Iodine is a mineral your thyroid needs to make its hormones, and those hormones run your metabolism and, before birth, build the fetal brain. Get enough and the system works, and most people in countries that iodize salt already get enough from salt, dairy, and seafood. Too little is the problem the world spent a century fixing: iodine deficiency causes goiter and an underactive thyroid, and in pregnancy it is the leading preventable cause of intellectual disability worldwide. Universal salt iodization is one of public health's great successes.
The catch is that more is not better. Above what the thyroid needs, extra iodine from kelp, seaweed, or high-dose drops can push the thyroid the other way, into an underactive or overactive state, most easily in people with autoimmune thyroid disease. The response runs as a U, with harm at both ends and health across the wide middle. For most people the goal is to get enough rather than to supplement, and giving iodine to people who already get enough has not been shown to help.
Findings & Outcomes
What It Is
Iodine is a trace mineral, and your body has exactly one essential use for it: building thyroid hormone. The thyroid gland pulls iodine out of the blood and assembles it into the two hormones that set the pace of your metabolism, T4 (four iodine atoms) and T3 (three). Without enough iodine the gland cannot make enough hormone, and everything those hormones govern, from body temperature to heart rate to, before birth, the wiring of the brain, is affected.
You get iodine from food. The reliable everyday sources are iodized salt, dairy (cows pick up iodine from feed and from the disinfectants used in milking), eggs, and seafood, especially fish, shellfish, and seaweed. The soil in many inland regions carries little iodine, so before salt was iodized, whole populations far from the sea were chronically short. An adult needs about 150 micrograms a day, a pregnant woman about 220 to 250, and the tolerable upper limit for adults sits around 1,100 micrograms a day.
The single fact that organizes this whole topic is that both too little and too much cause thyroid trouble. Most of what follows is about where the safe middle is and how to stay in it.
What It Does
Iodine does not so much do things as let the thyroid do them. Its effects sort by whether you are getting too little, enough, or too much, and the shape that connects them is a U: problems at the low end, health across a broad middle, and different problems at the high end.
At the low end sit the classic deficiency disorders, goiter and hypothyroidism, and the most serious of them is damage to a developing brain. Across the healthy middle, which is where most people in iodized-salt countries live, iodine simply keeps the thyroid supplied and there is nothing to add. At the high end, an intake well above what the gland needs can itself trigger an underactive or overactive thyroid, most readily in people whose thyroids are already vulnerable. Each card below grades one part of that picture at the strength of its own evidence.
The Research & Studies
Everything here is based on the research we have collected and checked, sorted into groups and ordered with the strongest evidence first. Click any claim to open the studies behind it.
How it works
Thyroid hormones T3 and T4 are built from iodine, and the body has no other essential use for it
Your thyroid uses iodine as the raw material for its hormones, and building those hormones is the one job iodine has in the body. Without enough of it, the gland cannot make enough hormone.
This is established thyroid physiology rather than an inference. Iodide is actively trapped from the circulation by the sodium-iodide symporter, oxidized, and bound to tyrosine residues on thyroglobulin to form the iodinated hormones T4 and T3, which regulate basal metabolic rate and, critically in fetal and early life, brain development. Recommended intake for non-pregnant adults is about 150 micrograms a day, rising to roughly 220 to 250 in pregnancy, with a tolerable upper level for adults near 1,100 micrograms a day.
The mechanism explains why both a shortfall and a large excess of iodine show up as thyroid disease, but it cannot tell any individual where they sit; that depends on intake and on how vulnerable a given thyroid is.
The studies · 2
Zimmermann, Jooste, Pandav. Iodine-deficiency disorders · Lancet 2008;372(9645):1251-1262
Leung, Braverman. Consequences of excess iodine · Nat Rev Endocrinol 2014;10(3):136-142
Counts once: this finding and 3 others here come from the same source, so they are one body of evidence, not separate confirmations.
Thyroid
Iodine deficiency is the main cause of goiter and preventable hypothyroidism worldwide
Not getting enough iodine makes the thyroid swell (a goiter) and can make it underactive. Getting sufficient iodine, usually through iodized salt, prevents and reverses this.
