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Iodine
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In one pass Iodine is the raw material the body uses to make thyroid hormone: too little and the neck swells (a goiter), too much and it can harm the thyroid from the other side.
Educational content, not medical advice — consult a clinician.
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Chapter 1
Sea & iodized salt
Iodine is the raw material the body uses to make thyroid hormone: too little and the neck swells (a goiter), too much and it can harm the thyroid from the other side. It is the kind of element you cannot do without and cannot overdo.
It comes from the ocean's cycle: kelp (see Kelp (Kombu)), Nori (Laver) (see Nori (Laver)), sea fish (White Fish · see Shrimp), shellfish, dairy, eggs, and iodized salt are all sources. The most reliable public-health tool is iodized salt, not because salt is healthy, but because almost everyone eats it and the dose is easy to standardize. Be careful with seaweed, whose iodine content varies enormously; a little nori is usually fine, but using kelp or seaweed powder as a supplement easily overshoots what you need.
One situation is an emergency: in someone whose overactive thyroid is not under control, a large dose of iodine (such as iodine contrast dye or a lot of kelp) can trigger a thyroid storm; if high fever, a very fast heartbeat, agitation, or confusion appear, go to the emergency department immediately.
It comes from the ocean's cycle: kelp (see Kelp (Kombu)), Nori (Laver) (see Nori (Laver)), sea fish (White Fish · see Shrimp), shellfish, dairy, eggs, and iodized salt are all sources. The most reliable public-health tool is iodized salt, not because salt is healthy, but because almost everyone eats it and the dose is easy to standardize. Be careful with seaweed, whose iodine content varies enormously; a little nori is usually fine, but using kelp or seaweed powder as a supplement easily overshoots what you need.
One situation is an emergency: in someone whose overactive thyroid is not under control, a large dose of iodine (such as iodine contrast dye or a lot of kelp) can trigger a thyroid storm; if high fever, a very fast heartbeat, agitation, or confusion appear, go to the emergency department immediately.
Numbers · How much iodine different foods carry
Iodine sources are concentrated in the ocean, and food chains far inland are low in iodine. Rough contents are below; values for the same food vary widely with origin, feed, and processing, so treat them as orders of magnitude.Seaweed (the highest and least predictable):
In testing cited by the US Office of Dietary Supplements (ODS), commercial whole or sheet seaweeds ranged from 16 µg to 2984 µg of iodine per gram, nearly a two-hundredfold spreadDried kelp is the iodine-richest type; a small piece can exceed the adult Tolerable Upper Intake Level () of 1100 µg a day, which makes it one of the most accident-prone ingredients in folk iodine supplementingNori: about 30–60 µg per gram; in the ODS table, 5 grams of dried flaked nori holds about 116 µg, and the amounts usual in sushi and soup are not a problemWakame: 40–100 µg per gram, moderateSea cucumber: 500–2000 µg per 100 g, highly variable
Sea fish and shellfish (moderate):
Cod ~100 µg per 100 gShrimp ~35 µg per 100 gSalmon ~15 µg per 100 gCanned sardines ~23 µg per 100 g
A few surprising moderate sources in the modern food chain:
Milk ~50–100 µg per liter, from iodine added to cow feed and iodine-containing disinfectants on milking equipment; it varies a lot between countriesEggs ~24 µg each, depending on the hens' feedYogurt ~50–80 µg per cup
Almost none: inland poultry, grains, fruits and vegetables, and legumes (unless the soil is iodine-rich or irrigation water comes from seawater evaporation areas).
Iodized salt is the public-health workhorse. China's standard for table salt averages about 25 mg of iodine per kg (counted as iodine, usually added as potassium iodate), or 25 µg per gram of salt; at 6 g of salt a day, that gives roughly 150 µg of iodine (some is lost in cooking). In the US, iodized salt is labeled at 45 µg per gram, the same order of magnitude. Sea salt, pink salt, Himalayan salt, and kosher salt are not iodized by default; swapping iodized salt for these healthy salts is a common way modern households unknowingly cut off their main iodine source.
In practice, check whether the salt at home is iodized; if it is not, and you rarely eat seafood, dairy, or eggs, the risk of iodine deficiency is quietly there.
Chapter 2
Raw material for thyroid hormone
Thyroid hormones carry iodine in their very names: has 4 iodine atoms, has 3.
