Iodine for vegans: the easiest nutrient to get wrong in both directions
Iodine is the nutrient where plant-based advice most often goes wrong, because the obvious plant source, seaweed, is unreliable in the dangerous direction. Dairy is a large iodine contributor in some countries only because of iodine-based teat disinfectants and supplemented feed — an accident of farming, not a property of milk. A pinch of iodised salt or a cheap tablet replaces it precisely.
Key facts
- Daily target used here
- 150 mcg
- Food sources listed
- 8
- Richest source
- Kombu / kelp — 2500 mcg
- Supplement
- The 150 mcg tablet is the boring right answer
How much you need
US RDA and EFSA adequate intake
US RDA — critical for fetal brain development
US RDA
US RDA
Excess can cause both hypo- and hyperthyroidism
Where it comes from
| Food | Serving | mcg | % of target |
|---|---|---|---|
| Kombu / kelpCan exceed the daily upper limit many times over | 1 g dried | 2500 | 1667% |
| Iodine supplement (potassium iodide)The most predictable option | 1 tablet | 150 | 100% |
| WakameVariable; keep portions small | 1 g dried | 150 | 100% |
| Iodised saltMost sea salt and pink salt are NOT iodised | 1/4 tsp (1.5 g) | 71 | 47% |
| Potatoes with skinDepends heavily on soil iodine | 1 medium | 60 | 40% |
| Fortified plant milkOnly some brands add iodine | 250 ml | 55 | 37% |
| Nori sheetsThe safest seaweed — low and fairly consistent | 1 sheet (3 g) | 50 | 33% |
| CranberriesHighly variable | 100 g | 35 | 23% |
Getting it absorbed, not just eaten
Absorption is not the issue — supply is
Dietary iodide is absorbed almost completely. The problem is that soil iodine varies hugely by region, so the same vegetable can be a source in one country and not in another.
Goitrogens are only a concern at low iodine
Raw cruciferous vegetables and soy contain compounds that mildly interfere with thyroid iodine uptake. With adequate iodine, normal amounts are harmless; cooking reduces them further.
Iodised salt loses potency
Iodine evaporates from open containers over months and degrades with heat and humidity. Keep it closed, and do not count on an old box for your whole intake.
The 150 mcg tablet is the boring right answer
A daily 150 mcg potassium iodide supplement (or a multivitamin containing it) gives a known dose. Avoid kelp-based 'natural iodine' products entirely — their labelling is often wildly inaccurate. In pregnancy, choose a prenatal that lists iodine, since many do not.
Signs of running low
- Goitre — visible thyroid swelling in the neck
- Fatigue, cold intolerance and weight gain (hypothyroid pattern)
- Dry skin and hair loss
- Brain fog and low mood
- In pregnancy, impaired fetal neurological development — the most serious risk
What to test
- Urinary iodine concentration
- The standard population measure; a single sample is noisy for individuals but still informative.
- TSH and free T4
- Detects thyroid dysfunction from either too little or too much iodine.
- Review your salt
- Simply checking whether your salt is iodised answers the question for most people in seconds.
Common mistakes
Using pink Himalayan or sea salt exclusively
Both are usually uniodised. Switching to them is a common accidental way to remove iodine from the diet.
Eating kelp regularly
Kelp and kombu can carry thousands of micrograms per gram, and repeated large doses cause thyroid dysfunction. Use kombu to flavour a broth and remove it, rather than eating it.
Assuming a prenatal contains iodine
Many do not. Check the label explicitly during pregnancy.
Questions people ask
Do vegans need an iodine supplement?
Unless you use iodised salt daily or eat nori regularly, yes — a 150 mcg supplement is the simplest way to be sure. Intakes in vegans are frequently measured below recommendations.
Is seaweed a good iodine source?
Nori is reasonable and predictable. Kelp and kombu are not: iodine content varies by orders of magnitude and can easily exceed safe limits.
Can too much iodine be harmful?
Yes. Excess can trigger both underactive and overactive thyroid, and thyroiditis. The adult upper limit is 1,100 mcg a day, which a single gram of kelp can exceed twice over.
Does soy affect iodine or the thyroid?
Soy has a mild goitrogenic effect only when iodine intake is inadequate. With sufficient iodine, normal soy consumption does not impair thyroid function.
Which salt should I buy?
Any salt labelled iodised. Availability varies by country — in the UK, for instance, iodised salt is uncommon, so a supplement is often the more practical route.
References
Other nutrient guides
Reviewed 2026-08-23 by the Veg.ac editorial team · Data: NIH ODS — Iodine fact sheet · Free to reuse under CC BY 4.0 with a link back.
General information, not medical advice. Talk to a clinician or registered dietitian about your own situation.