Enel Health®Atlas
Sign in
Enel Health®Atlas
Browse libraries

Not medical advice. The Atlas is not a substitute for a clinician who can examine you.

Iodine

The Thyroid and Brain-Development Mineral

Evidence grade 3 of 5, Mixed. Read how the Atlas grades evidence.

10 human trials, about 37 studies, Cochrane 2017 (14 trials) + observational data. The written rationale for this grade is still being prepared.

FDA
Regulated as a dietary supplement; iodized salt is a public-health measure
WADA
Not prohibited

Reference only. Not medical advice.

What can go wrong

Both too little and too much iodine can disturb thyroid function, so more is not better: excess can cause the same goiter, elevated TSH, and hypothyroidism seen in deficiency, and the tolerable upper limit for adults, including in pregnancy, is 1,100 micrograms per day. High-dose kelp and seaweed products are a particular concern because their iodine content is erratic and often exceeds the label, and case reports link them to hyperthyroidism. People with an existing thyroid condition, and anyone deciding whether and how much to supplement during pregnancy, should be guided by their obstetric provider or a qualified clinician rather than self-dosing, and this dossier is not a protocol.

Iodine is the raw material the body uses to build thyroid hormones, and those hormones steer fetal brain and nervous-system development, especially in the first trimester before the fetal thyroid is working. The requirement rises in pregnancy to a US RDA of 220 micrograms per day, with WHO recommending 250. Severe iodine deficiency in pregnancy causes serious, well-documented harm and remains the leading preventable cause of intellectual disability worldwide. The harder question is mild-to-moderate deficiency, which is what most people in countries with iodized salt actually face: professional bodies recommend a 150 microgram daily supplement as a precaution, but the randomized evidence that this improves a child's neurodevelopment is limited and inconsistent. People who avoid iodized salt, dairy, or seafood, including many who eat plant-based, are more likely to fall short.

Iodine builds thyroid hormones that steer fetal brain development, and need rises in pregnancy, yet many prenatal supplements still leave it out.

What people take it for

These are the reasons people give for taking it. They are not effects it has been shown to have.

  • Supplying the iodine the body needs to make thyroid hormones during pregnancy
  • Supporting fetal brain and nervous-system development, which depends on maternal thyroid hormone in early pregnancy
  • Covering dietary gaps for people who avoid iodized salt, dairy, or seafood, including many who eat plant-based
  • Preventing the severe and well-documented consequences of iodine deficiency, still the leading preventable cause of intellectual disability worldwide

What people get wrong

  • Often saidIodine supplements in pregnancy are proven to raise a child's IQ.

    ActuallyIn severely deficient regions the harm of too little iodine is beyond dispute, and iodine there reduces or eliminates cretinism. But in mild-to-moderate deficiency the randomized evidence that supplementation improves neurodevelopment is limited and inconsistent: the 2017 Cochrane review and Dineva's 2020 meta-analysis of 10 RCTs found no clear benefit, and Levie's 2019 meta-analysis found no verbal-IQ association below 150 micrograms per gram of urinary iodine. The precautionary recommendation is reasonable; the strong IQ promise outruns the trials.

  • Often saidSea salt and pink salt are good sources of iodine.

    ActuallyNon-iodized sea salt and specialty salts contain essentially no iodine; NIH lists non-iodized sea salt at 0 micrograms, versus about 45 micrograms per gram in iodized salt. Iodized table salt, dairy, and seafood are the reliable dietary sources, and shifts away from iodized salt have nudged some groups toward insufficiency.

  • Often saidMore iodine is better, so kelp supplements are a safe way to load up.

    ActuallyBoth too little and too much iodine disturb the thyroid; excess can cause the same goiter, elevated TSH, and hypothyroidism as deficiency. Kelp products are erratic: ConsumerLab testing found some delivering far more iodine than labeled, and case reports document kelp-triggered hyperthyroidism. The tolerable upper limit for adults, including in pregnancy, is 1,100 micrograms per day.

  • Often saidA prenatal vitamin always covers iodine.

