Iodine: The Overlooked Mineral in Pregnancy and Plant-Based Diets

Nutrition · Published 2026-10-24

Grilled white fish fillets with lemon and salad on a plate — Grilled white fish with lemon.

Iodine deficiency is the world's leading preventable cause of brain damage in children, according to the World Health Organization. Salt iodisation has made severe deficiency rare in many countries, but mild deficiency persists in some high-income countries, including the UK, and plant-based diets can make it more likely.

What iodine does

The thyroid gland uses iodine to make thyroid hormones (thyroxine, T4, and triiodothyronine, T3). These hormones regulate metabolism and are essential for normal growth and for brain development in the fetus and young child. When iodine intake is too low, the thyroid cannot make enough hormone; it may enlarge (a goitre) as it tries to capture more iodine, and hypothyroidism can follow.

The most serious consequences occur in pregnancy and early childhood. Severe deficiency in pregnancy can cause miscarriage, stillbirth and congenital hypothyroidism with intellectual disability. Milder deficiency is associated with smaller but measurable effects on child development.

Reference intakes and upper limits

Body	Adults	Pregnancy	Breastfeeding	Upper limit (adults)
US National Academies (NIH ODS)	RDA 150 mcg	220 mcg	290 mcg	1,100 mcg
EFSA (EU)	AI 150 mcg	200 mcg	200 mcg	600 mcg
WHO	150 mcg	250 mcg	250 mcg	—
UK reference nutrient intake	140 mcg	No increment set		—

The NHS notes that taking 0.5 mg (500 micrograms) or less of iodine a day from supplements is unlikely to cause harm.

Where iodine comes from

Iodine content in food depends on the iodine in soil and water, and on farming practices. Major sources are:

iodised salt, the main source in countries with salt iodisation programmes;
milk and dairy products, which in the UK are the largest source of iodine, largely because of iodine in cattle feed and hygiene practices;
white fish and shellfish (cod, haddock and prawns are good sources);
eggs;
fortified plant drinks, though many are not fortified.

Fruit, vegetables and grains generally contain little iodine, especially where soils are iodine-poor.

Salt iodisation

The WHO and UNICEF recommend universal salt iodisation as the main strategy to prevent iodine deficiency, and most countries now have programmes. The UK is one of the countries without one: iodised salt is not widely sold, and population intake relies mainly on dairy. Public health advice to reduce salt intake is compatible with iodisation, because the amount of iodine added can be adjusted.

Who is at risk

Pregnant and breastfeeding people, whose needs rise by about 50% according to US values.
People who avoid dairy and fish, including vegans and people with milk allergy, particularly in countries without iodised salt. Studies in the UK have found low iodine intakes in people following vegan diets.
People living in iodine-deficient regions without access to iodised salt.

Evidence on mild deficiency in pregnancy

An analysis from the UK Avon Longitudinal Study of Parents and Children (Bath et al., The Lancet 2013) found that children of mothers with mild-to-moderate iodine deficiency in early pregnancy were more likely to have lower verbal IQ and reading scores at ages 8 and 9. This was an observational study, so it cannot prove cause and effect, and randomised trials of iodine supplements in mildly deficient pregnant women have produced inconsistent results. The finding has nonetheless shaped advice to make sure pregnant people get enough iodine from diet.

Recommendations differ: the American Thyroid Association recommends a daily supplement with 150 micrograms of iodine for pregnant and breastfeeding women in the US and Canada, while the NHS does not routinely recommend iodine supplements in pregnancy and focuses on dietary sources. Anyone who is pregnant and does not eat dairy or fish should raise iodine with their midwife or doctor.

Too much iodine

Excess iodine can also disturb thyroid function, causing hypothyroidism or hyperthyroidism, particularly in people with existing thyroid disease, in older people and in newborns. Typical diets do not cause this, but some products can:

Seaweed, especially brown seaweeds such as kelp (kombu), can contain very large and variable amounts of iodine. The NHS advises against eating seaweed more than once a week, particularly in pregnancy.
Kelp supplements can contain far more iodine than the label suggests and have been linked to thyroid problems.
Iodine-containing medicines and contrast agents (such as amiodarone) can affect thyroid function and are monitored by doctors.

People with thyroid conditions should not take iodine supplements without medical advice.

In short

Adults need about 140–150 micrograms of iodine a day; needs rise in pregnancy and breastfeeding.
Where salt is not iodised, dairy and fish are the main sources, so vegans and people who avoid both may fall short.
Advice on iodine supplements in pregnancy differs between countries; ask your midwife or doctor.
Seaweed and kelp supplements can supply excessive amounts; the US upper limit is 1,100 micrograms and EFSA's is 600 micrograms a day.

This page is general information, not medical advice. If you have a thyroid condition, are pregnant or follow a vegan diet, ask a doctor, midwife or registered dietitian about iodine.

Sources

NIH Office of Dietary Supplements, Iodine fact sheet for health professionals.
NHS, Iodine.
World Health Organization and UNICEF, guidance on universal salt iodisation and iodine deficiency.
Bath SC et al. Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). Lancet 2013;382(9889):331–337.
Alexander EK et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid 2017;27(3):315–389.
EFSA, Dietary reference values finder.