Iodine
What it is
Iodine is an essential trace element with a single well-defined physiological function: it is a structural component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones regulate metabolic rate, cardiovascular function, growth, and neurological development throughout the lifespan. Because the body cannot synthesise iodine, adequate dietary intake is obligatory. The thyroid gland concentrates iodine with remarkable efficiency, maintaining intrathyroidal stores that can sustain hormone production for weeks to months when intake falls, but these reserves are finite and chronic inadequacy leads to progressive dysfunction.
In regions where iodine is not present in sufficient quantities in soil and water, deficiency has historically been a major public health problem. Salt iodisation programmes, introduced across much of the world from the mid-twentieth century onwards, dramatically reduced the burden of iodine deficiency disorders including goitre and cretinism. However, iodine status remains a live concern in populations that do not use iodised salt, consume diets low in iodine-rich foods such as dairy, seafood, and eggs, or live in regions with poor soil iodine. The United Kingdom is a notable example of a developed country with mild population-level iodine insufficiency, partly because UK salt is not universally iodised and the traditional dietary contribution from dairy has declined among certain groups.
Iodine is present in a small number of food categories. Dairy products and seafood are the primary sources in Western diets; eggs contribute modestly. Plant foods are generally poor iodine sources because the iodine content of soil and water, which varies considerably by geography, determines the iodine content of crops. Seaweed contains iodine in highly variable and sometimes extremely high concentrations and is not a reliable supplementation strategy. Iodine supplements are available as potassium iodide or potassium iodate, with potassium iodide being the preferred pharmacological form; the iodine content of supplements should be checked carefully, as products marketed for thyroid support frequently contain amounts that exceed recommended intakes by a wide margin.
What the evidence shows
The "Strong" primary rating assigned here applies specifically to the prevention of deficiency disorders in severely deficient populations and does not generalise to iodine supplementation in iodine-sufficient or mildly deficient populations, where the evidence is considerably weaker. The evidence for iodine is unusual in that its strongest pillar is not a randomised trial but a century of natural experiment, epidemiology, and mechanistic understanding so robust that the fundamental causal relationship between severe iodine deficiency and thyroid dysfunction, goitre, and cretinism is not scientifically contested. Where RCT evidence exists, it confirms and quantifies what observational data established: iodine supplementation in severely deficient populations prevents cretinism, improves motor development in children, normalises thyroid volume, and reduces goitre prevalence. The evidence for these outcomes in populations with severe deficiency (median urinary iodine concentration below 50 micrograms per litre) is strong.
The clinical picture becomes more complex and the evidence considerably weaker when the question shifts to supplementation during pregnancy in populations with mild to moderate iodine deficiency, which is the scenario most relevant to the UK and similar countries. A 2020 systematic review by Dineva et al. (American Journal of Clinical Nutrition) concluded that there is insufficient good-quality evidence to support current recommendations for iodine supplementation in pregnancy in areas of mild to moderate deficiency. The six RCTs conducted in mildly to moderately deficient pregnant women focused primarily on thyroid function biomarkers rather than child neurodevelopmental outcomes, and none showed consistent benefits on the outcomes that matter most clinically. The PINK trial, the most ambitious attempt to generate high-quality evidence in this area, was aborted early after recruiting only 59 of a planned 1,098 participants due to funding withdrawal, leaving the field without the definitive trial it needs.
Observational studies consistently associate mild to moderate maternal iodine deficiency with poorer neurodevelopmental outcomes in children, but translating these associations into supplementation recommendations is complicated by confounding, heterogeneity of iodine measurement methods, and the absence of a clearly established intervention effect in RCT data. This distinction between association and demonstrated intervention effect is critical and remains unresolved in mildly deficient populations.
There is also a frequently underappreciated risk on the other side of the intake curve. Iodine has a genuinely U-shaped dose-response relationship with thyroid health: both too little and too much impair thyroid function. Abrupt increases in iodine intake can suppress thyroid hormone synthesis through the Wolff-Chaikoff effect, and in individuals with pre-existing thyroid disease or prior exposure to deficiency, excess iodine can precipitate hypothyroidism, hyperthyroidism, or thyroid autoimmunity. Kelp supplements and high-dose iodine products are a particular concern; many contain iodine in amounts several times above the tolerable upper intake level, and several clinical studies have documented thyroid dysfunction following their use.
