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Iodine Levels in Seaweeds

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This article aims to clarify some of the confusion about iodine and its safe limits, as there is widely varying opinion and evidence. More information can also be found in the article Thyroid and Iodine by Dr. Craig Rose. Learn more here.


Iodine Levels in Seaweeds

What is the RDA for iodine? How much iodine is there in seaweeds? What is the safe dose of iodine?

This article aims to clarify some of the confusion about iodine and its safe limits, as there is widely varying opinion and evidence. More information can also be found in the article Thyroid and Iodine by Dr. Craig Rose.

By Monica Wilde. February 2013. Updated Oct. 2016

Seaweed is a good source of iodine. Iodine has been approved by the E.U. for the following claims when it is present in foods and food supplements. 

  • Iodine contributes to the normal production of thyroid hormones and normal thyroid function. 
  • Iodine contributes to normal cognitive function 
  • Iodine contributes to normal energy yielding metabolism 
  • Iodine contributes to the maintenance of normal skin 
  • Iodine contributes to normal functioning of the nervous system. 

Iodine is one of the naturally occurring minerals found in seaweeds which are used in several Napiers products: knotted kelp Ascophyllum nodosum (also called rockweed, Norwegian kelp, knotted kelp, knotted wrack or egg wrack) and bladderwrack Fucus vesiculosus (also called kelp, black tang, rockweed, bladder fucus, sea oak, black tany, cut weed, dyers fucus, red fucus, and rock wrack).


We monitor the amounts of iodine that are in our seaweed products and how this relates to what is absorbed by the body (as an average of 33% is excreted in our urine within 24 hours of eating it) and to safe daily levels.


Napiers Hebridean Seagreens Organic Kelp capsules ( Ascophyllum nodosum) contains 350 (micrograms) of chelated iodine per capsule. This is not a standardised amount but the closest consistent average from testing every batch of this seaweed produced over many years.

Of this 350 mcg, only a percentage of that will be absorbed and used by the body. This varies according to how lacking in iodine (insufficient) you are. In people that our study (Combet et al., 2014) found to be iodine insufficient, they excreted, on average, 31% of the iodine ingested by taking the seaweed. With those who had sufficient iodine, they excreted around 46%.

The capsules are designed to be a one a day or two a day dietary supplement. However, some people take more seaweed especially when adding it to shakes and smoothies. And others find that 4 to 6 capsules can make quite a difference to their general wellbeing - equivalent to about 2-3 teaspoons of powdered Ascophyllum. In these cases, it is still probable that they will still not exceed the World Health Organisation's safe upper limit guidance of 1100 mcg iodine per day.


The World Health Organisation considers 1100 mcg iodine per day to be the safe upper limit. 

The US Institute of Medicine has set 1100 mcg iodine per day as a safe upper limit. 

The UK Department of Health recommends that a daily intake of 1000 mcg iodine should not be exceeded. 

However, there is a lot of controversy over these fairly arbitrary figures, and an amount of iodine by a person’s weight has also been proposed. The allowance for a person who weighs 80 kg would be 2400 micrograms per day (mcg/day) – nearly 4 grams of Ascophyllum. See more about this at the UN’s FAO website.  


The safe upper limits above are also usually based on iodine in the form of potassium iodide which is used to enrich salt as an iodine supplement. One teaspoon of iodised salt contains about 400 mcg of iodine from potassium iodide. 

Seaweed is a natural source of iodine, called chelated iodine, which is treated very differently by the body. Seaweed iodine has a lower bioavailability than potassium iodide with an average of 33% excreted in urine within a 24 hour period. 


The current RDA (Recommended Daily Allowance) (minimum intake) is low at 150 mcg per day (220 mcg for pregnant women and 290 mcg for breastfeeding women). 

The daily RDI (Dietary Reference Intake) recommended by the United States Institute of Medicine is between:

  • 110 and 130 µg for infants up to 12 months
  • 90 µg for children up to eight years
  • 130 µg for children up to 13 years
  • 150 µg for adults
  • 220 µg for pregnant women 
  • 290 µg for lactating mothers.

The UK Department of Health’s Report on Health and Social Subjects: Dietary Reference Values for Food Energy and Nutrients for the United Kingdom, states, to summarise, that intakes of up to 5000 mcg per day (i.e. over 7 grams of Ascophyllum) has shown only few cases with any issues of mild effects. 

In cases where seaweed has been linked with thyroid problems, such as the 6 - 12% of Japanese fishermen who have goitre, it has been found that their intakes are 10 - 20 mg/d (10,000 - 20,000 mcg) of iodine (i.e. 14 - 28.5g of Ascophyllum) ~ extremely high levels of intake indeed! 

This is equivalent to consuming between 30 to 60 capsules of Napiers Hebridean Seagreens Organic Kelp capsules per day!!

The UK Report's chapter on iodine states the following: 

"Guidance on Intakes: High iodine intakes can cause toxic modular goitre and hyperthyroidism. Few cases of toxicity have been reported in people with intakes of less than 5000 mcg/d although transient mild effects have been demonstrated in previously deficient individuals receiving only 150 - 200 mcg/d. 

Normal subjects with intake of 1,000 - 2,000 mcg/d showed an increased iodine concentration in the thyroid gland, but no other changes. 

An intake of 10 - 20 mg/d (iodine not seaweed) in Japanese fishermen resulted in an incidence of iodine goitre in 6 - 12%. There appears to be a weak relationship between consistently high iodine intakes and thyroid cancer. The placenta is permeable to iodine and the foetus is more susceptible to iodine–induced hyperthyroidism than the adult. Transient hyperthyroidism has been reported in neonates following high iodine exposure in the mother, particularly in areas of iodine deficiency. Because there remains a small number of elderly people in the UK who may be sensitive to high intakes, the Panel recommend that the safe upper limit on intakes of 17 mcg/kg be retained, or not more than 1,000 mcg/day."


If you have a history of Hashimoto's Disease, an auto-immune thyroid condition, or Grave's Disease you should speak to your doctor before increasing iodine in your diet, or taking products high in iodine, as you may be sensitive to iodine, and could aggravate your condition.

Also speak to your doctor if you are taking a medication for underactive thyroid (not caused by the above conditions) such as levothyroxine. This is because, if your diet corrects an underlying mineral deficiency and improves the functioning of your thyroid, then your medication dose may need to be adjusted accordingly.


It is important to remember that iodine is a vital mineral, vital to the thyroid which controls so much of our body's metabolic and basal systems. 

If there was a nationwide calcium deficiency in the UK, there would be a national campaign to encourage people to drink more milk and dairy products. If would not be ignored until medication was required!

Iodine deficiency can be corrected by increasing your intake of fish, seafood and seaweed (or iodine fortified products). If this is not possible, then taking a safe seaweed supplement  from a reputable company is a sensible solution.


Leung, A. M., & Braverman, L. E. (2014). Consequences of excess iodine. Nature Reviews. Endocrinology10(3), 136–142. 

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