The thyroid actively pulls iodine out of the blood into its cells, concentrating it to twenty to forty times the level in plasma; the protein that does this is the sodium-iodide symporter (NIS). Once inside, iodine is attached by thyroid peroxidase () to thyroglobulin (Tg) to build the precursors of T4 and T3.
Without enough iodine, the raw material runs short: the pituitary sends out more thyroid-stimulating hormone (), the hurry-up signal, and the thyroid grows in an effort to trap more iodine, which is how a goiter forms. When iodine is severely short, no amount of TSH can produce enough hormone.
The thyroid actively pulls iodine out of the blood into its cells, concentrating it to twenty to forty times the level in plasma; the protein that does this is the sodium-iodide symporter (NIS). Once inside, iodine is attached by thyroid peroxidase () to thyroglobulin (Tg) to build the precursors of T4 and T3.
Without enough iodine, the raw material runs short: the pituitary sends out more thyroid-stimulating hormone (), the hurry-up signal, and the thyroid grows in an effort to trap more iodine, which is how a goiter forms. When iodine is severely short, no amount of TSH can produce enough hormone.
Mechanism · How the thyroid makes its hormones
The full process by which the thyroid makes and can be seen in seven steps:1. Iodine travels in the blood as iodide (I⁻)
2. The sodium-iodide symporter (NIS, Na⁺/I⁻ symporter) on the base of the thyroid follicle cells actively concentrates iodine, bringing the level inside the cells to 20–40 times that of plasma
3. Thyroid peroxidase (), using hydrogen peroxide, oxidizes I⁻ into a reactive form of iodine
4. Tyrosine residues on thyroglobulin (Tg) are iodinated, forming monoiodotyrosine (MIT) and diiodotyrosine (DIT)
5. TPO couples them: MIT plus DIT makes T3, DIT plus DIT makes T4, and most of the hormone exists as T4
6. Thyroglobulin loaded with T4 and T3 is stored in the follicle cavity as a gel called colloid
7. Stimulated by , the cells take the colloid back in, break down the thyroglobulin in lysosomes, and release T4 and T3 into the blood
Several common drugs act at different points: methimazole and propylthiouracil (PTU) block TPO and are used for Graves' hyperthyroidism; high-dose iodine briefly suppresses the thyroid through the Wolff-Chaikoff effect and is used before surgery and in thyroid storm, but over the long term it can cause hypothyroidism; lithium suppresses the release of T4 and T3, a side effect in the treatment of bipolar disorder; levothyroxine (L-T4) replaces the hormone directly and is the first-line treatment for hypothyroidism.
The division of labor between T4 and T3 is worth remembering: the thyroid mainly secretes T4 (about 80%) plus a little T3, and in the body's tissues T4 is converted into the truly active T3 by deiodinases, with DIO 1 and 2 activating and DIO 3 inactivating, and the whole family contains selenium. So a severe shortage of either iodine or selenium can upset this regulation; the thyroid is the central metronome of metabolism, and its key enzymes depend on these two trace elements (the deiodinases are covered in more detail in the Selenium story).
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Chapter 3
Setting how fast the body runs
Thyroid hormone is the metronome of metabolism: basal metabolic rate, heat production, heart rate, gut movement, cholesterol metabolism, and nervous-system development all follow its beat. Without enough iodine, not enough hormone is made, and the whole body's tempo slows.
The symptoms of hypothyroidism (an underactive thyroid) are very nonspecific: fatigue, feeling cold, slow weight gain, constipation, dry skin, a duller mind; especially in older people, they are easily taken for aging or depression. Diagnosis cannot rest on how you feel; it needs a blood test for thyroid-stimulating hormone (). In regions with enough iodine, the most common cause of hypothyroidism is Hashimoto's thyroiditis, not iodine deficiency.
The symptoms of hypothyroidism (an underactive thyroid) are very nonspecific: fatigue, feeling cold, slow weight gain, constipation, dry skin, a duller mind; especially in older people, they are easily taken for aging or depression. Diagnosis cannot rest on how you feel; it needs a blood test for thyroid-stimulating hormone (). In regions with enough iodine, the most common cause of hypothyroidism is Hashimoto's thyroiditis, not iodine deficiency.
