For most of the twentieth century, Americans got more iodine than they needed without ever thinking about it. That is no longer true. National survey data show median iodine levels in US adults fell from 153 to 116 micrograms per litre between 2001 and 2020 — and in the 1970s, the figure was around 320 [7]. The story of how iodine quietly entered the food supply, raised IQ across an entire nation, and then quietly started leaking back out again is one of the strangest in public health. Here is what the research shows, and what it means for how much iodine you should be getting each day.
Table of Contents
- What Does Iodine Do?
- The Goitre Trial That Started It All
- Did Iodized Salt Really Raise IQ?
- Why Iodine Levels Are Falling
- Which Salts Contain Iodine?
- How Much Iodine Do You Need Per Day?
- Can a Test Tell You If You're Low?
- Getting Enough Iodine Without Eating More Salt
- References
What Does Iodine Do?
Iodine is a trace mineral the thyroid gland uses to build its hormones, thyroxine (T4) and triiodothyronine (T3). Those hormones set the pace of metabolism throughout the body, and a developing brain depends on them: fetuses, infants and children need adequate iodine to reach their full cognitive potential [3].
When intake runs low, the thyroid enlarges as it works harder to trap what little iodine is circulating. That swelling at the front of the neck is a goitre. The National Institutes of Health (NIH) fact sheet on iodine puts the stakes plainly: iodine deficiency is the most common preventable cause of intellectual disability in the world [3].
A century ago, goitre was everyday life across the "goitre belt" — the band of northern US states where ice-age glaciers had scraped the iodine out of the soil. First World War draft boards examining young men from those counties regularly found swollen necks. The condition was so normal that shirt collars in some towns were cut larger to fit it.
The Goitre Trial That Started It All
Marine had been circling the idea for years. In 1909 a trout hatchery called him in about fish believed to be dying of thyroid cancer; the growths turned out to be goitre, and adding iodine to the water made them vanish. Taking the idea to humans was harder — iodine had a reputation as a poison, and the local school board initially refused him permission to test it on children.
The evidence that changed everything came from a trial in Akron, Ohio. In 1917, pathologist David Marine and physician O.P. Kimball began giving schoolgirls short courses of sodium iodide dissolved in water, repeated every six months, after first surveying the town to document how widespread goitre was [1]. Girls were chosen because they developed goitre at twice the rate of boys.
By 1922, the results were unambiguous. Among girls who started with normal thyroids and received nothing, more than 25% developed thyroid enlargement. Among those who received iodide, the figure was 0.2% — two girls out of roughly nine hundred [2].
Delivering iodine to a whole population was the next problem. Pills get forgotten, especially by children. The solution came from Michigan paediatrician David Murray Cowie: put the iodine in table salt. Everyone buys salt, it never spoils, and rich and poor eat about the same amount. From 1924, American table salt carried potassium iodide at 100 mg per kilogram [2]. Goitre collapsed, Switzerland and Italy saw the same result with their own programmes, and the worldwide cost of salt iodization still runs at just two to five cents per person per year.
Did Iodized Salt Really Raise IQ?
The goitre prevention was the intended effect. The effect on intelligence took ninety years to surface.
In 2013, economists James Feyrer, Dimitra Politi and David Weil compared cognitive test scores from American military draft records across two generations: men who fought in the First World War, born before iodized salt existed, and men drafted for the Second World War, born just after salt was iodized in 1924. For the quarter of the population living in the most iodine-deficient areas, the intervention raised IQ by approximately one standard deviation — roughly fifteen points [6].
A natural experiment like this is not a randomized trial, but the finding sits on top of consistent evidence. A 2009 randomized controlled trial of 184 mildly iodine-deficient schoolchildren in Dunedin, New Zealand found that iodine supplementation improved overall cognitive scores by 0.19 standard deviations, with clear gains in perceptual reasoning [5].
The economists calculated that the gain was large enough to explain roughly one decade's worth of the long-running upward trend in American IQ scores known as the Flynn effect [6]. Modern reviews estimate moderate-to-severe childhood iodine deficiency costs around 12 to 13.5 IQ points. The mineral in a salt shaker turned out to be one of the cheapest cognitive interventions ever deployed.
Why Iodine Levels Are Falling
US salt iodization has been voluntary since 1924. It was never written into law, and for decades that didn't matter, because two accidental iodine sources were quietly propping up the national intake.
The first was dairy. Cows receive iodine in their feed, and dairies long used iodine-based sanitisers on udders and milking equipment. Some of that iodine ends up in the milk [3].
The second was bread. Industrial bakers used iodate as a dough conditioner for decades because it made dough easier to machine.
