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I9.0
Iodine Research
Mixed evidenceModerate safety
28 peer-reviewed studies
What the evidence says
Mixed evidence
Studies are split: Iodine helped in 4 of 11 cases, with the rest inconclusive or showing no benefit.
Most evidence is from high-quality meta-analyses and randomised trials published 2009–2026 with a typical study size of 651 participants.
Based on 28 studies · 20 meta-analyses · 4 RCTs · 60,922 total participants
Confidence
High
What the studies found
4helped4unclear3didn't help· 17 more without graded effect data
By outcome
Thyroid function
Mixed evidence27 studies
Women's healthSupports healthy estrogen metabolism and hormonal equilibrium · 4-8 weeks · Provides essential nutrients for healthy pregnancy · Ongoing
Limited support15 studies
Cognitive function
Mostly mechanism / observational7 studies
Safety profile
Limited support5 studies
Diet & iodine status
Probably helps4 studies
Therapeutic & clinical
Too few graded studies2 studies
By the numbers
Pulled from 24 studies with measurable effects
Likely real effects
50%
across studies
People studied
61k
typical study: 651 people
Strongest designs
24
20 pooled, 4 randomised
Showed benefit
36%
4/11 studies
Populations Studied
Pregnant women5
General population3
Vegan and vegetarian populations1
Case-control participants1
Active research area
20 studies in the last 5 years · Latest meta-analysis: 2026
200920172026
1Birth weight difference by maternal iodine statusMeta-AnalysisCited 13×n=42,269 · very large study2023
Whilst associations were modest-sized, we recommend maintaining iodine sufficiency in the population, especially for women of childbearing age on restricted diets low in iodide.
Greenwood DC et al. · Nutrients (2023)
No clear effect
← WorseNo effectBetter →
Could be chance
Birth weight was similar between UIC ≥ 150 μg/L and <150 μg/L (difference = 30 g, 95% CI −22 to 83, p = 0.3, n = 13, I2 = 89%) with no evidence of linear trend (4 g per 50 μg/L, −3 to 10, p = 0.2, n = 12, I2 = 80%).
I:Cr was similar, but with nonlinear trend suggesting I:Cr up to 200 μg/g associated with increasing birthweight (p = 0.02, n = 5).
Birthweight was 2.0 centiles (0.3 to 3.7, p = 0.02, n = 4, I2 = 0%) higher with UIC ≥ 150 μg/g, but not for I:Cr.
2Iodine intake in vegan dietsMeta-AnalysisCited 23×n=4,421 · very large study2023
Further research into the usefulness of mandatory fortification of vegan appropriate foods is required.
Eveleigh ER et al. · The British journal of nutrition (2023)
Noticeable benefit
← WorseNo effectBetter →
Vegan diets had the poorest iodine intake (17·3 µg/d) and were strongly associated with lower iodine intake (P = < 0·001) compared with omnivorous diets.
Lower intake in vegan diets was influenced by sex (P = 0·007), the presence of voluntary or absence of Universal Salt Iodisation (USI) programmes (P = 0·01 & P = < 0·001), and living in a country with adequate iodine nutrition (P = < 0·001).
Vegetarians and particularly vegans living in countries with no current USI programme continue to have increased risk of low iodine status, iodine deficiency and inadequate iodine intake.
3Papillary thyroid cancer occurrenceMeta-AnalysisCited 13×n=6,544 · very large study2022
The 10 case-control included studies involved a total of 6,544 participants.
Zhang X et al. · Frontiers in endocrinology (2022)
Huge harm
← WorseNo effectBetter →
Likely real
According to the analysis of the included studies, excessive iodine intake (UIC≥300ug/L) was positively associated with the occurrence of PTC patients compared with healthy controls (OR4.05, 95%CI 1.64-10.02, P=0.002).
The 10 case-control included studies involved a total of 6,544 participants.
