During the World War I draft, a Michigan physician named Simon Levin examined 583 registrants and found that 30.3 percent of them had an enlarged thyroid. Many of the swellings were big enough to disqualify a man from military service. Surveys run the following year with the state’s public health commissioner put goiter rates as high as 64.4 percent in parts of Michigan.
Those figures come from a 2012 history of American iodine fortification published in the journal Nutrients by Angela Leung, Lewis Braverman and Elizabeth Pearce. Before the 1920s the affected region had a nickname: the goiter belt, running across the Great Lakes, Appalachia and the Northwest. Inside that belt, the same paper reports, between 26 and 70 percent of children had a visibly swollen neck.
Iodized salt reached Michigan grocery shelves on 1 May 1924. The goiter belt was effectively gone within a generation. It is one of the cheapest public health victories in American history.
And nobody ever made it mandatory. A 1948 bill from the U.S. Endemic Goiter Committee that would have required iodized salt in all states was defeated, and no federal requirement has replaced it. Whether the salt in your shaker contains iodine is still a decision each salt company makes on its own.
The mandate that never arrived
The Office of Dietary Supplements at the National Institutes of Health states this plainly on its iodine fact sheet for health professionals, last updated 5 November 2024. Salt manufacturers have been adding iodine to table salt since the 1920s, the agency says, and the practice is still voluntary.
That makes the United States unusual rather than typical. Leung and colleagues report that roughly 120 countries, including Canada and parts of Mexico, require iodization of all food-grade salt. The same paper notes that the Food and Drug Administration does not require iodine content to appear on food packaging at all.
Voluntary iodization worked anyway, which is the genuinely interesting part. The proportion of American households using only iodized salt has held steady at 70 to 76 percent since the 1950s, according to Leung’s review. The International Council for the Control of Iodine Deficiency Disorders has estimated that household access now tops 90 percent, though the authors are careful to add that data on actual usage is limited.
So the country solved a serious deficiency through a product choice rather than a law. The cost of that approach is that nothing holds the floor in place. There is no agency monitoring whether the share of iodized salt in the food supply drifts down, because nothing requires it to stay up.
Most of the salt you eat is not iodized
This is the misconception worth correcting first, because it reverses what most people assume. If you eat a lot of salt, you are not necessarily eating a lot of iodine.
The NIH Office of Dietary Supplements puts it directly: “most salt intake in the United States comes from processed foods, and food manufacturers almost always use noniodized salt in these foods.” Leung’s review reaches the same conclusion independently, stating that the majority of American salt consumption comes from processed foods made primarily with non-iodized salt.
There is one labeling hook, and it is small. ODS notes that when a manufacturer does use iodized salt in a food, the ingredient list has to identify the salt as iodized. That means the information exists on some packages, but only on the ones where the answer happens to be yes. The word salt on its own, with no modifier, tells you nothing either way.
The result is a strange gap in the American diet. The salt that carries iodine is the salt in the shaker, which is a small share of what most people consume. The salt that supplies most of the sodium in a typical diet carries essentially none.
How much iodine is in iodized salt
Here the published figures do not line up neatly, and both belong in the record.
ODS reports that according to its label, iodized salt in the United States contains 45 mcg of iodine per gram of salt. That is the same as 45 mg per kilogram. Leung’s review gives the same fortification figure in the other unit, 45 mg iodide per kilogram, and then reports something the label figure does not reveal: the FDA’s recommended range is 46 to 76 mg iodide per kilogram. The standard fortification level sits a hair below the bottom of the recommended range. ODS adds a figure that complicates it: measured samples averaged 47.5 to 50.7 mcg iodine per gram, which is inside the recommended range. An average inside a range and half of sampled brands below it are both possible at once.
Measured product does worse. The same review reports that of 88 table salt brands sampled, 47 contained less than the FDA’s recommended range. More than half the brands tested came in under the target.
Neither source is wrong. The label claim, the recommended range and the measured contents are three different things, and the only one printed on a package is the first.
Who the iodine ceiling applies to, and what it covers
These are reported values from federal tables, not a target this site is setting for anyone.
