscarriage and preterm birth.
Insufficient iodine intake is considered the most common preventable cause of delayed brain development. “It’s fundamental to a healthy baby,” says Lind.
A developing baby’s thyroid gland doesn’t start making thyroid hormones until the second trimester of pregnancy. During pregnancy, your free T4 thyroid hormones contribute to the baby’s total thyroid hormone levels. Further, babies are born with little stored iodine and “rely on breast milk or infant formula to meet their iodine needs to continue making T4,” Ghoshal writes.
To accommodate these requirements, the RDA for iodine increases to 220 mcg per day during pregnancy and 290 mcg per day while nursing.
Anyone who’s pregnant, planning to get pregnant, or lactating should supplement their diet with 150 mcg per day of iodine in the form of potassium iodide to ensure they meet these minimum levels.
Anyone who’s pregnant, planning to get pregnant, or lactating should supplement their diet with 150 mcg per day of iodine in the form of potassium iodide to ensure they meet these minimum levels, according to the American Thyroid Association, the Endocrine Society, and the American Academy of Pediatrics.
Still, because the window of tolerance is so small, many experts suggest talking with your doctor before adding supplemental iodine to your diet.
“Pregnant women are also more sensitive to big shifts in iodine,” writes Ghoshal, referencing a study suggesting that “women who consumed more iodine-rich foods and who took an iodine-containing multivitamin had higher urine iodine levels and higher incidences of thyroid disorder than those with low or even more than adequate iodine levels.”
Why Is Iodine Controversial?
Some integrative-health practitioners see the recent rise in thyroid-related diseases — including hypothyroidism and thyroid cancer — as evidence that we need more iodine than public health guidelines suggest.
“Many feel the current adult RDA of 150 mcg per day was provided as a baseline to prevent goiter and is not an appropriate level for ideal iodine balance to allow for its full range of benefits,” explains Ghoshal.
Some also argue that our increased exposure to the iodine-blocking effects of halogens (including chlorine) and halides (such as fluoride) may require us to consume dramatically more iodine in order to push out these blockers and free up iodine-receptor sites.
But an approach like this can pose dangers, especially to people with hyperthyroidism, hypothyroidism, or autoimmune thyroid conditions such as Graves’ disease.
“If you have a preexisting thyroid condition or have experienced iodine deficiency in the past, you may be more vulnerable to thyroid disorders … if you take in too much iodine,” explains Ghoshal. “This may happen even with small increases in iodine intake, but it’s particularly risky if you’re exposed to high levels of iodine for a long time or experience an iodine overdose.”
This increased vulnerability may be due to something called the Wolff-Chaikoff effect. “Basically, your thyroid has a safety switch,” Christianson explains in a blog post. “If too much iodine comes in, rather than make lethal amounts of thyroid hormone, your thyroid shuts off. It is a great example of homeostasis and has been used … to slow the thyroid when it is overactive.”
For most people, the Wolff-Chaikoff effect reverses once iodine levels normalize, but for people with thyroid problems, it can be permanent.
Christianson recommends that people avoid iodine supplements, and he views universal iodization to prevent goiter as an overcorrection. He points to research that suggests universal salt iodization has increased hypothyroidism, hyperthyroidism, and autoimmune thyroiditis in susceptible populations.
“The paradox is that populations that run on the low side of iodine have the lowest rates of autoimmune thyroid disease,” he states.
“The benefits and the harms of iodine are a U-shaped curve,” adds Lind. “If you have too little, it’s a problem; and if you have too much, it’s a problem.”
How Can I Learn My Iodine Status?
If you experience symptoms of thyroid disease, your practitioner may assess your iodine levels. But unless you receive a full thyroid workup, getting an accurate measurement of your own iodine level can be tricky.
“There are many tests available for iodine, but none of them are perfect … because there’s so much variability from reading to reading,” says Christianson.
Some tests, such as spot urine iodine elimination tests, are good for providing a snapshot of population health, but they’re not accurate for assessing individual iodine levels. Other urine iodine tests can be used for individuals but are often burdensome and expensive.
“The best way of understanding somebody’s iodine status is a dietary history,” says Lind.
How Can I Strike a Balance?
Most Americans currently get enough iodine. Still, some remain at risk of deficiency, including women of reproductive age, children, and those without access to healthy foods.
Vegans face a higher risk because their diets don’t include dairy, seafood, or eggs — all rich sources of iodine. Consuming a lot of soy and cruciferous vegetables may also exacerbate iodine deficiency because these foods contain compounds called goitrogens, which can block thyroid-hormone production.
On the other hand, it’s also possible to get too much iodine. A quarter teaspoon of iodized salt contains nearly half the RDA of iodine, while many multivitamins deliver a full day’s worth. Sea vegetables are exceptionally high in iodine, so if seaweed is a routine feature of your diet, you may actually be exceeding the sweet spot.
Further, iodine is hidden in the manufacturing and processing of some foods. Commercial bread manufacturers use dough conditioners with iodine. Commercial milk may include iodine from animal feed and from certain disinfectants used in processing. The Food and Drug Administration does not regulate iodine in most food products.
Making it even more difficult to gauge how much iodine you consume, soil-iodine levels vary regionally, affecting the foods grown in those areas. Seafoods and seaweeds range widely in iodine levels depending on where in the ocean they originated.
Most experts suggest eating a well-balanced diet and being mindful of general trends in your iodine intake. Correcting an iodine deficiency — or surplus — may require nothing more than small dietary adjustments.
If you’re not on a restrictive diet, suggests Ghoshal, your iodine intake is probably right where you need it to be.
This article originally appeared as “Iodine Intelligence” in the May/June 2025 issue of Experience Life.
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