Iodine is unusual among supplement ingredients: the evidence for what it does is genuinely settled, and the health claims are authorised rather than aspirational. It is also one of the easiest nutrients to get wrong — in both directions.
This is part of our guide to choosing a seaweed supplement.
What iodine actually does
Your thyroid gland uses iodine to make thyroid hormones, which regulate your metabolism — the rate at which your body uses energy. Without enough iodine, the thyroid cannot make enough hormone.
Because that role is well established, iodine carries several authorised GB health claims, usable where a product provides enough to qualify and meets the conditions of use:
- 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
- Iodine contributes to the maintenance of normal skin
These are permitted precisely because the science behind them is strong — rare in the supplement world, and worth valuing.
How much you need
The UK reference intake for adults is around 140 micrograms (µg) per day. Requirements rise in pregnancy and breastfeeding, when iodine is important for the baby's development.
On the upper side, the NHS advises that iodine supplements providing 500µg or less per day are unlikely to cause harm. That is not a blanket guarantee for every individual, because what matters is your total iodine from food and all supplements combined — the gap between "enough" and "too much" is smaller for iodine than for many nutrients, which is why the amount on the label matters.
Where UK diets get iodine
The main sources in a typical UK diet are:
- Dairy — milk and yoghurt are the biggest contributors for most people
- Fish and shellfish — particularly white fish
- Eggs
- Seaweed — very high, but very variable
Notice what is missing: plant foods are generally poor iodine sources. This is why vegans and people who avoid dairy and fish are at real risk of low iodine intake — a documented gap rather than a marketing invention. Plant milks are often not fortified with iodine, which widens it further.
The two ways to get it wrong
Too little
Mild iodine deficiency has re-emerged in parts of the UK population, especially among young women and those on plant-based diets. Low intake affects thyroid hormone production, and in pregnancy is particularly important.
Too much
This is the one seaweed marketing tends to skip. Excess iodine can also affect thyroid function, and can trigger or worsen thyroid disorders in susceptible people. High-dose kelp supplements have been documented as a cause — some deliver well above the daily requirement in a single capsule, with content varying batch to batch because the seaweed itself is uncontrolled.
The lesson: a controlled, stated iodine dose from a quality-tested source gives you far more certainty than a "natural" kelp product with no figure on the label.
Who should take advice first
Speak to your GP before taking iodine or seaweed if you:
- Have any thyroid condition (overactive or underactive)
- Take thyroid medication such as levothyroxine or carbimazole
- Are pregnant or breastfeeding — you need more iodine, but the amount and source matter, so get advice
- Have been advised to limit iodine for any medical reason
The NHS recommends seeking professional advice when considering iodine supplementation generally.
Practical takeaways
- If you eat dairy, fish and eggs regularly, you probably get enough iodine already.
- If you are vegan or avoid dairy and fish, iodine is one of the nutrients genuinely worth attention — check whether your plant milk is fortified.
- If you supplement, choose a controlled source with a stated microgram amount, and avoid stacking multiple iodine sources.
- If you have a thyroid condition, do not self-supplement — talk to your GP.
Our Organic Seaweed, Greens & Adaptogens Complex uses Seagreens — a certified food-grade seaweed with controlled iodine — and provides 192µg of iodine per recommended daily serving, equivalent to 128% of the nutrient reference value, so the amount is declared and consistent rather than left to chance. Iodine contributes to the normal production of thyroid hormones and normal thyroid function. As above: if you have a thyroid condition or take thyroid medication, please check with your GP first.
Read next: how to choose a seaweed supplement and what chlorella and spirulina actually are. Or browse our Energy & Vitality range.
Always read the label. Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle. This article is general information, not medical advice.
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