The results of this meta-analysis showed excessive iodine intake, that is, UIC≥300ug/L was associated with the occurrence of PTC but not with BRAF mutation and LNM while adequate iodine intake (100≤UIC<200ug/L) may be one of the protective factors for PTC.
4Urinary iodine concentration increaseMeta-AnalysisCited 23×n=4,317 · very large study2019
The evidence on the effect of iodine fortification of foods, beverages, condiments, or seasonings other than salt on reducing goitre, improving physical development measures, and any adverse effects is uncertain.
Santos JAR et al. · The Cochrane database of systematic reviews (2019)
Noticeable benefit
← WorseNo effectBetter →
This is equivalent to an increase of 38.32 µg/L (95% CI 24.03 to 52.61 µg/L).
This effect was not observed in the meta-analysis of non-RCTs (SMD 0.25, 95% CI -0.16 to 0.66; 3 non-RCTs, 262 participants; very low-quality evidence).
The evidence on the effect of iodine fortification of foods, beverages, condiments, or seasonings other than salt on reducing goitre, improving physical development measures, and any adverse effects is uncertain.
6Thyroid nodule developmentMeta-AnalysisCited 3×n=31 · small study2025
This systematic review and meta-analysis indicate that iodine deficiency increases moderately the risk of developing thyroid nodules, while more than adequate and excessive iodine intake does not show a consistent effect.
Gräfe W et al. · Journal of endocrinological investigation (2025)
Noticeable harm
← WorseNo effectBetter →
The pooled odds ratio for iodine deficiency was 1.24 (95% CI [1.16-1.33], I2 = 0.00), while more than adequate and excessive iodine intake showed no significant association.
This systematic review and meta-analysis indicate that iodine deficiency increases moderately the risk of developing thyroid nodules, while more than adequate and excessive iodine intake does not show a consistent effect.
However, the heterogeneity of study results and the predominance of cross-sectional designs limit definitive conclusions.
7Neonatal thyrotropin concentrationMeta-AnalysisCited 11×n=110 · medium study2016
Our findings reveal that, compared with heel blood samples, neonatal thyrotropin in samples collected from the cord are more sensitive to the iodine status of mothers; however, further investigations are required in this regard.
Nazeri P et al. · The American journal of clinical nutrition (2016)
Our findings reveal that, compared with heel blood samples, neonatal thyrotropin in samples collected from the cord are more sensitive to the iodine status of mothers; however, further investigations are required in this regard.
9Correlation between drinking water iodine and urinary iodineMeta-AnalysisCited 3×2023
Azevedo FM et al. · Biological trace element research (2023)
Noticeable benefit
← WorseNo effectBetter →
The study identified median iodine concentration in drinking water from 2.2 to 617.8 μg/L and the correlation between iodine concentration in drinking water and urinary iodine concentration was 0.92, according to meta-analysis.
Furthermore, the iodine status was correlated to the iodine content in water.
The determination of a cutoff point can contribute to the implementation of iodine consumption control measures.
10Prevalence of excessive iodine intakeMeta-AnalysisCited 9×2023
Candido AC et al. · Biological trace element research (2023)
No clear effect
← WorseNo effectBetter →
The prevalence of excessive iodine intake in 10,736 pregnant women in different regions of the world was 52%.
Therefore, the prevalence of iodine excess was 52%, with high heterogeneity among studies, explained by trimester of gestation and FT4 level; therefore, the farther the trimester of gestation and the lower the FT4, the higher the prevalence of iodine excess.
In addition, drinking water with high iodine intake contributed to excessive iodine intake.
11Prevalence of insufficient iodine intakeMeta-AnalysisCited 26×2022
Despite the progress in iodine fortification policies and periodic monitoring of the iodine nutritional status of the population worldwide, salt iodination alone may not be sufficient to provide adequate iodine status to pregnant women.
Patriota ESO et al. · European journal of clinical nutrition (2022)
No clear effect
← WorseNo effectBetter →
The population consisted of 163,021 pregnant women adults and adolescents, and the overall prevalence of insufficient iodine intake was 53% (95% confidence interval [CI]: 47-60; I2 = 99.8%).