The Food and Nutrition Board’s Recommended Dietary Allowance for iodine, as published by ODS, is 150 mcg per day for men and women aged 19 and over. It rises to 220 mcg per day during pregnancy and 290 mcg per day during lactation. The World Health Organization’s figure for pregnancy and lactation, cited in Leung’s review, is 250 mcg of total daily intake, which is higher than the US pregnancy value and lower than the US lactation value.
The Tolerable Upper Intake Level is 1,100 mcg per day for adults 19 and older, including during pregnancy and lactation. The scope matters as much as the number. ODS states that the Food and Nutrition Board set iodine ULs for food and supplement intakes, so the ceiling covers both routes combined, not supplements alone. It also carries an explicit carve-out: the ULs do not apply to people receiving iodine for medical treatment, who are under a physician’s care. The upper limit as a concept belongs to article 323.
One more number shows up on labels. FDA’s labeling regulation at 21 CFR 101.9 puts iodine’s Daily Value at 150 mcg, which covers adults and children from age 4 up. That is the figure any percent-of-daily-value claim on a package is measured against.
Both too little and too much damage the same gland
This is the fact that makes iodine different from almost every other nutrient, and the cleanest statement of it comes from the federal fact sheet itself. ODS writes that “High intakes of iodine can cause some of the same symptoms as iodine deficiency,” and then names them: goiter, elevated TSH levels and hypothyroidism.
Read that twice. Goiter is the classic sign of not enough iodine. It is also a sign of too much.
On the deficiency side, ODS describes goiter as usually the earliest clinical sign, with intakes below roughly 10 to 20 mcg per day leading to hypothyroidism. The agency is blunt about timing: during pregnancy and early infancy, iodine deficiency can cause irreversible effects, including major neurodevelopmental deficits and growth restriction in the fetus when deficiency is severe. Milder deficiency in infants and children can still produce neurodevelopmental deficits.
On the excess side, ODS states that iodine-induced hyperthyroidism can result from high intakes, and that studies have shown excessive iodine intakes cause thyroiditis and thyroid papillary cancer. The mechanism it describes for susceptible people is that excess iodine inhibits thyroid hormone synthesis, which is why the result can look like deficiency.
American medicine learned this the hard way during the first fortification push. Leung’s review records that in 1926 a physician named Hartsock described an outbreak of thyrotoxicosis among adults in the Great Lakes goiter belt who had started using iodized salt. The phenomenon has a name, Jöd-Basedow, and fear of it was one reason iodine prophylaxis was resisted for decades before 1924.
A 2014 review in Nature Reviews Endocrinology by Angela Leung and Lewis Braverman sets out who is vulnerable: people with pre-existing thyroid disease, older adults, fetuses and newborns, and people with other risk factors. The authors add two details that make this hard to catch. The resulting thyroid dysfunction can be subclinical rather than obvious, and the source of the excess iodine may not be apparent.
Iodine in pregnancy
ODS reports that overt iodine deficiency is now uncommon in North America. The agency is equally direct that this general statement hides a specific problem: a substantial portion of pregnant women in the United States are iodine insufficient.
The population measure here is urinary iodine concentration, which works for groups and not for individuals. Levels swing day to day with what a person ate, so a single reading describes a population, not a patient. ODS lists median urinary iodine concentrations for American pregnant women from several NHANES windows: 181 mcg/L in 2003 to 2004, 153 mcg/L in 2001 to 2006, 125 mcg/L in 2005 to 2008 and 144 mcg/L in 2007 to 2014. The World Health Organization’s adequacy range for pregnancy, as reported by ODS, is 150 to 249 mcg/L.
The 2005 to 2008 figure traces to a 2011 analysis in Thyroid by Kathleen Caldwell and colleagues at the Centers for Disease Control and Prevention. In that sample of 184 pregnant women, the median was 125 mcg/L and 56.9 percent fell below 150 mcg/L. Across the general population in the same cycles the median was 164 mcg/L, and the authors concluded that overall iodine nutrition in the United States had stabilized while certain groups still fell short.
How many pregnant women take iodine, and what the supplements contain
Two authoritative US sources give different answers, and the honest thing is to report both.