Five questions
Does low iodine status cause harm? Yes, clearly and dose-dependently. Severe deficiency causes cretinism, goitre, hypothyroidism, and impaired neurodevelopment in offspring. Moderate deficiency is associated with subclinical hypothyroidism, goitre, and probable neurodevelopmental effects in children. Mild deficiency is associated with some degree of thyroid adaptation and possible effects on foetal brain development, though the clinical magnitude is less clearly established. Iodine deficiency remains one of the most prevalent and preventable causes of intellectual impairment worldwide.
Does supplementation prevent disease? In severely deficient populations, yes: supplementation prevents cretinism, reduces goitre prevalence, and improves child motor and neurodevelopmental outcomes. In mildly to moderately deficient populations, the evidence for disease prevention through supplementation is weaker. Current public health guidance in the UK and similar countries recommends supplementation in pregnancy and lactation, and this is a commonly recommended and precautionary position given the plausibility of harm from mild deficiency and the safety of supplementation at recommended doses, but it rests partly on precautionary reasoning rather than definitive RCT evidence for clinical benefit in these populations.
Does it affect biomarkers? Iodine supplementation in deficient individuals normalises urinary iodine concentration, reduces TSH, reduces thyroid volume, and in some studies alters free thyroxine concentrations. These are expected physiological responses to correcting a substrate-limited process. In mildly deficient pregnant women, supplementation raises urinary iodine concentration but effects on maternal TSH and free thyroxine have been inconsistent across trials, with some showing small reductions in free thyroxine in the supplemented group, the clinical significance of which is unclear.
Does it help clinical populations? Iodine supplementation is a standard recommendation for pregnant and breastfeeding women, particularly those with low dietary iodine intake, and is incorporated into most prenatal micronutrient formulations. For individuals with documented iodine deficiency (confirmed by urinary iodine concentration), targeted supplementation to reach adequacy is appropriate across the lifespan. For individuals with established hypothyroidism who are stable on levothyroxine, iodine supplementation is generally not indicated and may interfere with thyroid hormone management.
Does it benefit healthy individuals? For individuals who are already iodine-sufficient through dietary intake, additional supplementation offers no established benefit and carries the theoretical risk of excess. The primary value of iodine supplementation is in correcting or preventing deficiency. For people with adequate iodine status, the question is not whether to supplement but whether their dietary pattern is maintaining adequacy -- an important distinction given the growing evidence that plant-based diets in non-iodised countries are associated with lower iodine status.
Individual variation
Iodine requirements vary substantially by life stage. Pregnant women need 220 to 250 micrograms per day to support foetal thyroid hormone synthesis and neurological development; breastfeeding women need 250 to 290 micrograms per day to maintain adequate breast milk iodine content. These are the populations most clearly at risk from insufficient intake and the groups for whom supplementation is most strongly supported by public health guidance.
Individuals following plant-based diets, particularly vegans, have consistently lower urinary iodine concentrations than omnivores in studies conducted in countries without mandatory salt iodisation. This reflects the near-absence of iodine in most plant foods and the growing replacement of traditional iodine-rich dairy with plant-based alternatives that contain negligible iodine. A 2025 review in Frontiers in Endocrinology confirmed that vegans and vegetarians are at elevated risk of both iodine deficiency and, paradoxically, iodine excess from uncontrolled use of seaweed-based products or high-dose supplements. A 2025 UK Biobank cohort study (BMC Medicine) found a moderately higher risk of hypothyroidism in vegetarians after BMI adjustment, though the association was observational and causality cannot be established.
Individuals with pre-existing thyroid conditions require specific attention. Those with Hashimoto's thyroiditis, Graves' disease, nodular goitre, or a history of thyroid surgery or radioiodine treatment may respond poorly to changes in iodine intake, even within the nominally safe range. Excess iodine can trigger thyroid autoimmunity flares, and several small clinical trials have documented subclinical hypothyroidism developing in Hashimoto's patients supplemented with modest doses of potassium iodide. Individuals in these groups should not begin iodine supplementation without medical assessment.
The elderly may be less tolerant of sudden changes in iodine intake in either direction and are more likely to have subclinical thyroid nodules or autonomy that can be destabilised by iodine loading.
