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Iodine

Iodine is needed to make thyroid hormones, which regulate growth, development and metabolism, and low intake early in life impairs cognition and growth. Adults following vegan diets have the lowest iodine intake and status of any dietary group studied, especially where salt is not routinely iodised. Population salt iodisation programmes are linked with adequate iodine status, while trials of iodine supplements in pregnancy in mildly to moderately deficient settings show better maternal iodine status but no clear infant benefit. Too much iodine, including from some seaweeds such as kelp, is also linked with thyroid dysfunction.

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Evidence retrieved up to

What the research shows

Chronically excessive iodine intake, from sources such as poorly monitored iodised salt, iodine-rich water, some seaweeds or supplements, is associated with thyroid dysfunction, with people who have existing thyroid conditions or past iodine deficiency most susceptible.

Low certaintyFurther research is likely to change this.
Take care: see the safety noteRisk

Who this is about

adults with chronically excessive iodine intake

Practical note

Graded low: the adult meta-analysis pooled 3 cross-sectional studies, with odds ratios of 2.78 for overt and 2.03 for subclinical hypothyroidism in excess versus adequate intake populations, and the authors rated the included studies low quality; narrative reviews agree on the direction.

Vegan note

A 2020 systematic review found that seaweed contributed largely to the diets of the vegans with excessive iodine intake.

Safety note

Brown seaweeds such as kelp are a concentrated and variable iodine source and product labelling may be insufficient; a 2021 review cautions that even small amounts can have antithyroid actions. Iodine-containing supplements add to intake from food and salt.

Sources

Adults following a vegan diet have lower iodine intake and lower urinary iodine than omnivores, and the gap is greatest in countries without a universal salt iodisation programme.

Moderate certaintyFurther research may change the size of the effect.
No known concerns at usual amountsRisk

Who this is about

adults following a vegan diet

Practical note

Graded moderate: observational evidence from a 2023 meta-analysis of 11 studies with 4421 adults and a 2020 systematic review of 15 studies starts low and is upgraded for a large, consistent difference; in the 2023 meta-analysis vegan diets had a pooled intake of 17.3 micrograms a day. A 2021 South Australian pilot study found lower urinary iodine in plant-based eaters, with both groups below the WHO threshold.

Vegan note

Vegans rely on iodised salt, fortified foods such as bread where it is fortified, or supplements for iodine; a few vegans with heavy seaweed use have excessive intakes instead.

Sources

Iodine is needed to make thyroid hormones, which regulate growth, development and metabolism; low iodine intake early in life impairs cognition and growth, and iodine status also shapes thyroid health in adults.

Moderate certaintyFurther research may change the size of the effect.
No known concerns at usual amountsRisk

Who this is about

infants, children and adults with low iodine intake

Practical note

Graded moderate: the sources here are reviews restating well-established physiology and population evidence rather than a pooled analysis of outcomes. Requirements rise during pregnancy and breastfeeding.

Vegan note

Groups with low intake include people following vegan diets; see the vegan status claim.

Sources

In settings with mild to moderate iodine deficiency, iodine supplements during pregnancy improved iodine status in mothers and infants, but trials showed no clear effect on infant growth or neurodevelopment.

Low certaintyFurther research is likely to change this.
Take care: see the safety noteUnclear

Who this is about

pregnant women in settings with mild to moderate iodine deficiency

Practical note

Graded low: a 2017 Cochrane review of 11 trials with over 2700 women graded its primary outcomes low or very low quality and found insufficient data for meaningful conclusions; a 2021 meta-analysis of 5 trials found no difference in infant cognitive, language or motor development in the first 2 years. Almost all trials were in mild or moderate deficiency settings.

Vegan note

Pregnant vegans start from lower iodine status, which these trials did not study separately.

Safety note

The Cochrane review reported more digestive intolerance in pregnancy with supplements, from one trial with very low quality evidence. Supplement use in pregnancy is a decision for the woman with her health professional.

Sources

Countries with sustained salt iodisation programmes are associated with adequate population iodine status and lower goitre rates.

Low certaintyFurther research is likely to change this.
No known concerns at usual amountsBenefit

Who this is about

general populations in countries with salt iodisation programmes

Practical note

Graded low: a 2021 systematic review of 23 mostly cross-sectional and before-after studies in 13 countries, without pooling, so the evidence is observational with heterogeneous designs.

Vegan note

A 2023 meta-analysis found vegans living without a universal salt iodisation programme had the highest risk of low iodine status.

Safety note

Poorly monitored salt iodisation can push intakes into excess; see the excess-intake claim.

Sources