Iodine

Iodine has a narrower useful range than almost anything else on a supplement shelf. The thyroid uses it to build its hormones, and it goes wrong at both ends.

The numbers

The NHS puts it plainly: “Adults need 140 micrograms (μg) of iodine a day.”

At the other end: “Taking high doses of iodine for long periods of time could change the way your thyroid gland works. This can lead to a wide range of different symptoms, such as weight gain.” And a practical ceiling for supplements: “Taking 0.5mg or less a day of iodine supplements is unlikely to cause any harm.”

Between 140 micrograms and 0.5 milligrams is a factor of about three and a half. That is not a wide margin, and it is unusual for a mineral to have its floor and its practical ceiling so close together.

Where the shortfall comes from

The NHS names cows’ milk and dairy products, eggs, sea fish and shellfish as good sources. Iodine is in plant foods too, but “the levels vary depending on the amount of iodine in the soil where the plants are grown”, which makes plant sources unreliable rather than absent.

So a diet without dairy, eggs or fish is the configuration where a shortfall is likely, and the NHS says so: someone “following a strict vegan diet” who eats none of those “may want to consider foods fortified with iodine or consider taking an iodine supplement”.

This is the second nutrient after Cyanocobalamin where a plant-based diet needs a specific plan rather than general good eating, and it is much less discussed than B12.

The label detail that matters: some plant-based drinks are fortified with iodine and many are not, and there is no way to tell without reading. The NHS says to check. Organic versions of the same drink frequently omit it.

Where the excess comes from

Almost always seaweed.

Kelp and other seaweed supplements can carry iodine in quantities far above both figures above, and the amount per capsule is often not stated in a form you can use, or varies between batches because the raw material is a harvested organism rather than a manufactured compound.

Someone taking a kelp supplement for general wellbeing can exceed the ceiling many times over without any number on the pack telling them so. That is the specific failure mode this page exists for.

Iodised salt is a different matter and is the intervention that eliminated iodine deficiency disorders across much of the world. The amounts are calibrated for that purpose.

If you have a thyroid condition already

The advice changes and it is not general. Someone with an existing thyroid condition, on medication, or with a nodule, can respond to iodine differently from someone without one, and the direction is not always predictable.

That is a conversation with whoever manages it, and the useful thing a label can do is tell you what is in a multivitamin or a seaweed capsule so you can say what you are taking. The other supplement-side thyroid questions, including Biotin and blood test interference, are on the thyroid page.

On the label

In supplements it appears as potassium iodide or sodium iodide, usually with the elemental amount in micrograms.

In seaweed products it may appear only as kelp, laminaria, bladderwrack or a similar name, with no iodine figure at all. That absence is the thing to notice, because it is the case where you cannot do the arithmetic even if you want to.