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I8.5
Iodine Research
Mixed evidence
137 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 1986–2026 with a typical study size of 651 participants.
Based on 137 studies · 42 meta-analyses · 71 RCTs · 63,789 total participants
Confidence
High confidence
What the studies found
4helped4unclear3didn't help· 126 more without graded effect data
By outcome
Thyroid function
Mixed evidence107 studies
Women's healthSupports healthy estrogen metabolism and hormonal equilibrium · 4-8 weeks · Provides essential nutrients for healthy pregnancy · Ongoing
Limited support49 studies
Cognitive function
Mostly mechanism / observational17 studies
Safety profile
Limited support9 studies
Diet & iodine status
Probably helps4 studies
Therapeutic & clinical
Mostly mechanism / observational3 studies
Topical antisepsis (povidone-iodine)
Too few graded studies1 study
By the numbers
Pulled from 29 studies with measurable effects
Likely real effects
50%
across studies
People studied
64k
typical study: 651 people
Strongest designs
113
42 pooled, 71 randomised
Showed benefit
36%
4/11 studies
Populations Studied
Pregnant women5
General population4
Vegan and vegetarian populations1
Case-control participants1
Active research area
42 studies in the last 5 years · Latest meta-analysis: 2026
198620062026
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.