Pregnant women who live in insufficient iodine status country had a higher prevalence (86%; 95% CI: 78-93; I² =97.0%) of inadequate iodine nutritional status than to those living in country considered sufficient (51%; 95% IC: 45-57; I² = 99.8%).
Despite the progress in iodine fortification policies and periodic monitoring of the iodine nutritional status of the population worldwide, salt iodination alone may not be sufficient to provide adequate iodine status to pregnant women.
12Iodine status and neurodevelopmental outcomes in pregnancyMeta-AnalysisCited 28×2021
Iodine supplementation during pregnancy can improve the iodine status in pregnant women and their offspring; however, according to our meta-analysis, there was no evidence of improved growth or neurodevelopmental outcomes in infants of iodine-supplemented mothers.
Nazeri P et al. · European journal of endocrinology (2021)
Iodine supplementation during pregnancy can improve the iodine status in pregnant women and their offspring; however, according to our meta-analysis, there was no evidence of improved growth or neurodevelopmental outcomes in infants of iodine-supplemented mothers.
13Iodine deficiency prevalence during pregnancyMeta-AnalysisCited 6×2021
The prevalence of iodine deficiency during pregnancy using urine iodine is considerably high in Ethiopia.
Kabthymer RH et al. · BMC pregnancy and childbirth (2021)
No clear effect
← WorseNo effectBetter →
The meta-analysis of seven studies that included 2190 pregnant women showed a pooled prevalence of iodine deficiency during pregnancy to be 68.76% (95% CI: 55.21-82.31).
In a subgroup analysis, the prevalence in Oromia region is 71.93% (95% CI: 54.87-88.99) and in Amhara region is 60.93% (95% CI: 57.39-64.48).
Iodized salt use (AOR = 0.18; 95% CI: 0.08-0.44) and 1st trimester pregnancy (AOR = 0.68; 95% CI: 0.47-0.99) were found to have a significant association with iodine deficiency.
There is insufficient good-quality evidence to support current recommendations for iodine supplementation in pregnancy in areas of mild-to-moderate deficiency.
Dineva M et al. · The American journal of clinical nutrition (2020)
Meta-analyses of 2 RCTs showed no effect on child cognitive [mean difference (MD): -0.18; 95% CI: -1.22, 0.87], language (MD: 1.28; 95% CI: -0.28, 2.83), or motor scores (MD: 0.28; 95% CI: -1.10, 1.66).
There is insufficient good-quality evidence to support current recommendations for iodine supplementation in pregnancy in areas of mild-to-moderate deficiency.
Well-designed RCTs, with child cognitive outcomes, are needed in pregnant women who are moderately deficient (median UIC < 100 µg/L).
15Thyroglobulin concentration and maternal iodine statusMeta-AnalysisCited 9×2020
Further studies are warranted to determine the sensitivity of Tg at different degrees of iodine deficiency during pregnancy.
Nazeri P et al. · Thyroid : official journal of the American Thyroid Association (2020)
Likely real
The pooled mean Tg values, and 95% confidence intervals were estimated in a population of women with UIC <150 and UIC ≥150 μg/L during pregnancy.
Dose-response meta-analyses revealed a significant nonlinear association between maternal UIC and Tg concentration during pregnancy.
Among populations of pregnant women, an inverse association was found between UIC values <100 μg/L and Tg concentration (p-linearity = 0.007; p-nonlinearity = 0.027); however, higher values of UIC were not associated with Tg concentration.
16Thyroid disease prevalence across iodine intake levelsMeta-AnalysisCited 15×2017
Thyroid nodules are the most easily detectable thyroid disease.
Weng W et al. · Medicine (2017)
The prevalence of thyroid nodules was 22.3% (95% confidence interval [CI]: 20.6%-24.1%) for the low-iodine group, 25.4% (95% CI: 20.8%-28.8%) for the medium-iodine group, and 6.8% (95% CI: 2.8%-11.5%) for the high-iodine group.