The Lancet review summarizes decades of population data: iodine deficiency is the commonest cause of endemic goiter and of preventable hypothyroidism, and its effects trace directly to inadequate thyroid hormone production. Iodine status is monitored at population level through median urinary iodine concentration, goiter rate, newborn TSH, and thyroglobulin. The intervention is inexpensive and effective, which is why salt iodization is the recommended control strategy in nearly all affected countries.
The benefit here is the benefit of adequacy: correcting a real shortfall. It does not imply that adding iodine on top of a sufficient intake helps, which is a separate question the excess-iodine findings on this page address.
The study · 1
Zimmermann, Jooste, Pandav. Iodine-deficiency disorders · Lancet 2008;372(9645):1251-1262
Counts once: this finding and 3 others here come from the same source, so they are one body of evidence, not separate confirmations.
Universal salt iodization is one of the cheapest, most effective public-health measures ever deployed
Adding a tiny amount of iodine to ordinary table salt fixed iodine deficiency for whole countries. It is one of the cheapest and most successful public-health measures ever done.
Salt iodization works because salt is consumed in fairly steady amounts across a population and is cheap to fortify. Introducing iodized salt to a chronically deficient region can transiently raise the number of thyroid disorders as the population adjusts, but the review notes that the small risks of iodine excess are far outweighed by the substantial risks of deficiency. International progress in extending iodization has slowed, leaving a portion of the world's population still deficient.
The take-home for an individual is that iodized salt is the reliable, near-free route to sufficiency. Where iodized salt is displaced, by specialty salts or by processed foods made with non-iodized salt, intake can quietly drop.
The study · 1
Zimmermann, Jooste, Pandav. Iodine-deficiency disorders · Lancet 2008;372(9645):1251-1262
Counts once: this finding and 3 others here come from the same source, so they are one body of evidence, not separate confirmations.
Higher iodine intake raised five-year subclinical hypothyroidism from 0.2% to nearly 3% (China cohort)
In a large Chinese study, people in areas with high iodine intake developed an underactive thyroid and thyroid autoimmunity more often than people in a mildly deficient area over five years. Too much iodine, not only too little, tracked with thyroid trouble.
This prospective cohort is a key piece of evidence that the iodine-thyroid relationship is U-shaped. The regions differed by iodine intake (median urinary iodine 84, 243, and 651 micrograms per litre), and the higher-intake regions showed more new subclinical hypothyroidism and autoimmune thyroiditis. Because the comparison is between whole regions rather than randomized individuals, other regional differences could contribute, but the gradient is consistent with the known biology of iodine excess.
Who this may not transfer to:The cohort included both sexes; the excess-iodine effect on thyroid function is not specific to one sex.
The signal is at the population level and modest in absolute terms, but it is the empirical basis for not pushing iodine intake above sufficiency, particularly for people prone to thyroid autoimmunity.
The study · 1
Teng, Shan, Teng et al. Effect of iodine intake on thyroid diseases in China · N Engl J Med 2006;354(26):2783-2793
Unfortified plant-based milks carry about 2% of the iodine in cows' milk
Plant-based milks contain almost no iodine unless they are fortified, roughly 2% of what cows' milk has. In countries where dairy is a main iodine source, swapping to these drinks quietly removes a lot of iodine.
Where dairy is a principal dietary iodine source, as in the UK, replacing cows' milk with an unfortified plant drink meaningfully lowers iodine intake, and the authors flag consumers of these drinks as a group at risk of deficiency unless they get iodine elsewhere. Iodine content was measured by ICP-MS; a minority of products were fortified and reached iodine levels comparable to cows' milk. This is a snapshot of the products on shelves rather than a study of health outcomes.
If you have largely swapped dairy for plant-based milks, check whether yours is iodine-fortified, and if not, make sure another source (iodized salt, seafood, eggs, or a modest supplement) covers the gap.
The study · 1
Bath, Hill, Infante, Elghul, Nezianya, Rayman. Iodine concentration of milk-alternative drinks available in the UK in comparison with cows' milk · Br J Nutr 2017;118(7):525-532
Neurodevelopmental
Iodine deficiency is the leading preventable cause of intellectual disability worldwide
Before birth, thyroid hormone builds the baby's brain, and a serious iodine shortage in pregnancy causes lasting intellectual disability. This makes iodine deficiency the biggest preventable cause of intellectual disability in the world.