Clinical · Why hypothyroidism looks like aging
Hypothyroidism is one of the endocrine diseases most often missed in clinical practice, especially in older people, because its symptoms are nonspecific and it comes on slowly:Fatigue, sleepiness, and slow weight gain, often put down to getting older or depressionFeeling cold, with cold hands and feetConstipationDry skin, brittle hair, hair lossSlower thinking and low moodIn young women, heavier or irregular periodsA slow heart rate and raised total cholesterol, with low-density lipoprotein () cholesterol high on a checkup and no obvious reason
In the population, it is far more common in women than in men and becomes more common with age; among older people, subclinical hypothyroidism ( raised while free is still normal) is not unusual. In regions with enough iodine, the main cause is Hashimoto's thyroiditis (the immune system attacking the thyroid); in iodine-deficient regions, the main cause is still lack of iodine.
Diagnosis cannot rest on how you feel:
TSH (thyroid-stimulating hormone) is the first-choice screening test and rises in hypothyroidism; most laboratories set the upper reference limit at around 4–5 mIU/LFree T4 (the part of T4 in the blood that is not bound to protein and can act directly) below the lower reference limit means overt hypothyroidism; raised TSH with normal free T4 is subclinical hypothyroidismIf Hashimoto's is suspected, anti- antibodies can be addedWhen needed, a thyroid ultrasound, a urine iodine test, and a look at the diet
In practice: from middle age on, if you notice the cluster tired all the time lately, always cold, unexplained weight gain, constipation, a TSH test is usually cheap and a missed diagnosis costly; do not buy thyroid hormone and take it on your own.
Clinical · How hypothyroidism is treated
The standard treatment for hypothyroidism is levothyroxine (L-), a synthetic form of T4:The dose is set by a doctor. Full replacement is often estimated by body weight at about 1.6 µg/kg/day; older people and people with heart disease usually start low and increase slowlyTake it in the morning on an empty stomach, about 30 min before breakfast, and not at the same time as calcium or iron tabletsT4 has a half-life of about 1 week and takes 4–6 weeks to reach a steady level, so after each dose change is rechecked 6–8 weeks later before deciding whether to adjust again; the aim is to bring TSH back into the range your doctor sets for you
Do not buy thyroid hormone online on your own, and do not raise or lower the dose yourself: too much speeds the heart and thins the bones, and too little brings the symptoms back.
Chapter 4
Pregnancy & brain
Iodine is the nutrient through which one generation shapes the next. In early pregnancy the fetus's own thyroid is not yet fully developed, and its brain development depends entirely on the mother's thyroid hormone; when the mother is severely iodine-deficient, neuron migration, myelination, and synapse formation are all affected, and the damage is irreversible: iodine given after birth cannot undo brain-development problems that have already happened.
Mild insufficiency affects far more people, but it is not visible at a glance the way cretinism is. In pregnancy, keep two numbers apart: how much you should get in total each day, and how much belongs in a supplement. People who are planning a pregnancy, pregnant, or breastfeeding usually do enough by using iodized salt at home and choosing a prenatal vitamin that contains iodine; anyone with a history of Hashimoto's or Graves' disease should ask an endocrinologist before taking iodine.
Mild insufficiency affects far more people, but it is not visible at a glance the way cretinism is. In pregnancy, keep two numbers apart: how much you should get in total each day, and how much belongs in a supplement. People who are planning a pregnancy, pregnant, or breastfeeding usually do enough by using iodized salt at home and choosing a prenatal vitamin that contains iodine; anyone with a history of Hashimoto's or Graves' disease should ask an endocrinologist before taking iodine.
Evidence · What low iodine in pregnancy does
Iodine is the nutrient through which one generation shapes the next: the cognitive ability of the next generation depends in part on the iodine intake of today's mothers during pregnancy.Mechanistically, the fetal brain depends mainly on the mother's thyroid hormone through early and mid-pregnancy (the fetus's own thyroid only starts working at 12–16 weeks, and its output is limited in the first 20 weeks); severe iodine deficiency in the mother leaves too little thyroid hormone, which affects fetal neuron migration, myelination, and synapse formation all at once, and the damage is irreversible: even iodine given immediately after birth cannot reverse brain-development problems that have already happened.
The most severe outcome is cretinism: severe intellectual disability (IQ < 50), short stature, deafness and inability to speak, spastic limbs, and an abnormal gait. Historically, the Swiss Alps, the Andes, and the Himalayas all had villages where iodine deficiency passed down through the generations.
The subclinical impact is actually the bigger public-health problem: it is not visible at a glance the way cretinism is, but it reaches far more people. The UK ALSPAC cohort (Bath 2013) followed mothers' iodine status in pregnancy against their children's later cognition and found that children of iodine-insufficient mothers were more likely to fall into the lowest quartile for verbal IQ. Note what that is: a shift in risk, not every child loses N points; the lost IQ points figures that circulate online mostly come from severely deficient populations and do not carry over to mild insufficiency. The other side needs saying too: two randomized trials of iodine supplements in pregnancy, summarized by the US ODS (run in populations with broadly adequate iodine), found no difference in the children's cognition at age one to two, so a benefit of supplementing mildly insufficient populations has not yet been shown in trials.