Leung’s 2012 review, drawing on NHANES data from 2001 to 2006, reports that only 20.3 percent of pregnant women routinely take an iodine-containing supplement. The American Academy of Pediatrics, in a 2014 policy statement in Pediatrics by Walter Rogan and colleagues, writes that only about 15 percent do. Different survey windows, both from credible sources, and no basis for splitting the difference.
What is in the bottle is a separate question. A 2019 study in Thyroid by Aneesh Patel and colleagues examined the best-selling US multivitamins sold between July 2016 and July 2017. Of 59 prenatal products analyzed, 34, or 57.6 percent, contained any iodine at all. Among those that did, the median was 150 mcg per daily serving, but the range ran from 25 to 290 mcg. Of the iodine-containing prenatals, 73.5 percent used potassium iodide and 23.5 percent used kelp. Two of the study’s six authors were affiliated with the Council for Responsible Nutrition, the supplement industry’s trade association, which is worth knowing when reading the conclusions.
The 2014 AAP statement adds the complaint that explains why that matters: supplements may not contain enough iodide and may not be labeled accurately. The academy called on the FDA to address mislabeling of iodine content in prenatal and lactation supplements.
What the professional bodies say
Two positions stand, and both are older than they look.
The American Thyroid Association issued a public health statement on the risks of excess iodine on 5 June 2013; the page carries a later date of 3 March 2024. It recommends that women take a multivitamin containing 150 mcg of iodine daily in the form of potassium iodide during preconception, pregnancy and lactation. The American Academy of Pediatrics statement from 2014 recommends a daily supplement providing at least 150 mcg of iodine along with iodized salt. Those are published positions of professional bodies, reported here as such. Neither is advice this article is giving to any individual reader, and neither is a substitute for a conversation with the clinician managing a pregnancy.
There is a wrinkle in how that recommendation gets written down, and it is a useful lesson in label arithmetic. The ATA’s own statement specifies 150 mcg of iodine in the form of potassium iodide. Leung’s 2012 review describes the same guidance from the ATA and the Endocrine Society as 150 µg of potassium iodide containing 114 µg of iodine. Potassium iodide is a compound; iodine is one element inside it, and the two numbers are not interchangeable. Both renderings are in the published literature, and a label can legitimately report either quantity.
Kelp, seaweed and the supplements that overshoot
If iodine has a genuine excess risk in the American supplement aisle, this is where it lives.
ODS reports that commercially available seaweeds in whole or sheet form have iodine concentrations ranging from 16 mcg per gram to 2,984 mcg per gram. That is a 186-fold spread between the weakest and strongest products. Do the arithmetic at the top of the range and a single gram delivers about 2.7 times the 1,100 mcg upper limit for adults.
The American Thyroid Association’s 2013 statement says many iodine, potassium iodide and kelp supplements contain iodine at up to several thousand times the daily tolerable upper limits. Its stated position is to advise against ingesting iodine and kelp supplements containing more than 500 mcg of iodine daily, for children, adults and during pregnancy and lactation. That ceiling is less than half the federal UL, which tells you how the association reads the risk.
A 2004 review in the European Journal of Clinical Nutrition by Michael Zimmermann and François Delange makes the delivery-form argument explicitly. Kelp and seaweed-based products, the authors wrote, should be avoided because the iodine content varies unacceptably. That is a European review addressing European products, so it is evidence about the form rather than about US regulation.
The clinical end of this is documented but thin. A 2019 case report in Medicine by Adriana Gherbon and colleagues describes a 70-year-old woman with no previous thyroid disease who developed a heart rate above 100 beats per minute, insomnia, anxiety and a 6 kilogram weight loss over three months after taking kelp-containing tablets for three months. Her thyroid function returned to normal after the tablets stopped and she completed three months of antithyroid treatment. One case report is one person. It establishes that this can happen, not how often it does.
When this stops being a label question
The useful signal here is a change in the body that a clinician needs to see, not a number on a bottle.
- A new lump or swelling at the base of the neck, or an old one that grows, is the classic thyroid sign and it points in both directions at once.
- A cluster of racing heartbeat, trouble sleeping, anxiety and unexplained weight loss is the pattern described in the kelp case above, and it is worth reporting along with a full list of everything being taken.
- Fatigue, cold intolerance and unexplained weight gain sit on the other side of the same gland.