Both pillars gave way. Dairies switched to other sanitisers, and around 1980 bakers swapped iodate for bromate. The NIH reports that urinary iodine levels in Americans fell by more than 50% between 1971–1974 and 1988–1994 — from roughly 320 to 145 micrograms per litre — driven by exactly these changes [3].
Australia ran the same experiment on a faster clock. When its dairy industry phased out iodine sanitisers in the 1990s, iodine deficiency re-emerged in Tasmania within five years [4]. Australia and New Zealand responded in 2009 by making iodized salt mandatory in bread.
The United States did not. NHANES survey data covering 2001 to 2020 show median urinary iodine in adults falling from 153 to 116 micrograms per litre, and in children from 243 to 166 [7]. Over the same period, the proportion of children with iodine intake below the estimated average requirement rose from 15.4% to 27.6%, and in adults from 15.0% to 17.9%. Females were affected more often than males at every age, and people who rarely consumed milk products were far more likely to fall short — consistent with dairy's role as an accidental iodine source [7].
The sufficiency threshold for a population is 100 micrograms per litre. America is not yet a deficient country — what it has lost is its buffer, and the trend line points the wrong way.
Which Salts Contain Iodine?
Only salt labelled "iodized" reliably carries iodine. The premium options — sea salt, kosher salt, pink Himalayan, fleur de sel — are not usually iodized and carry essentially none [3].
The bigger catch is processed food. Most of the salt Americans actually eat arrives inside packaged and restaurant food, and manufacturers almost always use non-iodized salt. Swapping the home salt shaker for pink crystals while eating a typical modern diet removes one of the last iodine sources without replacing it.
Seafood and eggs help but cover less than many people assume. An egg contributes a meaningful amount, canned tuna carries only about seven micrograms, and most fruit, vegetables and grains sit at one to three micrograms per serving — against a daily requirement of 150.
How Much Iodine Do You Need Per Day?
The recommended daily allowance (RDA) for adults is 150 micrograms. Requirements rise by about half in pregnancy, because thyroid hormone is a raw material for building the fetal brain. Clinical guidelines therefore recommend a 150 microgram iodine supplement during pregnancy, alongside folate.
Pregnancy is also where the American numbers look worst. Median iodine in pregnant women has sat below the adequacy line in every national survey since the mid-2000s, and pregnant women who had eaten no dairy in the previous day showed a median of 100 micrograms per litre against an adequacy range that starts at 150.
Can a Test Tell You If You're Low?
Not reliably. Spot urine iodine tests describe populations, not individuals — a single person's reading swings with every meal, and the NIH is explicit that these measurements cannot be used to diagnose individual iodine deficiency [3]. Even TSH (thyroid-stimulating hormone), the standard thyroid blood test, can sit entirely normal while intake is low, because rising TSH drives the thyroid to trap more of whatever iodine is available and compensates for a shortfall long before the test moves [3].
With no practical way to test yourself, the sensible approach is to make sure intake is adequate in the first place.
Getting Enough Iodine Without Eating More Salt
Eating more iodized salt is the wrong fix for most people. A meta-analysis combining 85 trials found that reducing sodium intake lowers blood pressure across the range of intakes studied [8].
The large SSaSS trial in China went further: 600 rural villages were randomized to either regular salt or a substitute made of 75% sodium chloride and 25% potassium chloride. Over a mean 4.74 years of follow-up, the salt-substitute villages had a 14% lower rate of stroke, along with fewer major cardiovascular events and deaths [9]. Adequate potassium matters, and for most people so does eating less salt, not more.
Cutting back on bread — sensible for other reasons — removes yet another iodine source in countries that add iodine to it.
That leaves food-first planning plus, where the diet falls short, low-dose supplementation aimed at the RDA rather than megadoses. Reaching 150 micrograms a day is the goal; exceeding it many times over is not.
From the MicroVitamin range
MicroVitamin includes 75 micrograms of iodine per daily dose — half the 150 microgram Daily Value — as a low-dose top-up alongside dietary sources, in keeping with its no-megadose approach: MicroVitamin Capsules.
Iodization remains one of the best returns on investment in public health history. Keeping your own intake at the recommended level is considerably cheaper still.
References
1. https://pubmed.ncbi.nlm.nih.gov/2405081/
2. https://jn.nutrition.org/article/S0022-3166(22)10117-3/fulltext
3. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/
5. https://pubmed.ncbi.nlm.nih.gov/19726593/
6. https://academic.oup.com/jeea/article-abstract/15/2/355/2691480
7. https://www.liebertpub.com/doi/10.1089/thy.2022.0103
8. https://www.ahajournals.org/doi/10.1161/CIRCULATIONAHA.120.050371