Globally, the push for universal salt iodization since the 1990s has greatly reduced iodine deficiency and is regarded as one of the most successful micronutrient public-health programs. But the protection of iodized salt can be quietly removed: people who switch to non-iodized salt in the name of healthy salt, in households that eat little seafood, cut off their main source of iodine.
Three numbers from three bodies get read as one; they do not conflict, but they are not saying the same thing:
US (IOM): 220 µg/day in pregnancy and 290 µg/day while breastfeeding (150 for other adults)WHO: sets the requirement at 250 µg/day for both pregnancy and breastfeeding, one notch above the US RDAATA: recommends an extra 150 µg/day as a supplement on top of the usual diet; this is a supplement amount, not a total intake, proposed because many prenatal vitamins contain no iodine at all
So seeing 150 and 250 does not mean someone made a mistake: one is what belongs in the pill, the other is what the day should add up to. People with a history of Hashimoto's or Graves' disease should consult an endocrinologist before taking iodine in pregnancy, because it may worsen the disease.
Background · Why iodized salt matters so much
Cretinism is one of the most dramatic examples in medical history of a single nutrient shaping a whole life: a shortage of one trace element, acting through the mother, affects a child's cognition for life. The US ODS calls iodine deficiency the world's most common preventable cause of intellectual disability, and the WHO says the same.This is also why the WHO gives population-level iodine nutrition more weight than individual diagnosis: the subclinical effect is the larger public-health burden, and a measure as simple as iodizing salt has changed the brain-development curve of hundreds of millions of people.
If you are planning a pregnancy, pregnant, or breastfeeding: switching your household salt back to iodized salt and choosing a prenatal vitamin that contains iodine (ask your doctor first if you have a history of Hashimoto's or Graves' disease) is usually enough. This matters more than any brain-boosting supplement.
Chapter 5
Too little or too much
Iodine is the kind of element you cannot do without and cannot overdo. A large dose of iodine, at once or over several days, makes the thyroid temporarily shut down iodine organification (the step that attaches iodine to thyroglobulin), which keeps hormone synthesis from running away; a healthy thyroid recovers after a few days, while an already damaged thyroid (in Hashimoto's, for example) often cannot, stays suppressed, and ends up underactive.
The opposite direction is real too: a thyroid starved of iodine for a long time can tip into overactivity when it suddenly meets a large amount. Iodized salt used normally is safe for healthy people; the risk lies with people who have Hashimoto's, an overactive thyroid that is not under control, or thyroid nodules and who meet iodine contrast dye or a lot of kelp. An uncontrolled overactive thyroid hit with a large dose of iodine can trigger a thyroid storm: if high fever, a very fast heartbeat, agitation, or confusion appear, go to the emergency department immediately.
The opposite direction is real too: a thyroid starved of iodine for a long time can tip into overactivity when it suddenly meets a large amount. Iodized salt used normally is safe for healthy people; the risk lies with people who have Hashimoto's, an overactive thyroid that is not under control, or thyroid nodules and who meet iodine contrast dye or a lot of kelp. An uncontrolled overactive thyroid hit with a large dose of iodine can trigger a thyroid storm: if high fever, a very fast heartbeat, agitation, or confusion appear, go to the emergency department immediately.
Safety · Why too much iodine also hurts the thyroid
Iodine is a U-shaped-curve nutrient: too little causes an underactive thyroid, and too much can also throw thyroid function off. The US ODS notes that with very high iodine intake some people develop the same signs as iodine deficiency (goiter, raised , an underactive thyroid), and iodine-induced hyperthyroidism can also occur, including when iodine is given to treat deficiency; people with autoimmune thyroid disease such as Hashimoto's, and people who were iodine-deficient, can run into trouble at intakes considered safe for the general population.The Wolff-Chaikoff effect: a large dose of iodine far beyond everyday needs (such as iodine contrast dye, a lot of kelp, or a high-iodine supplement) makes the thyroid temporarily shut down iodine organification to keep hormone synthesis from running away. A normal thyroid escapes after a few days and returns to normal; a thyroid with Hashimoto's or other damage often fails to escape, stays suppressed, and eventually becomes clinically underactive.
