Iodine: food sources, supplements and thyroid health

The thyroid gland, at the front of the neck, uses iodine as a raw ingredient for thyroid hormones. NHS inform describes those hormones as helping to keep cells healthy and the metabolism working. Because the body cannot make iodine itself, it has to come from food or drink.

Iodine in food reflects the sea and the soil. Seawater is the major reservoir, which is why sea fish and shellfish are rich in it. Crops and grazing animals pick up iodine from the soil they grow in, so levels in grains and in milk vary by region and by farming practice.

The NHS puts the adult requirement at 140 micrograms a day and says most people can get all the iodine they need by eating a varied and balanced diet. The foods it highlights are cows milk and dairy products, eggs, sea fish and shellfish, with cereals and grains contributing depending on the soil.

In practice, milk and dairy foods carry much of the load for many UK households. Someone who drinks little milk and rarely eats fish may find their intake is thinner than they assume, and swapping dairy for plant drinks changes the picture unless the plant drink is fortified with iodine.

• Cows milk, yoghurt and cheese.

• Eggs.

• Sea fish, including white fish, and shellfish.

• Iodine-fortified soya or oat drinks (check the label).

The NHS highlights strict vegans who do not eat fish, eggs or dairy as a group to consider fortified plant-based drinks or a supplement, after a conversation with a healthcare professional. People with very restricted diets, those who avoid milk because of allergy or intolerance, and anyone eating little fish are also more likely to have low intake.

Needs rise in pregnancy and breastfeeding because the baby depends on the mother for iodine. The NHS iodine page does not give pregnancy-specific figures, so a midwife or GP is the right person to advise on whether a pregnancy multivitamin containing iodine would suit you, and whether any other supplements need to be adjusted at the same time.

Iodine is a mineral where both too little and too much can upset the thyroid. The NHS says taking excessive iodine for a long period can affect thyroid function and cause symptoms such as weight gain, and that 0.5mg or less a day of iodine supplements is unlikely to cause harm.

Seaweed and kelp products can contain large and highly variable amounts of iodine, so they are not a predictable way to top up intake. Anyone with an existing thyroid condition, goitre or a history of thyroid treatment should discuss iodine supplements or seaweed with their doctor first. Check labels so that a separate iodine tablet is not added on top of a multivitamin that already contains it.

In the UK, underactive thyroid is most often caused by an autoimmune condition rather than a dietary shortage, so taking iodine will not fix it. A GP tests thyroid function with a blood test. Treatment for an underactive thyroid is levothyroxine, not iodine.

Swelling of the thyroid, known as a goitre, can occur with iodine deficiency but has other causes as well. A lump or swelling in the neck, or changes in voice and swallowing, should be assessed by a GP.

The simplest approach is food. Include milk or yoghurt, eggs and fish in the week if your diet allows. If you use plant drinks in place of milk, choose one that states it is fortified with iodine and check the label.

If you are vegan, the NHS advice is to consider fortified drinks or a supplement and to speak to a healthcare professional rather than guess at a dose. A registered dietitian can review your usual intake properly.