The prevalence of 8.3% (95% CI: 3.8%-17.3%) for subclinical hypothyroidism in the high-iodine group was significantly higher than the low- and medium-iodine groups (P < .01).
The prevalence of hypothyroidism in the medium-iodine group was 0.2% (95% CI: 0.1%-0.4%), and was lower than the prevalence of the other 2 groups (P < .01).
17Dose-response relationship between iodine intake and biomarkersMeta-AnalysisCited 12×2014
The model of dose-response quantification used to describe the relationship between iodine intake and biomarkers of iodine status may be useful for providing complementary evidence to support recommendations for iodine intake in different population groups.
Ristić-Medić D et al. · Nutrition reviews (2014)
Noticeable benefit
← WorseNo effectBetter →
The results of pooled randomized controlled trials indicated that the doubling of dietary iodine intake increased urinary iodine concentrations by 14% in children and adolescents, by 57% in adults and the elderly, and by 81% in pregnant women.
The dose-response relationship between iodine intake and biomarkers of iodine status indicated a 12% decrease in thyroid-stimulating hormone and a 31% decrease in thyroglobulin in pregnant women.
The overall pooled regression coefficient (β) and the standard error of β were calculated by random-effects meta-analysis on a double-log scale, using the calculated intake-status regression coefficient (β) for each individual study.
Suboptimal maternal iodine status during pregnancy was associated with modest decrements in child neurodevelopmental performance, with exploratory dose-response analyses suggesting that the fitted curves approached their maximum within a mid-range of dietary iodine intake.
Luo Q et al. · Nutrients (2026)
Binary outcome analysis confirmed increased odds of adverse neurodevelopmental outcomes (OR = 1.19, 95% CI: 1.03 to 1.39, p = 0.026).
Subgroup analysis by iodine exposure indicator showed directionally consistent negative effects across dietary intake (g = -0.11), UIC (g = -0.11) and UI/Cr (g = -0.28), with no significant between-subgroup difference (p = 0.237).
Suboptimal maternal iodine status during pregnancy was associated with modest decrements in child neurodevelopmental performance, with exploratory dose-response analyses suggesting that the fitted curves approached their maximum within a mid-range of dietary iodine intake.
Autoimmune thyroid diseases are associated with a dyslipidemic profile and with gut microbiota dysbiosis; the latter characterized by increased potentially pathogenic bacteria and reduced beneficial bacteria such as Lactobacillus and Bifidobacterium.
van Wees-Jansen ERPC et al. · Nutrients (2026)
Long-term remission following drug therapy is achieved in no more than approximately 50% of patients, while all treatment modalities carry potential adverse effects.
Indeed, observational studies have shown inverse associations between iodine intake and remission rates, as well as achievement of euthyroidism, medication requirements and thyroid autoantibody titers.
These findings suggest that dietary iodine restriction may enhance treatment efficacy and reduce medication-related risks.
20thyroid cancer riskMeta-Analysisn=651 · large study2026
Considering the current limitations, future large-scale, multicenter, prospective investigations are anticipated to provide further validation.
Li S et al. · Cancer control : journal of the Moffitt Cancer Center (2026)
Huge harm
← WorseNo effectBetter →
Borderline
The findings revealed a correlation between high urinary iodine concentrations and an increased risk of TC (odds ratio [OR]: 6.43, 95% confidence interval [CI]: 2.72-15.22, P < .05).
The elevated risk was observed for both PTC (OR: 7.56, 95% CI: 1.6-35.78, P < .001) and PTMC (OR: 8.96, 95% CI: 5.89-13.64, P < .001).
However, there was no significant association between dietary iodine intake and TC risk (OR: 0.75, 95% CI: 0.37-1.52, P > .05).ConclusionThis meta-analysis demonstrated a definitive link between high urinary iodine excretion and an increased risk of TC.