Testing and status assessment
Urinary iodine concentration (UIC) is the standard population-level measure of iodine status and is used in large surveys to classify populations as deficient, sufficient, or excess. At the individual level, however, spot UIC is an unreliable measure because iodine excretion varies substantially with day-to-day dietary intake, hydration, and time of collection. A single spot measurement cannot reliably classify an individual's iodine status. Repeated measures or 24-hour urine collections are methodologically preferable but impractical in routine clinical settings.
Serum thyroglobulin is a sensitive early marker of iodine deficiency at the population level but is influenced by thyroid disease and is not used routinely for individual status assessment. TSH is not a sensitive indicator of mild iodine deficiency; TSH may remain within the normal range even when intake is low, because the thyroid adapts through increased iodine trapping efficiency before TSH rises.
In practice, the most useful clinical approach is dietary assessment. For individuals eating a typical Western diet that includes dairy products and some seafood, iodine adequacy is likely in most cases. For individuals following strict plant-based diets in countries without mandatory iodisation, supplementation with 150 micrograms per day of potassium iodide is a reasonable precaution that brings intake toward the recommended dietary allowance without approaching the upper threshold.
Safety
Iodine has a narrow therapeutic window relative to many micronutrients. The tolerable upper intake level for adults is 1,100 micrograms per day (US Institute of Medicine), and the American Thyroid Association advises against supplements containing more than 500 micrograms per day. During pregnancy and lactation the upper limits are more conservative, ranging from 500 to 1,100 micrograms per day depending on the guideline source. These limits exist because excess iodine intake can cause hypothyroidism through failure to escape the Wolff-Chaikoff effect, hyperthyroidism through the Jod-Basedow phenomenon in susceptible individuals with thyroid nodules, and thyroid autoimmunity through incompletely understood mechanisms.
Kelp and seaweed supplements are a particular hazard. The iodine content of these products is highly variable and frequently not accurately reflected on the label; some preparations contain several thousand micrograms per serving. Cases of thyroid dysfunction, including severe hypothyroidism and thyrotoxicosis, have been reported following regular kelp consumption or supplementation. These products should not be used as an iodine source.
Individuals with Hashimoto's thyroiditis should exercise caution. While the evidence that supplementation at recommended doses significantly worsens autoimmune thyroid disease in iodine-sufficient individuals is mixed, several clinical studies have shown that even modest supplementation can precipitate subclinical hypothyroidism in those with pre-existing TPO antibody positivity. Medical supervision before supplementing is warranted in this group. The same principle applies to those with Graves' disease, nodular goitre, or any history of thyroid disease or thyroid surgery.
Iodine at standard supplemental doses (up to 150 micrograms of potassium iodide per day) is generally safe in healthy adults without thyroid disease, in pregnant and breastfeeding women meeting but not exceeding recommended intakes, and in children receiving age-appropriate doses. Adverse effects at these levels are uncommon in individuals without pre-existing thyroid disease.
Interactions with medications are worth noting. Amiodarone contains substantial iodine (approximately 75 micrograms of free iodine per 200 mg tablet) and commonly causes thyroid dysfunction. Lithium and interferon-alpha impair thyroid function independently and may interact with iodine status. Anti-thyroid drugs and levothyroxine therapy should be managed in relation to iodine intake under medical supervision.
What can reasonably be concluded
Iodine is an essential micronutrient with a clear and well-established role in thyroid function and foetal neurological development. The evidence for the harm of severe deficiency and the benefit of correcting it is among the strongest in nutritional science. The case for supplementation in pregnancy and in populations with structural dietary gaps, particularly those following plant-based diets in non-iodised countries, is clinically reasonable and supported by public health guidance, though the RCT evidence for neurodevelopmental benefit in mild-to-moderate deficiency is weaker than is often implied.
The single most important caution with iodine is its narrow therapeutic window. Supplementation at appropriate doses in genuinely deficient individuals is safe and beneficial. Supplementation at high doses, or in individuals who are already iodine-sufficient, or in people with pre-existing thyroid conditions without medical oversight, carries real risk. High-dose iodine products and kelp supplements sold for thyroid support are among the most likely ways for consumers to inadvertently exceed safe intake levels and should be treated with particular scepticism.
Where evidence is limited or outcomes are uncertain, conclusions should be treated as provisional and subject to revision as the evidence base develops.