Walk into any grocery store aisle and you'll see it. Rows of round, blue or white canisters sitting there for about a dollar. Most of them have a little label that says "This salt supplies iodide, a necessary nutrient." We don't really think about it. It’s just salt. But that tiny addition of iodine is actually one of the most successful public health interventions in human history. Seriously.
So, why is table salt iodized in the first place?
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It isn't for flavor. It definitely isn't a preservative. It's there because, about a hundred years ago, people in the United States—specifically in the Great Lakes, Appalachians, and Northwest—were developing massive, unsightly swellings in their necks. These were goiters. They weren't just an aesthetic problem; they were a sign of a massive nutritional failure that was affecting the brain development of an entire generation.
The Goiter Belt and the Michigan Experiment
Back in the early 20th century, there was a region of the U.S. known as the "Goiter Belt." Soil in these areas was notoriously low in iodine because prehistoric glaciers had essentially scraped the minerals away. Since the soil lacked iodine, the crops lacked iodine. Since the crops lacked iodine, the people eating them became deficient.
David Murray Cowie, a pediatrician at the University of Michigan, saw what was happening. He noticed that in some Michigan counties, nearly 64% of school-aged children had enlarged thyroid glands. That is a staggering number. Imagine two out of every three kids in a classroom having a visible physical deformity caused by their diet.
Cowie had heard about how Switzerland was experimenting with adding potassium iodide to salt. Salt was the perfect vehicle. Everyone uses it. It’s cheap. It doesn't spoil. In 1924, after some heavy convincing, the Michigan State Medical Society backed a plan to get salt producers to add iodine to their product. By 1924, Morton Salt began distributing iodized salt nationwide.
The results were almost immediate. Within a decade, the prevalence of goiters plummeted. But the benefits went way beyond just fixing necks.
Your Thyroid is a Hungry Engine
To understand why this matters today, you have to look at how the thyroid works. It’s a tiny, butterfly-shaped gland in your neck. Its only job is to take iodine, which you get from food, and turn it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3).
These hormones are basically the "gas pedal" for your cells. They control your metabolism, heart rate, and body temperature. Without enough iodine, the thyroid can't make these hormones. The pituitary gland then freaks out and sends a signal (TSH) to the thyroid to "work harder." The thyroid tries to comply by growing larger, hoping more surface area will help it trap more iodine. That’s your goiter.
But the real tragedy of iodine deficiency isn't the goiter. It's what happens to a developing brain.
The Intelligence Factor
Iodine is critical for neurodevelopment during pregnancy and early infancy. Historically, severe iodine deficiency led to a condition formerly called "cretinism," characterized by profound intellectual disabilities and stunted growth.
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Even mild deficiency can be a problem. Research from the National Bureau of Economic Research actually looked at the data from the 1920s and found that the introduction of iodized salt led to a significant jump in IQ scores in the Goiter Belt—some estimates suggest a 15-point increase in the most deficient areas. It fundamentally changed the economic trajectory of those regions because people were literally smarter and more capable of working.
The Modern Dilemma: Fancy Salts and Processed Foods
Here is the weird part. Even though we solved this problem a century ago, we might be sliding backward.
Most people think that if they eat a lot of salty food, they're getting plenty of iodine. That's a myth. Most processed foods—the stuff in boxes, the fast food burgers, the frozen pizzas—are made with non-iodized salt. Why? Because it’s cheaper for manufacturers, and they don't have to worry about the slight (though mostly imperceptible) metallic taste that iodine can sometimes impart in large-scale food processing.
Then there’s the "artisanal salt" trend.
If you’ve switched to Himalayan pink salt, sea salt, or those big flaky Kosher salt crystals, you’re likely not getting iodine from your salt shaker. These salts are often marketed as "natural" or "mineral-rich," but they usually contain negligible amounts of iodine. Unless the package specifically says "iodized," it isn't.
Who Is at Risk Now?
It’s not just about the type of salt. Our diets have changed.
- Vegans and Vegetarians: Some of the best sources of iodine are seafood and dairy. If you cut those out and don't use iodized table salt, you're at risk.
- The "Clean Eating" Crowd: If you're avoiding processed foods (good!) but also avoiding standard table salt in favor of sea salt (potentially bad!), you might be missing the mark.
- Pregnant Women: The demand for iodine spikes during pregnancy. The American Thyroid Association actually recommends that pregnant and breastfeeding women take a daily vitamin containing 150 micrograms of iodine because the stakes for the baby are so high.
Is Too Much Iodine a Thing?
Yeah, you can overdo it, but it’s pretty hard to do through salt alone. The Tolerable Upper Intake Level (UL) for adults is about 1,100 micrograms per day. A teaspoon of iodized salt contains about 250 micrograms. You’d have to eat a lot of salt to hit toxic levels, and by then, the sodium would likely be a bigger problem for your blood pressure than the iodine would be for your thyroid.
However, for people with pre-existing thyroid conditions like Hashimoto's or Graves' disease, sudden spikes in iodine can sometimes trigger or worsen symptoms. It's a delicate balance. The thyroid is sensitive.
The Global Perspective
While we take it for granted in the U.S., iodine deficiency remains the leading cause of preventable intellectual disability worldwide. Organizations like the IGN (Iodine Global Network) work with governments to implement Universal Salt Iodization (USI). In countries where people don't have access to diverse diets, that little blue box of salt is quite literally a lifesaver.
It’s one of those rare cases where a massive health problem was solved with a solution that costs pennies.
How to Make Sure You're Covered
If you're worried about your iodine levels, you don't necessarily need to start eating spoonfuls of salt. There are plenty of ways to keep your thyroid happy without overdoing the sodium.
- Check your salt label. If you love your pink Himalayan salt for the crunch, that’s fine. Just make sure you have a container of the cheap iodized stuff for your everyday cooking or baking.
- Eat from the ocean. Fish like cod and tuna are great sources. Seaweed (nori, kelp) is an iodine powerhouse—sometimes too much of one, so don't go overboard on the kelp supplements.
- Don't skip the dairy. Milk, yogurt, and cheese are solid sources, largely because of the iodine cleaners used on dairy cows and iodine-enriched feed.
- Eggs are your friend. The yolk contains a decent amount of iodine.
- Multi-vitamins. Check your daily vitamin. Many contain potassium iodide, which is a great insurance policy if you prefer non-iodized salt for cooking.
The bottom line is that why is table salt iodized comes down to a simple reality: your body can't make iodine on its own. It’s an essential element. We figured out a way to get it into the population without anyone having to think twice about it.
In a world of complicated health advice and expensive supplements, iodized salt is a throwback to a time when a simple, science-backed fix actually worked. It's not a marketing gimmick. It's a hundred-year-old success story that's still keeping us healthy today.
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Keep a container of the iodized stuff in your pantry. Use it for your pasta water or your morning eggs. It’s a tiny habit with a huge historical pedigree, and your thyroid will thank you for it.