The developing brain is the tissue most sensitive to iodine deficiency because it depends on maternal thyroid hormone, especially in early gestation. In severely deficient regions the classic result was endemic cretinism, with profound, irreversible cognitive and motor impairment. The Lancet review states plainly that iodine deficiency is the most common cause of preventable mental impairment worldwide, and that salt iodization is the cost-effective way to prevent it.
This is the strongest reason iodine adequacy matters, and it is a benefit of preventing severe deficiency. It concerns severe deficiency; the milder end of the range, and whether supplementing a nearly-sufficient mother helps, is treated separately on this page.
The study · 1
Zimmermann, Jooste, Pandav. Iodine-deficiency disorders · Lancet 2008;372(9645):1251-1262
Counts once: this finding and 3 others here come from the same source, so they are one body of evidence, not separate confirmations.
In 6,180 pooled European pairs, mild maternal deficiency was not clearly linked to lower child IQ
When three European studies were pooled into one much larger analysis, mild maternal iodine shortage was not clearly tied to lower child IQ. Any hint of an effect was small, limited to verbal scores, and only for iodine measured very early in pregnancy.
This pooled analysis is larger and better-powered than any single cohort, and its more tempered result is why the mild-deficiency question is genuinely open rather than settled. It did find a curvilinear relationship suggesting the low end matters for verbal IQ, and that the vulnerable window is early gestation, but at the conventional deficiency threshold the differences did not reach statistical significance. As with any observational analysis, iodine was measured rather than assigned.
Who this may not transfer to:The exposure is maternal iodine status during pregnancy and does not transfer to men.
Held next to the single-cohort ALSPAC result and the null trial on this page, the honest summary is that severe deficiency clearly harms and mild deficiency is uncertain, which argues for being replete rather than for high-dose supplementation.
The study · 1
Levie, Korevaar, Bath et al. Association of Maternal Iodine Status With Child IQ: A Meta-Analysis of Individual Participant Data · J Clin Endocrinol Metab 2019;104(12):5957-5967
200 micrograms a day of iodine in mildly deficient pregnant women did not improve child IQ (-0.7 points)
In the one randomized trial of the question, giving iodine to mildly deficient pregnant women did not raise their children's IQ or thinking skills years later. Starting a supplement once a woman was already close to sufficient did not help.
This is the highest-quality design for the question, because women were randomly assigned to iodine or placebo, removing the confounding that limits the cohort studies. The population was only mildly deficient (schoolchildren in the same areas were iodine-sufficient), which matters: the trial speaks to supplementing a nearly-sufficient population, not to correcting severe deficiency. Compliance averaged 87%. The consistent finding of no benefit is the main reason routine iodine supplementation in mildly deficient, otherwise adequately-nourished pregnant women is not clearly supported.
Who this may not transfer to:The intervention is iodine supplementation in pregnancy and does not transfer to men.
The practical reading is to reach adequacy through diet and iodized salt before pregnancy rather than to expect a supplement started in a nearly-sufficient woman to add cognitive benefit. It does not apply to a woman with a genuine, severe shortfall.
The study · 1
Gowachirapant, Jaiswal, Melse-Boonstra et al. Effect of iodine supplementation in pregnant women on child neurodevelopment: a randomised, double-blind, placebo-controlled trial · Lancet Diabetes Endocrinol 2017;5(11):853-863
Cognition
Children of mildly iodine-deficient mothers were about 60% more likely to score in the lowest verbal-IQ band (ALSPAC)
In a large UK study, children whose mothers had low iodine in early pregnancy were more likely to land in the bottom group for verbal IQ and reading years later, even after accounting for many other differences between families.
This is an observational cohort, not a trial: iodine was measured, not assigned, so it shows association rather than proof that the low iodine caused the lower scores. The authors adjusted for 21 socioeconomic, parental, and child factors, and scores worsened stepwise across lower iodine bands. Even so, unmeasured differences between iodine-deficient and iodine-sufficient mothers could contribute. A larger pooled analysis of European cohorts, reported separately on this page, found a weaker signal, so this single-cohort result should be read as suggestive rather than settled.
Who this may not transfer to:The exposure is maternal iodine status in pregnancy, so the finding is inherently about pregnant women; it does not transfer to men.
The reading is not that a supplement fixes IQ, but that entering pregnancy already iodine-replete is worth doing. The interventional evidence for adding iodine to a nearly-sufficient mother is disappointing, which is covered by the trial finding on this page.