    ActuallyNot reliably. NIH notes that of best-selling prenatal supplements surveyed in 2016 to 2017, only 34 of 59 contained iodine, and among pregnant women taking supplements only about 18% took an iodine-containing product. A later 2021 market review found iodine in roughly 76% of products assessed, so it is worth checking the label rather than assuming.

Who profits, and who pushes back

  • American Thyroid Association & endocrine bodies

    endorsesProfessional organizations

    A 150 mcg daily iodine supplement is recommended before and during pregnancy as a precaution, particularly because not all prenatal supplements contain iodine.

  • NIH Office of Dietary Supplements

    endorsesAuthoritative reference

    The pregnancy RDA is 220 mcg; vegetarians, vegans, and people not using iodized salt are at higher risk of insufficient intake.

  • Cochrane Review (2017) authors

    mixedSystematic reviewers, 14 trials

    Maternal iodine supplementation reduced some markers of iodine status but did not consistently improve infant neurodevelopment in mild-to-moderate deficiency; adequately powered randomized trials are still needed.

  • Australian iodine RCT investigators

    mixed150 mcg/d vs placebo, Bayley-III at 18 months

    No differences in childhood neurodevelopmental scores between iodine and placebo groups were found, though the trial was underpowered after funding was withdrawn.

Where the research comes from

Most of the published research on this comes from one place. 55% of it is from Sufficient. Findings that no unrelated group has reproduced are weaker than the study count makes them look.

Share of published research by country of origin.
PlaceShare of published research
Sufficient55%
Mild-moderate deficiency45%

Research published from: Global.

How we got here

The link between the sea and the throat is old and not European. Chinese medical tradition recorded that seaweed and burnt sea sponge could shrink goiter, with the association attributed to texts as early as 2700 BC and written into the Shennong Bencao Jing around 300 BC; the physician Ge Hong (284 to 363 AD) and later Sun Simiao (652 AD) named seaweed formulas for goiter without any idea why they worked. Europe caught up much later. Bernard Courtois isolated iodine in Paris in 1811, and by 1820 Jean-Francois Coindet showed that iodine was the active ingredient in the old burnt-sponge remedy. Population-scale prevention came in the 1920s: David Marine and Oliver Kimball ran a large goiter-prevention trial in Akron, Ohio between 1917 and 1920, Switzerland adopted iodized salt, and Michigan followed in 1924, with Morton Salt taking iodized salt nationwide that year. In 1993 WHO and UNICEF endorsed universal salt iodization, now reaching most households worldwide.

  1. 2700 BCChinese tradition links seaweed to shrinking goiter; the remedy is later written into the Shennong Bencao Jing (~300 BC)
  2. 284-363 ADThe Chinese physician Ge Hong records seaweed formulas for goiter; Sun Simiao lists further seaweed prescriptions in 652 AD
  3. 1811Bernard Courtois isolates iodine in Paris; by 1820 Jean-Francois Coindet identifies it as the active agent in the burnt-sponge goiter remedy
  4. 1917-1920David Marine and Oliver Kimball run a large goiter-prevention trial in Akron, Ohio, with goiter developing in 0.2% of treated girls versus 14% of controls
  5. 1924Michigan introduces iodized salt and Morton Salt takes it nationwide in the US, following Switzerland's earlier program
  6. 1993WHO and UNICEF endorse universal salt iodization, later reaching most households worldwide
  7. 2017Cochrane review (Harding) finds no clear neurodevelopmental benefit of supplementation and flags increased digestive intolerance
  8. 2019-2020Levie individual-participant meta-analysis (6,180 pairs) and Dineva's AJCN meta-analysis (10 RCTs) both fail to show clear neurodevelopmental benefit in mild-to-moderate deficiency

At a glance

Pregnancy RDA
220 mcg/day
Common supplement
150 mcg
UL (19-50 y)
1,100 mcg/day
US median intake
~216 mcg/day

What this entry does not yet cover

What follows is unfinished work on our side, not a finding about Iodine. A short entry is not a safe entry.

  • Cost and market. We have not documented what it sells for or who sells it.
  • Supply chain. We have not mapped where it is made.
  • Who uses it. We have not documented the user groups.
  • What could change. No forward-looking summary yet.
  • How it compares. We have not set it against the alternatives.
  • Children and teenagers. We have not checked what is known about younger people.

Sources