Iodine

Iodine is the rate-limiting raw material for thyroid hormone. Without enough, the thyroid cannot make enough hormone, and over time the gland enlarges to compensate (a visible goitre, historically the "Derbyshire neck" of inland UK communities). The country-level UK story was largely solved by milk processing and the iodine-rich cattle feed used in commercial dairying; UK milk became, by the 1950s, the reliable everyday source.

The relevance now is for groups who do not drink milk: vegans, dairy-free families, people on lactose-free regimens. NDNS data shows iodine intakes in adolescent girls and women of reproductive age sitting close to or below the reference, and the pregnancy concern is meaningful: low maternal iodine in early pregnancy is associated with poorer neurodevelopment in the child.

At the other end, kelp and brown seaweed (particularly bladderwrack and some forms of dried kombu) can carry iodine at levels thousands of micrograms per gram of dry weight. Daily kelp supplements can deliver 5,000 to 30,000 micrograms or more in a single dose, far above the safe upper. Doctors see iodine-induced thyroid problems in patients taking these products, sometimes for years.

Values per 100g from USDA SR Legacy and McCance and Widdowson 7th edition; the site's food entries currently store macros only.

The vegan / dairy-free practical: if you do not eat dairy or white fish, the reliable iodine routes are fortified plant milk (UK brands vary, check the label), and either eggs or a deliberate iodine source. A standard pregnancy multivitamin formulated for the UK market typically includes around 100 to 150 micrograms of iodine; this is the most common practical answer for vegans planning a pregnancy.

UK Reference Nutrient Intake (RNI) for iodine is set by SACN (1991).

UK iodine deficiency in adults rarely causes severe goitre any more, because milk has been a reliable source for generations. The current concern is mild-to-moderate insufficiency, particularly in women of reproductive age and on plant-based diets.

Signs of deficiency

• Tiredness, low mood
• Weight gain, feeling cold
• Slow heart rate
• Goitre (an enlarged thyroid gland, visible at the front of the neck)
• In long-running deficiency: hypothyroidism (low thyroid hormone)
• In pregnancy: increased risk of poorer brain development in the baby; severe maternal iodine deficiency causes cretinism

At higher risk in the UK

• Vegans and dairy-free families. The biggest UK risk group. Iodine is not naturally in plant foods at meaningful levels.
• People who never eat white fish or seafood.
• Pregnant women, particularly those entering pregnancy on a low iodine intake.
• Adolescent girls. NDNS data has consistently shown this group with intakes near or below the RNI.

Iodine excess can cause its own thyroid problems: paradoxically, either an overactive thyroid (hyperthyroidism) or an underactive thyroid (hypothyroidism, particularly in people with existing autoimmune thyroid disease).

NHS supplement guidance: taking 500 micrograms or less a day of iodine from supplements is unlikely to cause any harm.

Kelp and brown seaweed supplements are the practical UK risk. Some products deliver 1,000 to 30,000 micrograms in a single capsule, far above safe levels. NHS standing guidance: do not regularly take kelp or seaweed supplements at high dose. Eating seaweed as a labelled food (nori in sushi, wakame in miso soup) once or twice a week is generally fine; daily concentrated kelp supplementation is not.

If you take a kelp or seaweed supplement and notice anxiety, palpitations, weight loss, or temperature intolerance (signs of hyperthyroidism), or fatigue and cold intolerance (signs of hypothyroidism), see a GP.

For most UK adults eating dairy or white fish, supplementation is not needed.

If you are vegan, dairy-free, or planning a pregnancy and are not regularly eating eggs or fish: a standard UK pregnancy multivitamin or a basic multivitamin that includes around 100 to 150 micrograms of iodine is a sensible backstop. Discuss with a GP or midwife at the booking appointment if planning pregnancy or in early pregnancy.

Do not use kelp or seaweed supplements as a "natural" iodine source. The dose is hard to control and frequently exceeds the safe upper.

People with existing thyroid conditions (overactive, underactive, autoimmune thyroid disease) should follow personalised advice from their GP or endocrinology team rather than self-supplementing iodine.

• Plant-based eating and iodine: Plant-based eating covers iodine for people not eating dairy or fish.
• Best food sources: Haddock, Cod, Milk, Plain yoghurt, Eggs.
• Other minerals important in pregnancy: Iron, Folate, Calcium, Vitamin D.

• NHS. Vitamins and minerals: Iodine. nhs.uk/conditions/vitamins-and-minerals/iodine.
• SACN. Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. Department of Health Report 41 (1991).
• National Diet and Nutrition Survey (NDNS) rolling programme reports. Public Health England / OHID.
• Public Health England. McCance and Widdowson's The Composition of Foods, 7th summary edition (2015).

This page is reference information for UK shoppers. It is not medical advice. Pregnancy supplement decisions and existing thyroid conditions should be discussed with a GP, midwife, or endocrinology team.