The study · 1
Bath, Steer, Golding, Emmett, Rayman. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children (ALSPAC) · Lancet 2013;382(9889):331-337
Why Both Too Little And Too Much Cause Harm
For most of history the iodine story was a story of shortage. Inland populations developed goiters, the visible swelling of a thyroid straining to work with too little iodine, and in the worst-affected areas children were born with severe, permanent intellectual and physical impairment. Iodine deficiency became, and worldwide remains, the leading preventable cause of intellectual disability. The fix was one of the cheapest in the history of public health: add a trace of iodine to table salt. Where that was done, goiter and its worst consequences receded within a generation.
The reason more is not simply better is that the thyroid protects itself against a flood of iodine, and that protection can misfire. Given a sudden large load, the gland briefly shuts down hormone production (the Wolff-Chaikoff effect); most people adjust within days, but some, especially those with autoimmune thyroid disease, stay stuck in that shut-down state and become hypothyroid. In a different failure mode, a big iodine load fed to a gland that already has autonomous nodules can drive it into overactivity (the Jod-Basedow effect). This is why populations that go from deficient to iodine-replete see a temporary uptick in thyroid disorders, and why the people most likely to be harmed by a high-dose iodine supplement are the very people who assume it will help their thyroid.
Where the reader lands is straightforward. If you use iodized salt or eat dairy and seafood, you are almost certainly sufficient, and the useful move is to keep it that way, not to add a supplement. The people who need more iodine are those with a clear shortfall, and the clearest case is pregnancy.
Pregnancy Is Where The Stakes Are Highest
Thyroid hormone builds the fetal brain, and in early pregnancy the fetus depends entirely on the mother's supply, so maternal iodine before and during those first weeks matters more than at any other time of life. Severe maternal deficiency causes irreversible harm to the child, which is beyond dispute. The live scientific question is what mild-to-moderate deficiency does, the kind now seen in parts of the UK, Europe, and among people who have drifted away from dairy and iodized salt.
The observational signal is modest, and the picture is more mixed than headlines suggest. In one UK cohort, children of mildly deficient mothers were more likely to fall in the lowest band for verbal IQ and reading. A larger pooled analysis of three European cohorts found only a weak, curvilinear link with verbal IQ that did not reach significance at the usual deficiency threshold. And when the question was put to a randomized trial, giving iodine to mildly deficient pregnant women did not measurably improve their children's development years later.
Read together, these say something careful: getting enough iodine before and during early pregnancy clearly matters, the deficient end of the range is the one to avoid, and starting a supplement once a woman is already close to sufficient has not been shown to buy extra points. The practical reading is to enter pregnancy already iodine-replete, through iodized salt, dairy, and seafood or a prenatal that contains about 150 micrograms, rather than to reach for a high-dose product.
How It Works
Anatomy of the Practice
1In the thyroid gland
The thyroid traps iodine from the blood through a pump called the sodium-iodide symporter, attaches it to a protein scaffold, and builds thyroid hormone: T4 carries four iodine atoms, T3 carries three. When iodine is scarce the gland enlarges to capture more of it, which is the goiter of deficiency; when it still cannot make enough hormone, the result is hypothyroidism.
2Before birth and in early life
Thyroid hormone directs how the fetal and infant brain grows and connects, and in the first weeks the fetus relies entirely on the mother's iodine and hormone. A severe shortfall in this window causes permanent intellectual and neurological impairment, which is why the developing brain is the tissue most sensitive to iodine deficiency.
3When intake climbs too high
Faced with a large iodine load the gland briefly stops making hormone to protect itself, the Wolff-Chaikoff effect, and most people escape it within days. People with autoimmune thyroid disease may not, and can stay hypothyroid. Separately, a heavy iodine load can push a nodular gland into overactivity, the Jod-Basedow effect. Both are why excess iodine, not only deficiency, shows up as thyroid disease.
The mechanism is unusually well established, because it is basic thyroid physiology worked out over a century rather than a hopeful inference. What that mechanism cannot tell you on its own is where any individual sits on the curve, which is a question of intake and of how vulnerable a given thyroid is.
Ways to Do It
For most people this is about getting enough, not taking more. The everyday food supply covers it; the reasons to be deliberate are avoiding dairy and seafood, and pregnancy.
