Short answer: iodine is needed to make thyroid hormones. UK adults need 140µg a day, while supplement labels use a 150µg Nutrient Reference Value (NRV) for percentage labelling. Both deficiency and excess can affect thyroid function, so the microgram amount matters.
What does iodine do?
Iodine helps make thyroid hormones, which help keep cells and the body’s metabolic rate functioning normally. When a product meets the conditions of use, authorised GB claims include:
- iodine contributes to the normal production of thyroid hormones and normal thyroid function;
- iodine contributes to normal energy-yielding metabolism;
- iodine contributes to normal cognitive function;
- iodine contributes to the normal functioning of the nervous system; and
- iodine contributes to the maintenance of normal skin.
These are normal-function claims. They do not mean an iodine supplement treats hypothyroidism, Hashimoto’s disease, fatigue or weight gain.
Why do 140µg and 150µg both appear?
The NHS says adults need 140µg of iodine a day. The NRV used on food-supplement labels is 150µg. NRV is the reference used to calculate the percentage printed on the label; it is not a personalised prescription.
For example, 192µg is 128% of the 150µg NRV. It is also more than the NHS adult daily-need figure. Always add iodine from every supplement rather than judging each product in isolation.
Where does iodine come from in the UK diet?
Good sources listed by the NHS include cows’ milk and dairy products, eggs, sea fish and shellfish. Iodine can occur in plant foods, but the amount varies with the soil. Some soya, oat and other plant-based drinks are fortified; check the nutrition label rather than assuming.
The NHS says most people should be able to obtain enough from a varied, balanced diet. People following a strict vegan diet without fish, eggs or dairy may want to consider iodine-fortified foods or a supplement, after seeking healthcare advice.
How much is too much?
The NHS states that taking 0.5mg (500µg) or less a day from iodine supplements is unlikely to cause harm, but it also warns that high doses taken for long periods can change how the thyroid works. “Unlikely to cause harm” is not a target and does not include food iodine.
Government toxicology guidance also notes that high consumers of seaweed may be exposed to iodine amounts that could pose a risk, particularly around pregnancy. Seaweed iodine can vary by species and source, which is why a clearly declared, controlled amount is preferable to an unnamed kelp powder with no iodine figure.
Who should ask for advice before supplementing?
- anyone with a thyroid condition or symptoms;
- people taking levothyroxine, carbimazole or other thyroid-related medicine;
- people who are pregnant, planning pregnancy or breastfeeding;
- children; and
- anyone already using a multivitamin, kelp, seaweed or iodine product.
The current GOV.UK food-supplement guidance advises checking with a healthcare professional before taking supplements during pregnancy or breastfeeding, with medical conditions or regular prescribed medicines.
How should a seaweed supplement be compared?
- Find iodine in micrograms per capsule and per recommended daily intake.
- Check the seaweed species and whether the iodine amount is standardised or tested.
- Look for batch contaminant testing or a credible certification statement.
- Add iodine from multivitamins and other products.
- Do not use “detox”, thyroid-treatment or weight-loss testimonials as evidence.
Our product as a worked example
Organic Seaweed Complex contains 192µg iodine per capsule (128% NRV) from 300mg of organic Ascophyllum nodosum and Fucus vesiculosus. The pot contains 100 capsules. Because the legacy label permits one to three capsules but three would supply 576µg, our current online guidance suggests one capsule daily unless a healthcare professional advises otherwise.
The product also contains a 290mg proprietary blend of spirulina, artichoke, chlorella, green tea, alfalfa and cayenne. Their individual weights are not declared. It contains naturally occurring sulphites.
Frequently asked questions
Does more iodine increase metabolism?
No. The authorised claim concerns normal energy-yielding metabolism; it does not mean that taking extra iodine speeds metabolism or causes weight loss.
Can iodine treat an underactive thyroid?
No food supplement should be used to self-treat an underactive thyroid. Too much iodine can also affect thyroid function. Seek medical assessment.
Is seaweed automatically safer than potassium iodide?
No. “Natural” does not remove the dose issue. Compare the declared iodine amount and quality controls.
Related guides
- UK seaweed and iodine supplements compared
- Chlorella and spirulina explained
- Which supplement health claims are legal?
Sources
- NHS: iodine
- GOV.UK Committee on Toxicity: iodine and pregnancy
- GOV.UK food-supplement guidance
- Great Britain Nutrition and Health Claims Register
Written by: Barry Lees, founder of The Health Improvers
Last reviewed: 2 August 2026
This guide is general information, not diagnosis or medical advice. Food supplements are not substitutes for a varied, balanced diet and a healthy lifestyle.
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