The Jod-Basedow effect runs the other way: long-term iodine deficiency followed by sudden iodine can trigger hyperthyroidism. Common scenarios include the transition period when a population switches from non-iodized to iodized salt, people with thyroid nodules who receive iodine contrast for a CT scan or coronary angiography, and long-term use of amiodarone (an iodine-containing drug for heart rhythm problems).
By group:
A healthy thyroid plus ordinary iodized salt: safeHashimoto's plus a lot of kelp or high-iodine supplements: higher risk of hypothyroidismUncontrolled Graves' hyperthyroidism plus a large amount of iodine: may trigger a thyroid storm. A thyroid storm is an emergency: if high fever, a very fast heartbeat, vomiting and diarrhea, agitation, or confusion appear, go to the emergency department immediatelyThyroid nodules plus a sudden large dose of iodine (contrast): Jod-Basedow hyperthyroidism
The safest practice: use iodized salt normally (6 g of salt a day gives about 150 µg of iodine, which meets the adult recommendation); eat seafood in reasonable amounts, where sushi nori is fine, and avoid kelp powder and concentrated iodine supplements; the adult Tolerable Upper Intake Level for iodine is 1100 µg a day. Do not treat high-iodine detox as wellness: iodine is not a detoxifier, and self-dosing at high levels carries real risk for the thyroid. Anyone with a thyroid history should tell their doctor before a planned large iodine exposure (contrast, kelp, supplements).
Background · Iodine tablets in nuclear emergencies
One scenario is easily confused: iodine tablets against nuclear radiation.A nuclear accident can release radioactive iodine (I-131) into the environment, raising the risk of thyroid cancer in exposed people, especially children; iodine-deficient thyroids take up more radioactive iodine, so their risk is higher. The tablets handed out near a nuclear accident are high-dose potassium iodide, meant to fill up the thyroid with stable iodine so radioactive iodine cannot enter thyroid cells and cause radiation damage. The US FDA has approved potassium iodide as a thyroid-blocking agent for such radiation emergencies, taken as public-health authorities instruct until the risk of significant exposure has passed.
This is a thyroid-protection measure for radiation emergencies, not everyday wellness, and it should not be taken on your own at other times; nor is it any basis for high-iodine detox.
Iodine is a U-shaped-curve nutrient, and high iodine equals detox is wrong; anyone with a thyroid history should tell their doctor before a planned large iodine exposure (contrast, kelp, supplements).
References · 5
- National Institutes of Health, Office of Dietary Supplements. (2022). Iodine — Fact Sheet for Health Professionals. Fact sheet (updated November 5, 2024; Wayback snapshot 16 September 2026): commercial seaweeds range from 16 to 2,984 mcg iodine per gram; dried nori, 2 tablespoons flaked (5 g), 116 mcg; US iodized salt is labelled 45 mcg iodine per gram (measured 47.5-50.7); two RCTs giving 150 or 200 mcg/day iodine from early pregnancy to delivery found no effect on child cognitive, language or motor scores at 1.5-2 years, and one found no benefit at 5-6 years either (fact sheet). ods.od.nih.gov/factsheets/Iodine-HealthProfessional
- Zimmermann, M. B. (2009). Iodine deficiency. Endocrine Reviews, 30(4), 376–408. Review: an estimated 2 billion people have insufficient iodine intake; about 50% of Europe remains mildly deficient; deficiency in pregnancy and infancy may impair growth and neurodevelopment; optimal intakes from iodized salt are about 150-250 micrograms/day for adults; the small risks of iodine excess are far outweighed by the risks of deficiency (abstract, PMID 19460960). 10.1210/er.2009-0011
- World Health Organization. Micronutrients: iodine deficiency disorders. WHO describes iodine deficiency as the world's most prevalent, yet easily preventable, cause of brain damage; pregnancy requirement ~250 mcg/day (WHO). www.who.int/health-topics/micronutrients
- American Thyroid Association (Becker, D. V., et al., 2006). Iodine supplementation for pregnancy and lactation. Thyroid, 16(10), 949-951. Recommends a 150 mcg/day iodine supplement during pregnancy and lactation, since many prenatal vitamins lack iodine. pubmed.ncbi.nlm.nih.gov/17042677
- Bath, S. C., Steer, C. D., Golding, J., Emmett, P., & Rayman, M. P. (2013). Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). The Lancet, 382(9889), 331-337. 10.1016/S0140-6736(13)60436-5