The cheapest and most reliable route, and the one that eliminated widespread deficiency. About a quarter teaspoon of iodized table salt supplies roughly a day's iodine. Specialty and gourmet salts, sea salt, and pink Himalayan salt are usually NOT iodized, so check the label if that is what you use, and note that most salt in restaurant and packaged food is non-iodized.
Dairy (milk, yogurt, cheese), eggs, and seafood are the dependable food sources, and fish and shellfish are naturally iodine-rich. A varied diet that includes these, plus a little iodized salt, keeps most people comfortably in the sufficient range without any thought.
Plant-based milks (soy, almond, oat, and the rest) carry almost no iodine unless fortified, so swapping cows' milk for them removes a major source. Vegans, and anyone off both dairy and seafood, are the group most likely to fall short. Choose an iodine-fortified plant milk, or a modest supplement or multivitamin providing about 150 micrograms, and read the label, since not all contain it.
Needs rise to about 220 to 250 micrograms a day. A prenatal that contains roughly 150 micrograms of iodine, on top of iodized salt and food, covers the increase. The aim is to be replete going in, not to take a high-dose product, and this is worth confirming with whoever provides your prenatal care.
Kelp tablets, sea-vegetable powders, and iodine drops such as Lugol's solution deliver many times, sometimes thousands of times, the daily requirement, with contents that vary wildly batch to batch. For someone already replete they add no benefit and carry the excess-iodine risks in the cautions below. Getting enough iodine is the goal, and more than enough is the thing to avoid.
Go Deeper
- Thyroid conditions: where iodine sits among the causes and treatments of an underactive or overactive thyroid, including Hashimoto's and Graves' disease.
- Irish sea moss: a marine plant sold as a mineral tonic, and the same iodine-overshoot question in a popular supplement.
- Vitamin D: another essential micronutrient where getting enough matters and more is not automatically better.
- Selenium: the other trace mineral the thyroid runs on, and the same U-shaped lesson that too much causes trouble.
- Micronutrient inadequacy: where iodine sits among the shortfalls worth correcting, and why the resurgence matters.
The Chinese Medicine View
Chinese medicine has no concept of iodine, and it would be an invention to claim otherwise. What the tradition has is a long, practical use of marine plants, and that is where it touches this topic. Kun Bu (kelp, Laminaria) and Hai Zao (sargassum seaweed) are classed as salty and cold, described as softening hardness, dissipating nodules, and transforming Phlegm. Their classic indication was goiter and neck swellings, the swellings that centuries later were often understood as the enlarged thyroid of iodine shortage. That these iodine-rich seaweeds were reached for is a convergence worth noticing; it is offered as a lens, not as evidence that the tradition identified iodine or its mechanism.
The tradition also used these substances with restraint, and its cautions line up with the modern picture rather than cheerleading. Salty, cold marine herbs were understood to burden the Spleen and Stomach and weaken digestion if used heavily or in someone already damp and sluggish, so they were dosed sparingly and fitted to the constitution. Hai Zao sits among the classical incompatibility cautions, traditionally kept apart from Gan Cao (licorice). A practitioner would not read more as better, and would match a marine substance to the person and their state. That instinct, that the right amount depends on who is taking it, sits beside a modern reading in which a modest amount fills a need and a careless amount causes harm.
Cautions For This Practice
Extra restraintEverything to be aware of is here, in one place. This practice suits most healthy people; a few situations call for real care.
Excess iodine can trigger an underactive or overactive thyroid, especially with autoimmune or nodular thyroid disease
The review lays out who is at risk and why. The Wolff-Chaikoff effect is a normal protective shutdown of hormone synthesis after a large iodine load; most people escape it in days, but people with autoimmune thyroid disease may not and become hypothyroid. Jod-Basedow is the opposite failure, in which excess iodine fuels an autonomous nodular gland into hyperthyroidism. Kelp and seaweed products are singled out for extremely variable, sometimes very high iodine content. The tolerable upper level for adults is about 1,100 micrograms a day, which concentrated products can exceed many times over.Leung, Braverman. Consequences of excess iodine
Autoimmune thyroid disease is the main reason not to high-dose
People with Hashimoto's thyroiditis or Graves' disease are the most sensitive to an iodine load, which can tip the thyroid into an underactive or, less often, overactive state. If you have autoimmune thyroid disease, kelp tablets, seaweed supplements, and iodine drops are the category to be careful with, and any deliberate iodine supplementation is a decision to make with whoever manages your thyroid rather than to self-start.