- Pregnancy, a pregnancy in planning and breastfeeding are the periods where the published figures change and where deficiency has consequences that do not reverse.
- Existing thyroid disease, or treatment with an antithyroid medication such as methimazole, changes how any iodine load behaves. ODS notes that high iodine doses can have an additive effect with these drugs.
- ODS also names an interaction that connects to potassium: taking potassium iodide alongside ACE inhibitors such as lisinopril, or potassium-sparing diuretics such as spironolactone, can raise the risk of hyperkalemia. Medication interactions are covered in article 677, and the ODS list is explicitly only a few examples.
Four questions make any iodine conversation shorter. Does the supplement list iodine at all, and in what form? If the form is kelp or seaweed, does the label give a measured iodine amount or just a weight of seaweed? What else in the cabinet contains iodine, including multivitamins, prenatal products and iodine-containing medications? And is the salt at home iodized, which the package will say only if the answer is yes?
The practical summary on iodine
Iodine is the clearest case in human nutrition where the curve bends at both ends. Too little produces goiter, a rising TSH and hypothyroidism. Too much produces goiter, a rising TSH and hypothyroidism. The federal fact sheet says so in its own words, which is the strongest possible attribution for a fact this counterintuitive.
Everything else follows from that shape. It explains why the upper limit exists at 1,100 mcg a day across food and supplements, why a professional body sets its own advisory ceiling at less than half that, and why a seaweed product with an unmeasured iodine content is a different kind of object from a tablet with a number on it.
And the American system gives you less information than you would expect. The iodization that fixed the goiter belt is voluntary, the salt in processed food almost never carries it, and the package is not required to tell you the iodine content at all. The country got a spectacular public health result out of an arrangement that still asks you to know this yourself.
Sources
- National Institutes of Health, Office of Dietary Supplements. “Iodine: Fact Sheet for Health Professionals.” ods.od.nih.gov
- PubMed (Nutrients). “Leung AM, Braverman LE, Pearce EN. "History of U.S. Iodine Fortification and Supplementation." Nutrients, 2012. PubMed. DOI: 10.3390/nu4111740.” doi.org
- American Thyroid Association. “ATA Statement on the Potential Risks of Excess Iodine Ingestion and Exposure.” thyroid.org
- PubMed (Pediatrics, American Academy of Pediatrics). “Rogan WJ, Paulson JA, Baum C, et al. "Iodine deficiency, pollutant chemicals, and the thyroid: new information on an old problem." Pediatrics, 2014. PubMed. DOI: 10.1542/peds.2014-0900.” doi.org
- PubMed (Thyroid). “Caldwell KL, Makhmudov A, Ely E, Jones RL, Wang RY. "Iodine status of the U.S. population, National Health and Nutrition Examination Survey, 2005-2006 and 2007-2008." Thyroid, 2011. PubMed. DOI: 10.1089/thy.2010.0077.” doi.org
- PubMed (Thyroid). “Patel A, Lee SY, Stagnaro-Green A, et al. "Iodine Content of the Best-Selling United States Adult and Prenatal Multivitamin Preparations." Thyroid, 2019. PubMed. DOI: 10.1089/thy.2018.0386.” doi.org
- PubMed (Nature Reviews Endocrinology). “Leung AM, Braverman LE. "Consequences of excess iodine." Nature Reviews Endocrinology, 2014. PubMed. DOI: 10.1038/nrendo.2013.251.” doi.org
- PubMed (European Journal of Clinical Nutrition). “Zimmermann M, Delange F. "Iodine supplementation of pregnant women in Europe: a review and recommendations." European Journal of Clinical Nutrition, 2004. PubMed. DOI: 10.1038/sj.ejcn.1601933.” doi.org
- PubMed (Medicine). “Gherbon A, Frandes M, Lungeanu D, Nicula M, Timar R. "Transient Hyperthyroidism following the ingestion of complementary medications containing kelp seaweed: A case-report." Medicine (Baltimore), 2019. PubMed. DOI: 10.1097/MD.0000000000017058.” doi.org
- U.S. Food and Drug Administration (Electronic Code of Federal Regulations). “21 CFR 101.9, Nutrition labeling of food (Reference Daily Intakes).” ecfr.gov