Nodular goiter and older adults
A gland that already contains autonomous nodules can be driven into overactivity by a big iodine load, the Jod-Basedow effect, and this is more common in older people and in those from previously iodine-deficient areas. A sudden high-iodine intake, from a supplement or from iodine-containing medicines and contrast dyes, is the trigger to be aware of.
Kelp, seaweed, and sea-vegetable supplements vary enormously
The iodine content of kelp and seaweed products is highly variable and can be extremely high, with kelp ranging from roughly 16 to more than 8,000 micrograms of iodine per gram, so a single serving can deliver many times the daily requirement or the upper limit. If you use seaweed foods or supplements, keep the amount small and measured, and be aware that some products also carry heavy metals such as arsenic.
Lugol's solution and high-dose iodine drops
Lugol's and similar concentrated iodine products deliver thousands of times a day's requirement in a few drops. They are sometimes promoted as a general tonic or a thyroid-support product, and for someone already iodine-replete they offer no established benefit while carrying the excess-iodine risks above. This is not a supplement to take on your own initiative.
Pregnancy is the one time to be deliberate in both directions
Too little iodine risks the developing brain, and too much can suppress the fetal thyroid, so pregnancy is where the U-shaped response matters most. The aim is the recommended amount, about 220 to 250 micrograms a day from food, iodized salt, and a prenatal with roughly 150 micrograms, rather than a high-dose product. If you are planning a pregnancy or are pregnant and unsure of your intake, this is worth raising with your prenatal care provider.
Supplementing when you are already sufficient
For people who are already sufficient in iodine from salt, dairy, and seafood, taking extra has not been shown to benefit the thyroid, and it moves you toward the harmful high end of the curve. The reason to add iodine is a shortfall, and the clearest one is a diet without dairy, seafood, or iodized salt.
Start slow, be smart, read the research, and consult a professional if you have any concerns. This is here to inform your choice, not make it for you.
Common Questions
Should I take an iodine supplement?
For most people, no. If you use iodized salt or eat dairy, eggs, and seafood, you almost certainly already get enough, and giving iodine to people who are already sufficient has not been shown to help. The exceptions are a diet without any of those sources (vegans and people who avoid both dairy and fish), and pregnancy, where needs rise. Even then the target is the recommended amount, roughly 150 micrograms for most adults, not a high-dose kelp or iodine-drop product, which can push a vulnerable thyroid into trouble.
Is iodine good or bad for the thyroid?
Both, depending on the amount, which is what makes iodine unusual. The thyroid cannot make its hormones without it, so too little causes goiter and an underactive thyroid, and in pregnancy it harms the developing brain.
An intake well above what the gland needs can itself trigger an underactive or overactive thyroid, most easily in people with autoimmune thyroid disease or nodular goiter.
The response is a U, with harm at both ends, so the goal is sufficiency, not as much as possible.
Does pink Himalayan or sea salt have iodine?
Usually not in any meaningful amount. Iodine is added to table salt on purpose; most specialty, gourmet, and sea salts, including pink Himalayan salt, are not iodized, and neither is most of the salt in restaurant and packaged food. If those are your main salt sources and you also skip dairy and seafood, you may be getting far less iodine than you assume, and iodized salt or another reliable source is the simple fix.
Can too much iodine cause a problem in pregnancy?
Yes, which is why pregnancy calls for the recommended amount rather than a megadose. Too little iodine risks the baby's brain, and that is the more common concern, but a very high maternal intake can suppress the fetal thyroid. The sensible aim is about 220 to 250 micrograms a day from food, iodized salt, and a prenatal with roughly 150 micrograms of iodine, and to leave high-dose kelp and iodine drops out of it.
Explore Related
Other pages this one connects to, by the evidence they share, the outcomes they touch, and the ground they cover.
All 11 sources on this page independently checked and cross-referenced.
Thomas Dehli, Founder & Editor, Sacred Lotus
Sacred Lotus has published Chinese medicine reference material since 2001. Integrative pages are held to the same standard as the herb and formula library: cite the source, grade the claim at its real strength, and say where the research has not looked. This page is educational and it is not medical advice. Last reviewed and updated August 16, 2026.
Evidence strength
How confidently the research supports a claim. Strength describes the evidence, not our endorsement.